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California Scientific
4011 Seaport Blvd
West Sacramento, CA 95691

Making More Horsepower

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Making horsepower can be addressed from two sides: engineering, and mythology. In this article I will focus on the engineering. I'm not an expert on mythology. Some of the claims I've heard are truly remarkable: "changing pipes on my Sportster added 35 horsepower", "my big twin is making 130 horsepower", "I can run my Softail at 7,500 rpm all day long". Apparently these bikes are not subject to the laws of physics. We'll see in this article that actually, none of these claims are believable.

Stock Harley Davidson engines are tuned to meet noise and emission standards. In the '70's and '80's, Harleys came with more power. The Harley engine can be re-tuned to make substantially more torque and horsepower with no loss in fuel mileage or reliability. After you do this, however, your bike will be somewhat louder and fail to meet emission tests.

In this article, I'll make specific recommendations. These recommendations are based on the premise that you want a reliable engine which makes good torque at low rpms, and reasonable horsepower at high rpm's. If you want to race, or want to ride around town with a racing engine, my recommendations are not for you. Of course, a racing engine will have relatively poor performance at low rpm, use a lot of gas, cost a lot more to build, need more maintenance, and will not last very long. But, they are a blast while they last.


In chart 4 below are the engine torque curves for five different motorcycles. The Harleys are V-twins, the BMW is a flat twin, the Honda is a V-4, and the Kawasaki is an in-line 4. We see that these different engines based on very different technologies all made about the same peak torque. If your engine is configured correctly, it will make about 1 foot pound of peak engine torque per cubic inch. Best case is about 1.1. That's pretty much it.

torque curves for 5 motorcycles

For peak torque, this leaves us with only a few things to play around with. First, there's the "configured correctly". Your engine is basically just a large air pump, and you want it to have a good pumping efficiency. On a stock Harley, there are two things which can be improved relating to pumping efficiency: carburetors and exhaust pipes.

Also, we see in chart 4 that, although the engines make about the same peak torque, they achieve this torque at very different rpms. The Kawasaki is the highest revving engine, making peak torque at 8500 rpm, and peak horsepower at 10,500 rpm. The California Harley makes its power at the lowest rpm: peak torque is at 2400 rpm, and peak horsepower is at 5000 rpm.

The horsepower curves for these bikes are in chart 5. We see that the bikes which make their torque at the highest rpm also make the most horsepower. This is a fact from physics: to make more horsepower, you need to make your torque at higher rpm.

horsepower curves for 5 motorcycles

You can choose the rpm at which your bike makes its peak torque by changing the cam shaft. At the same time as you change your cam shaft, you must consider making other modifications to your engine including push rods, valve springs, valve jobs, valve spring keepers, and head porting. Depending on the cam shaft you choose, you may need none of these extra modifications, or some, or all.

Finally, the Harley ignition system is again chosen for emission tests. You can improve the low-rpm power and over all performance of your bike by changing this.

Here's about the best we can do with a 80 inch Harley: about 85 foot-pounds at about 6000 rpm. This works out to 97 horsepower. To do better, we either need to over-rev the engine, bore or stroke the engine, or get an assist from the gods on Mount Olympus.


The stock carburetor is configured to meet smog and noise regulations, not to produce power. Stock Harleys run very lean for emissions purposes, which makes them run hot and get good gas mileage. When you tune your carburetor correctly, you will cool down the engine and lose about 2 miles per gallon. Also, the stock air cleaner is very restrictive and almost completely enclosed to reduce intake noise.

There are several approaches to fixing this. All approaches start with replacing the air cleaner. This must be followed by either rejetting the carburetor, or replacing it. Just replacing the air cleaner and fixing the carburetor increases horsepower by 10-15%. Rejetting can be done by adding a Dynajet kit. Or, the carburetor can be replaced with a Mikuni, S&S, or other high quality carburetor.

Some people prefer the stock CV Keihen carburetor because of its altitude compensating abilities: a constant vacuum carburetor is less sensitive to altitude changes than a slide carburetor like a Mikuni. If you tour through the Rockys and Sierras, the stock Keihen CV, rejetted, might be a good choice for you. If you have an older Harley with a non-CV Keihen, you should be able to buy a used CV Keihen at a swap meet for very little money.

