The Harley Tax
By Dave Murray
The "Harley Tax" should in fairness be called the "Officious, Nitpicking, Largely Ineffective, Expensive, and Intrusive Tax." It was imposed by a gubbamint which sureashell ought to have better things to do. The Motor Company has responded to ridiculous noise and exhaust emission regulations primarily by installing restrictive air cleaners and exhausts to reduce induction and exhaust noise, and by running very lean mixtures, which supposedly reduce emissions. This is pretty much the same approach taken by all of the motorcycle companies, and all brands of new bikes will respond positively to the sorts of modifications mentioned below--but this article is specifically about H-D Sportsters. If you ride something else, consult your dealer to pay the "Suzuki Tax," "Honda Tax," etc. (Or shove it off a cliff and buy a Harley . . . .)
You can achieve a considerable power increase fairly cheaply simply by reversing these efforts. A couple of caveats: my focus is on mid-1990s, 49 state Sportsters, and your mileage may vary. Emissions and noise level will increase, and you want to talk to local riders to find out just how draconian the local constabulary is on these issues, particularly in the Peoples' Republic of California.
A four-cycle motor is a pump. It pumps in fuel/air mixture, and pumps out exhaust. Any restriction in either system requires more power to do the pumping, leaving less at the rear wheel. Also, the valves are open for a finite time, so reducing restriction allows the intake stroke to get more fuel/air mix into the cylinder for a bigger bang, and allows the exhaust stroke to do a more effective job of removing the exhaust products. Any exhaust products remaining after the exhaust valve closes occupies space, which could be filled by fuel/air mix. It isn't quite this simple, for example, best power is achieved with a little back pressure, but it ain't rocket science, either.
The "Screaming Eagle" air filter and mufflers are good equipment, but not your only choice. There are many other makers of after market equipment, which, as always, falls into three categories; The Good, The Bad, and The Ugly. I have the SE air cleaner, which comes with a permanent K&N element, and I am well pleased. The exhausts are "Custom Chrome" drag pipes. (We doan need no steenkin' baffles!) They aren't quite the most powerful, but I like the sound.
You do have to re-jet the carb, both to offset the original over-lean condition, and to compensate for the leaning effect of higher airflow velocity through your new low restriction air filter. The CV carb should, in theory, compensate for this, but it doesn't. Maurice Riggins has a couple of superb articles in the tech section of Sportster.org. These are his numbers, and they worked for me.
First, change the stock #42 low speed jet to a #45.
Then change the high speed jets as follows. (These suggestions assume a low restriction air cleaner similar in flow to the SE, and pipes roughly similar in flow to the SE performance mufflers.)
883cc, stock: #160
1200cc (std.), stock: #170
1200 (sport), stock: #190
Re-jetting, plus your low restriction air filter, plus a low restriction exhaust should give you 8-10 more rear wheel horsepower and better throttle response, as well as reducing the exhaust gas temperature.
Your best resource is an independent mechanic with a good local reputation, who has installed and dynoed the equipment, and who can predict the results. Talk to other bikers, a pattern will emerge. When you have found him, BUY STUFF FROM HIM! Don't waste his time picking his brain, and then buy it online to save a few bucks. My favorite wrench, Geoff, has busted knuckles, gotten exhaust burns, fixed errors, diagnosed rattles, busted tires, etc, for twenty years. It is a marvel that I can buy his experience so cheaply. There are excellent dealers, too, but I suspect a built-in bias toward H-D products.
Ride paranoid, and have fun. (Is it paranoia if they really are trying to kill you?)
Copyright 2002 by Dave Murray. All rights reserved.