Some people prefer the Mikuni because of its excellent performance at all rpm. The 40 millimeter Mikuni works marginally on an 883 Sportster, and very well on 1000, 1100, and 1200 Sportsters and 74 and 80 inch big twins. The 42 millimeter Mikuni is a bit too big for a 1200 Sportster or a 1340 big twin, and should only be used on larger engines.

Finally, some people prefer the S&S because it has very good performance, many Harley mechanics are familiar with its jetting characteristics, and it's made in America.

A Screamin' Eagle air cleaner is about $40, and the Dynajet kit is about $75. Expect to pay about $100 to have them installed. A Mikuni carburetor kit, which includes an air cleaner, is about $300. Expect to pay about $100 to have it installed. An S&S Super E carburetor is about $340, including an air cleaner, plus about $100 for installation. You can also buy a Bendix carburetor for about $275.

Exhaust Pipes

Exhaust pipes must be correctly tuned in terms of head pipe diameter, head pipe length, and muffler back pressure. For example, in the '70's, Harley Davidson had two nearly identical racing pipes for the XR 750 - one was 1/2 inch longer and made seven more horsepower. Drag pipes are the worst. Staggered duals are generally best at low to mid rpm, and two into one pipes are generally best at higher rpm. Generally, better pipes are louder, although the loudest pipes are usually drag pipes, and these make terrible power.

The stock exhaust pipes are rather restrictive, although they are not nearly so bad as many people think. Simply drilling a 1 to 1.5 inch hole through the baffles of the stock Harley pipes makes about 93% as much horsepower as the best pipes, and more horsepower than a rather embarrassing number of after market pipes. This does not work on '95 California pipes with catalytic converters - you'll have to buy new pipes or new mufflers. California riders should be able to buy a set of stock '94 or earlier exhaust pipes for about $50 from your Harley shop or local swap meet. There's about a billion sets of these available, and about zero purchasers.

For power, Cycle Shack, RevTech, Python II and SuperTrap pipes are all excellent. Unless you have substantially more than 80 cubic inches, you should stick to 1 3/4 inch pipes - 2 inch pipes will not perform as well.

Cycle Shack pipes are $160 - $225. Python II pipes are about $275. SuperTrap pipes are $325 - $400. Installation of any of these pipes should be about $25.

Compression Ratio

Next, for torque, there's "brake mean effective pressure". BMEP is a rather complicated thing, but it is almost the same as compression ratio. Big twins are built with compression ratios of about 8.2 to 1. If you have 50-100 thousandths milled off the heads, your compression ratio will raise to 9-10 to 1. 9 to 1 compression is 10% higher than 8.2 to 1, and therefore will make 10% more torque than 8.2 to 1. Unfortunately, you can't just keep milling the heads. At about 9.5 to 1, most Harleys will start to detonate (that's bad), and at about 10.5 to 1, almost nobody can keep them running (yes, I know there are a few magicians out there).

Generally, removing 50 thousandths is completely safe, removing 80 thousandths is relatively safe, and removing 100 thousandths requires good ignition and carburetor tuning. Removing 80 thousandths will turn a 70 horsepower engine into an 80 horsepower engine.

Unfortunately, in order to mill the heads, you need to open up the engine. You should expect to pay about $200 for disassembly and assembly, and about $250 for machine work.

Alternatively, you can buy Wiseco forged high-compression pistons. These will cost about $200, plus $200 for assembly, plus $100 for boring the cylinders.

Larger Bore and Stroke

Finally, for more torque, there's more cubic inches. As they say in drag racing, "there's no substitute for cubic dollars." To increase the size of your engine, you can bore out the cylinders and use larger pistons, or trade crankshafts for a "stroker". As we've seen, engines make about 1 foot-pound of torque for each cubic inch, so if you increase your big twin from 80 to 90, 100, or 120 cubic inches, and the carburetor and exhaust pipes are correctly selected, you can raise your torque from 80 to about 90, 100, or perhaps as much as 120 foot-pounds. Again, there are trade-offs involved here.

At any given rpm, the pistons are moving up and down once per revolution. This means at 5000 rpm, the pistons are moving twice the length of the stroke per revolution, times 5000 revolutions per minute. On a stock big twin, which has a stroke of 4.25", the pistons are moving 4.25 x 2 x 5000 = 42,500 inches per minute = 3,542 feet per minute. This is pretty fast. Metal technology can only support running a piston up to about 4,200 to 4,500 feet per minute on a regular basis. Our Kawasaki ZX-11, for example, revs to almost 12,000 rpm, but has a stroke of only 2.28", and is therefore running its pistons at 4,370 feet per minute at 11,500 rpm. A big twin runs 4,300 feet per minute at 6,070 rpm. The Sportster, with a stroke of 3.8 inches, is running its pistons at 4320 feet per minute at 6,800 rpm.

You will hear of people who run their big twins up as high as 7,500 rpm, but they are putting drag racing loads on their engines. Remember: drag engines only have to live for about 10 seconds, and typically only run about a total of 5 minutes before they're shot. Most of us expect better longevity from our engines. I would not run a Harley big twin over 6,000 rpm. Although a Sportster piston can run almost up to 7,000 rpm, the exhaust valves start to float at about 6,500 rpm. This is bad. With stock valve springs, a Sportster should not be run over 6,500 rpm.

It is interesting to note that the Chevy 350 in a Corvette ZR-1 has a stroke of 3.48" and only goes 6,000 rpm, and the Chevy 454 has a stroke of 4" and only goes 4,600 rpm. The Sportster has a longer stroke then a 350 Chevy, and the big twin has a longer stroke than a GM 454. If you thought your Harley was engineered to the same standards as GM engines, you would not exceed about 6000 rpm on a Sportster and 5000 rpm on a big twin.

When you put a stroker crankshaft in your bike, this only gets worse. For example, a 100 inch stroker will typically have a 5.25" stroke. If you believe that your pistons should not exceed 4,300 feet per minute, then you believe that this engine should not exceed 5,000 rpm. Thus, while the torque from a stroker can be truly awe-inspiring, the maximum allowable rpm and therefore the maximum available horsepower will not break a world record. A 100" stroker putting out 100 foot-pounds of torque at 4000 rpm and 80 foot-pounds of torque at 5000 rpm is making 76 horsepower at both 4000 and 5000 rpm. This may pull your arms out of their sockets, but some well-built 80 inch twins will beat you in a race because of their ability to run at higher rpm.

Some engine builders will tell you that their strokers can exceed these rpm limits. David Ogilvy once said: "Advertising is to a business what engines are to an airplane: you can turn them off and coast for a while...". Well, you can exceed these rpm limits for a while, too.

Big bore kits and stroker cranks are expensive, require substantial engine work, and must be part of a complete package which includes carburetion and typically head work. Expect to pay $1500 to $2500. I would choose a mechanic with a lot of experience building racing engines. Some combinations of pistons and crankshafts work a lot better than others.

If you have an 883 Sportster, boring it out to a 1200 using either the Harley kit or the Wiseco kit is a very inexpensive way to get a lot more power. The Harley kit is, well, from Harley. It requires you to reshape the combustion chambers. This is not too hard. Also, the 1200 pistons are a little heavier than the 883 pistons, so your crankshaft will be a little out of balance and your bike will vibrate a little more. This is not a big deal. The Wiseco kit does not require combustion chamber reshaping, and the pistons are made to balance correctly with the 883 crankshaft. With either kit, if you leave in the 883 valves, you'll make better torque in the low end, and a little less horsepower in the top end. Changing to 1200 valves will give you the same power band the 1200 has.

Cam Shafts

Now that your Harley is making as much torque as it can, you can choose your power band. That is, you can choose the rpm where your engine makes its peak power. The most important component for choosing your power band is the cam shaft. The cam shaft determines when and how much your intake and exhaust valves open.

First, we'll discuss valve lift. Valve lift is the amount your valves open. On a stock evolution big twin build before 1992, the valve lift is .495 inches - about 1/2 inch. In 1992, Harley changed their cam. The valve lift is now .472 inches.

The more lift your cam has, the more fuel/air mixture gets in your engine. As usual, there's a limit. If your cam has more than 1/2 inch of lift, the stock valve springs bind. This is bad. Also, if your valve lift at piston top dead center is too much, the valves might hit the pistons, or bump into each other. This is very bad. Cams with more than 1/2 inch of lift should only be installed by expert racing mechanics, and generally are not suitable for street bikes.

Next, we'll discuss duration. The cam shaft determines when the valves open and close. This is actually more complicated than it seems at first.

On the compression and power strokes, nothing much interesting happens with the valves. They mostly just sit closed in their seats.

When the piston is almost at the bottom of the power stroke, the crankshaft pin is mostly moving front to back, and the piston is mostly not moving. Because of this, the piston has already extracted all the power from the burned fuel that it is going to get. So, we can open the exhaust valve a little before the piston reaches bottom.

When the engine is on its exhaust stroke, the piston is moving up, and pushing the exhaust gases out the exhaust pipe. While this is happening, the air in the exhaust pipe builds up a significant amount of momentum. Because of this momentum, the intake valve can be opened before the piston reaches the top.

When the cam shaft first opens the intake valve, the valve is only opened a small amount. So, it's best if the intake valve is busy opening while not much intake is happening. Also, the momentum in the exhaust pipes causes an effective suction at the intake valve, so even though the piston is still coming up, at high rpm the intake valve can usefully open.

When the piston is first starting to travel down on its intake stroke, the momentum in the exhaust pipes will continue to pull the burned fuel / air out the exhaust valve and in from the intake valve. So, for a little while, even though the piston is pulling down on its intake stroke, we leave the exhaust valve open. At some point in time, which depends on the engine rpm, the intake manifold, and the exhaust pipes, the intake charge reaches the exhaust valve. This is the time to close the exhaust valve.

This period when both the intake and exhaust valves are open simultaneously is called the overlap. Stock Harley cams have no overlap. Typical performance cams have 30 to 60 degrees of overlap - that is, for 30 to 60 degrees of crank shaft rotation, both valves are open. During the overlap period, particularly at low rpm, some of the intake charge will go out the exhaust valve unburned. This is very bad from an emissions standpoint.

When the piston reaches bottom on the intake stroke, there's still a lot of momentum in the intake manifold. Also, while the piston is near the bottom of the stroke, the crankshaft pin is mostly moving front-to-back, not up and down, so the piston is basically not moving for a while. Because of these two effects, the intake valve is left hanging open for a while. The higher the rpm at which you wish to make power, the longer you leave the valve open.

So, although one might feel intuitively that the valves are each open for 180 degrees, in fact with most cams the valves are open significantly longer than that. The longer the valves are open, the higher the rpm at which the cam is effective.

The stock California cam keeps the valves open for about 178 degrees of crank shaft rotation, so we say it has about 178 degrees of duration. It makes peak torque at 2,400 rpm. The Crane Fireball 310 cam has about 238 degrees of duration, and makes peak torque at about 4,000 rpm. The Kawasaki ZX-11 cam has about 288 degrees of duration, and makes peak torque at 8,500 rpm.

So, we can roughly predict how much cam duration we want, depending on where we want to produce our peak torque. It must be emphasized that these figures all depend on the carburetor, intake tract, exhaust pipes, precise cam timing, rod to stroke ratio, and the flow characteristics of the head. These factors can change the peak torque rpm by 1000 rpm in a Harley. Never the less, we can still see a basic trend.

In chart 3, the graph shows us which cams will produce about what peak torque rpm. Since a Harley big twin should not rev much over 6,000 rpm, the "correct" cam for a big twin will produce peak torque at 3,000 to 5,000 rpm, and therefore have about 190 to 250 degrees of duration. Less duration will produce more low end torque and less top end horsepower; more duration will produce less low end torque and more top end horsepower.

Peak Torque vs Cam Duration

There are many popular cam shafts made which fall into this range, including stock Harley Davidson cams from pre-1992; the Andrews EV13, EV27, EV3, EV46, and EV35; the Crane Fireball 300, 310, and 316; the Sifton Interstate and Ascot; and the Bartels BP20 and BP40. Stock Harley cams from 92-95 are best used as door stops and paper weights. I cannot personally recommend throwing stock Harley cams through the windows of the EPA offices, but I admit I would be very amused to read about it happening.

When changing the cam in a Harley, your selection should be based on what power characteristics you want in your engine. The stock 88-91 cam, the EV13, and the Fireball 300 are all comparable, with about 225° duration, and an emphasis on low end torque, at the expense of some top end horsepower. The EV3, EV27, Fireball 310, Interstate, and BP20 are all comparable with about 235° duration, giving good low end and top end and strong mid range. The EV46, EV35, Fireball 316, Ascot, and BP40 are all comparable with about 246° duration, and an emphasis on mid range and top end, sacrificing some low end torque. This last group should probably only be used in the lighter big twins such as FXRs and DynaGlides. Their low end torque characteristics are less suitable for the heavier Softails and FLHs.

Strokers have a very different rod to stroke ratio than stock motors, and different stroke to intake tract length ratio, and accordingly they need different cam shaft durations to achieve the same results. If you are building a stroker, you should ask the crank shaft factory and the cam shaft factory for recommendations.

When you are changing the cam shaft in a big twin, there's a trick: if you have adjustable push rods, you don't need to take the valve covers off the bike to change the cams. You use a bolt cutter to remove the stock solid push rods, then remove the old cam, insert the new cam, and insert then adjust the new push rods. So, if you're not doing head work, you can change the cam shaft without even removing the gas tank. If you are doing head work, then you will need adjustable push rods to compensate for the new head height. So, basically, you should always use adjustable push rods.

Push rods come in aluminum and chrome moly. The aluminum works just fine for most motors; however, the chrome moly rods are only $10-$20 more, so whatever you choose will probably work out. Chrome moly rods should be used in high rpm high cam lift engines.

Head Work

The stock Harley heads flow air just fine up to 4,000 to 4,500 rpm. A lot of people spend $700 to $1200 to get ported and polished heads which promise to make more horsepower. Well, they do, but only at very high rpm, and often at the expense of low rpm torque.

If achieving maximum air flow was our only consideration, the Holley 780CFM 4 barrel carburetor would be a standard hop- up device for Harleys. In fact, if you bolted one of these Holley carburetors up to a Harley, the Harley would never get the air flowing through the carburetor fast enough for the carburetor to start working. Faster intake flow makes the carburetor and cam shaft work better. Smaller ports make faster flow. However, smaller ports will not support enough air flow at high rpm. So, as usual, there's a trade off between low end torque and high end horsepower.

If you do get your heads ported, the exhaust port should be polished to a mirror surface. The less drag in the exhaust ports, the better. Also, it's a good idea to have your exhaust pipes and exhaust ports matched up.

On the intake side, the ports should be finished to about a roughness of 250. This feels like the strike area on a match book. This roughness creates a small amount of turbulence in the intake charge, which helps the air and fuel mix for better combustion.

On stock Harley heads, there's a lump just above the intake valve seat on the inlet side, and another lump just above the exhaust valve seat on the outlet side. Smoothing out these lumps gets you most of the benefit available from porting the heads.

A really good, experienced person can re-shape the combustion chambers for more efficient burning. Good combustion chambers can allow you to run at higher compression ratios, which makes more torque everywhere. The stock Harley combustion chambers work pretty well up to about 9.5 to one compression. To get much past 9.5 to one, you need to improve the combustion chambers, or add dual spark plugs, or both.

You can build a good, reliable 70-75 horsepower engine with great low end torque for about $1000 without ever removing the valve covers. For another $1000, you can have the heads removed, milled, ported, polished, and re-installed, and pick up an extra 5-10 horsepower.

Depending on who ports your heads, you could also lose low-end torque due to excessive port size. And, ported heads are like cut hair: you can't put it back. Your local shop almost certainly knows somebody who "ports heads better than Jerry Branch". Interestingly, as near as I can tell, every city in America has some guy who "ports heads better than Jerry Branch", and another guy who is "15th degree red belt, and the emperor's personal bodyguard". If you're going to get your heads ported, I suggest you send away and get the real thing from Branch, Hardy, or RevTech.

I'm not real big on porting heads for street motors. However, since there's hundreds of dollars of shop time involved, many mechanics are real big on ported heads.


The ignition on a stock Harley is calibrated for EPA emission testing. In the real world, the stock ignition causes hard starting, poor low end response, back firing, and probably cancer and heart disease. The single biggest improvement you can make to a Harley, in my opinion, is to replace the ignition.

The Harley ignition is electronic. This means that attached to the end of the crankshaft, there's a little magnet. A small coil of wire picks up when the magnet sweeps by. Nearby, there's a little computer system, which looks at the pulses coming in from the coil and decides when the ignition should fire.

At low rpm, the spark plugs are to fire just a little bit before the pistons get to top dead center. As the rpm increases, the spark plugs must fire more and more ahead of the pistons reaching top dead center. This is because the flame front of the burning fuel in the combustion chamber takes a little time to work its way across the cylinder. Since this flame propagation time does not change, as the engine runs faster and faster the spark must come earlier and earlier so as to have the fuel completely burned when the piston is at the top of the cylinder.

Making the spark plugs fire earlier and earlier is called spark advance. The computer module, also known as the ignition module, has built into it a table: for various rpm, the table tells the computer how much to advance the spark. This table is called the ignition map or advance map. On some vehicles (but not Harleys) this map is built into a separate, replaceable chip; replacement chips are called power chips. Also, on fuel injected vehicles (such as Harley's '95 FLHTCI), there is another table in the chip which tells the computer when to fire the fuel injectors.

The Harley ignition map was designed to mollify EPA officials. From a rider's point of view, it's junk. Unfortunately, since the chip is sealed into the computer module, we have to throw away the entire computer system and replace it with a new one. Fortunately, computer modules are cheap these days. A new ignition module costs $100 to $250, depending on the brand and the options (sorry, no color monitors available).

There are any number of fine ignitions available. The Screamin' Eagle ignition module just plugs in: installation takes less than ten minutes and requires only screwdrivers. It costs about $100 and will fix all of the above listed problems.

For another $50, you can get the Screamin' Eagle ignition coils. These are not very important unless you're planning to run your engine above 5,000 rpm frequently.

For more money and more work, you can also get a fine ignition system from Dyna, Accell, Jacobs, CompuFire, or MC Power Arc.

Some of these ignition system are "single fire", and some are "dual fire". Dual fire ignition systems, like the Harley systems, use one coil for both cylinders and fire both spark plugs at the same time. This means one cylinder is getting a spark while it is about half way through compression. This sounds really bad, but actually it's no big deal: the mixture is not compressed enough to light up, so not much happens.

Single fire ignition systems have two coils - one for each cylinder - and fire each spark plug independently. Of course, this is just better. However, it's more expensive, and you have to find places to hide two coils. A single fire ignition will improve your idle and make your engine run more smoothly with less vibration, particularly at low rpm.

For most people, just using the Screamin' Eagle ignition module will work just fine. If you're building a high rpm high horsepower engine, you should probably get a single fire ignition.

Some engine builders remove the Harley ignition system altogether and replace it with points. This is insane. If you intend to ride your Harley more than 50 miles at a time, or get more than 50 miles away from home, don't do this. The only good things to say about points is that they are easy to understand, and easy to throw away.


The last thing you can do for the performance of your bike is change the gearing. Harley Davidson has made two front pulleys, 27 tooth and 29 tooth, and two rear pulleys, 61 tooth and 70 tooth. In 1994, 883s came with 27/70, and 1200 Sportsters and all big twins came with 29/70. '93 big twins came 27/61 (softail) and 27/70 (everything else). '95 big twins come 27/61 (softail) and 29/70 (everything else). The best combination for acceleration is 27/70. The combination which gets you the lowest rpm in fifth gear is 29/61. I consider 29/70 to be a pretty good compromise.

Table 1 shows the top speed in each gear for the various alternatives. The difference between 29/61 and 27/70 is almost a full gear. Each step is about 7%. A 7% reduction in gearing gives you a 7% improvement in rear-wheel torque. This is about the gain you get from the best exhaust pipes made. Softail's acelleration will particularly benefit from a switch to 29/70 or 27/70; however, there will be more engine rpm at 60 mph, and therefore more vibration.

Table 1: Road speed at 5500 rpm in each gear
Pulleys 29 / 6127 / 6129 / 7027 / 70
1st gear45 mph42 mph40 mph37 mph
2nd gear70 mph65 mph57 mph53 mph
3rd gear91 mph85 mph79 mph74 mph
4th gear119 mph111 mph104 mph97 mph
5th gear147 mph137 mph129 mph120 mph
rpm @ 602250 rpm2400 rpm2570 rpm2750 rpm

Trouble shooting

This list is by no means comprehensive, but it will help. There are several books on building Harley engines which can help, including the Clymers shop manual. Also, Mikuni sells a carburation manual for $7.95 which is indispensable.

Engine pops or backfires when you close the throttle and decelerate in gear.
The low speed jets in the carburetor are too lean.

Exhaust pipes are turning blue.
The carburetor is too lean, probably in the needle jet.

Exhaust pipes have black carbon buildup inside, which comes off on your finger.
The carburetor is too rich, probably needle jet or needle seat. You're blowing unburned gas out your exhaust pipes.

Engine detonation (sounds like loose change rattling around in the engine):

  1. Full throttle only, on hot days, at low rpm. Carburetor is a bit lean in needle seat or main jet. Not too serious. We all have this problem sometimes.
  2. Full throttle only, on hot days, at mid or high rpm. Carburetor is a bit lean in needle seat or main jet. This is more serious. Get on this problem soon.
  3. Happens even while cruising at a constant speed. Carburetion is way too lean, or compression ratio is too high, or the gas you just bought is Tijuana special, cut with #2 diesel. Pull off the road, call for a trailer. This is big time serious.

Black smoke on full throttle acceleration.
Main jet is too rich.

Grey smoke on full throttle acceleration.
Bad valve guide seals, or bad rings. The engine was mis-assembled.

Grey smoke when you start the engine, particularly when cold.
Bad valve guide seals. The engine was mis-assembled.

Ticking sound which goes faster and slower as the engine does.

  1. Bad spark plug connection to ignition coil (pray this is it).
  2. Stuck lifter.
  3. Improper valve to valve clearance, or improper valve to piston clearance.

Engine misses at idle or full throttle acceleration.
Bad connection from ignition coils to spark plugs.

Truly crummy gas mileage, as in less than 40 when cruising, or less than 35 around town.

  1. Too much cam duration. You're blowing gas out your exhaust pipes.
  2. Carburetor is way too rich, especially in the midrange.
  3. Accelerator pump on carburetor is adjusted too aggressively.

Oil drips out the air cleaner.
Your crank case vent hoses are improperly routed. Switch to the Screamin' Eagle part, or route the vent hoses up under the gas tank, then down to under the battery. Remember to plug up the holes in the air cleaner. This is a common problem with Mikuni and S&S carbs on '92 or later evolution motors.

Throttle sticks, particularly when turning sharply at slow speeds.
Throttle cables are improperly routed. Straighten them out.

Some guy with a ZX-11 just blew you off the highway.
Welcome to reality. Take comfort in the knowledge that women hate crotch rockets. Evolution will have its way in the end.


When I bought my first Harley, I knew that it was possible to get a lot more power out of the engine, but I didn't know much else. Since then, I've learned what is important, and what just costs money. If you want the absolute best motor money can buy, there are any number of very good, very competent speed shops, including XRV (North Hollywood, CA. 818/762-5407), and Performance Techniques (Huntington Beach, CA. 714/841-7971). Just bring money.

On the other hand, you can do very well on your own, either doing a little bit of wrenching, or paying a friend or a professional mechanic. The setups listed below will give you a lot of performance gain for relatively little money. #2 gives you the most bang for the buck. #3 is what I use on my bikes. #6 is very close to what most speed shops will build for you. Prices vary considerably, so shop around. Call local shops, check mail order ads, and ask manufacturers.

In this list, systems #1, #2, and #3 will improve your power at all rpm, make the engine run cooler and smoother, and have very little effect on your gas mileage. System #4 will have noticeably less low end power then #3, probably get 2-3 miles per gallon less, and produce noticeably more high rpm power. System #5 will probably require premium gas, and possibly an oil cooler. It should get good gas mileage - possibly the best of any system here. System #6 will noticeably reduce your low end power, require a bunch of premium gas, probably require an oil cooler, and will allow you to overheat the engine if you use the throttle excessively.

  1. $250-$350 basic improvements.
    Carburetor: Screamin' Eagle air cleaner, Dynajet kit
    Ignition: Screamin' Eagle ignition module
    Exhaust: drill out the stock baffles (except '95 California models)
    Cost: $250 for parts, $100 for installation.
    Your engine will make about 15% more power, get about the same gas mileage, and run much smoother and cooler.
    These parts can be installed by a moderately experienced home mechanic using the included instructions in about 2-4 hours.
  2. $400 - $600 Touring or Sport.
    Carburetor: Screamin' Eagle air cleaner, Dynajet kit
    Ignition: Screamin' Eagle ignition module
    Exhaust: drill out the stock baffles (except '95 California models)
    Camshaft: Andrews EV 13, or Crane Fireball 300 cam (heavy bike, conservative rider)
    Camshaft: Andrews EV 27, Bartels BP20, Crane Fireball 310 cam, or Sifton Interstate
    Camshaft: Andrews N2 (Sportsters only)
    Push rods: adjustable aluminum
    Cost: $400 for parts, $200 for installation.
    Your engine will make about 65-68 horsepower, get about the same gas mileage, and run much smoother and cooler. The EV 13 or Fireball 300 would be good for a more conservative rider on a heavier bike - for example, touring on an FLH, maybe pulling a trailer. The EV-27, BP 20, Fireball 310, or Interstate would be for a more aggressive rider.
    These parts can be installed by a moderately experienced home mechanic with a service manual in about 4-8 hours.
  3. $800 - $1000 Touring.
    Carburetor: Mikuni 40mm
    Ignition: Screamin' Eagle ignition module, Screamin' Eagle coil
    Exhaust: Screamin' Eagle, Cycle Shack, or Python II
    Camshaft: Andrews EV 27, Bartels BP20, Crane Fireball 310 cam, or Sifton Interstate
    Camshaft: Andrews N2 (Sportsters only)
    Pushrods: adjustable aluminum
    Cost: $800 for parts, $200 for installation
    Your engine will make about 69-73 horsepower, get about the same gas mileage, and run much smoother and cooler.
    These parts can be installed by a moderately experienced home mechanic with a service manual in about 4-8 hours.
  4. $900 - $1300 Sport.
    Carburetor: Mikuni 40mm
    Ignition: Screamin' Eagle ignition module, Screamin' Eagle coil
    Exhaust: Bartels, Cycle Shack, Python II, RevTech, or SuperTrap
    Camshaft: Andrews EV 46, Bartels BP40, Crane Fireball 316 cam, or Sifton Ascot
    Camshaft: Andrews N4, Bartels BP145, or Sifton Stingray (Sportsters only)
    Pushrods: adjustable chrome moly
    Cost: $900 - 1100 for parts, $200 for installation
    Your engine will make about 73-76 horsepower, and run much smoother. Your gas mileage and engine temperature will depend on how hard you run the engine. This setup sacrifices some low end torque for extra top end horsepower. It is most suitable for the lighter FXR and Dyna series.
    These parts can be installed by a moderately experienced home mechanic with a service manual in about 4-8 hours. These cams can cause piston-valve contact in some early evolution engines.
  5. $1750 - $2200 Touring or Sport.
    Carburetor: Mikuni 40mm or S&S Super E
    Ignition: Screamin' Eagle ignition module, Screamin' Eagle coils, or Dyna S, or Accell mega fire
    Exhaust: Cycle Shack, Python II, or RevTech
    Camshaft: Andrews EV 27, Crane Fireball 310 cam, or Sifton Interstate
    Pushrods: adjustable aluminum or chrome moly
    Head work: mill heads 50-80 thousandths
    Cost: $1000 - $1400 for parts, $250 for machining, $500 for installation
    Your engine will make 75-80 horsepower, and run much smoother. Your gas mileage and engine temperature will depend on how hard you run the engine.
    These parts should be installed by an experienced professional mechanic.
  6. $2500 - $3500 Sport.
    Carburetor: Mikuni 40mm or S&S Super E
    Ignition: Dyna S, or Accell mega fire
    Exhaust: Bartels, Cycle Shack, Python II, RevTech, or SuperTrap
    Camshaft: Andrews EV 46, Bartels BP40, Crane Fireball 316 cam, or Sifton Ascot
    Pushrods: adjustable chrome moly
    Valves: Andrews high-lift springs, titanium keepers, 5-angle valve job
    Heads: Branch, RevTech, or Hardy
    Cost: $2000 - $2500 for parts, $500 - $1000 for installation
    This engine should make 80-90 horsepower, and run much smoother. Your gas mileage, engine temperature, and engine life will depend on how hard you run the engine. This setup sacrifices some low end torque for extra top end horsepower. It is most suitable for the lighter FXR and Dyna series.
    These parts should be installed by a professional mechanic with experience on 90+ horsepower Harley engines. Choose carefully - everyone says they're the best. These cam shafts and heads can cause piston-valve contact and valve-valve contact, and must be checked carefully. Frequently, more machining is necessary. If I wanted this much performance, I'd go to an engine building shop.