(reprint from Mikuni Corporation)
Common Tuning Issues:
3. Backfires Through Carburetor
4. Exhaust Backfires
5. Detonation ("Spark Knock")
6. Poor Mileage
7. Which Manifold?
We are confident that you will agree with us that your new Mikuni carburetor is one of the best modifications you have made to your Harley-Davidson motorcycle. The Mikuni substantially increases power compared to the stock carburetor. However it is the dramatic improvement in throttle response that makes the Mikuni flatslide HSR carburetors such outstanding performers. No carburetor made today can match the Mikuni in this regard.
We have chosen tuning components that, for the great majority of applications, are correct. Chances are, you have had to do very little beyond adjusting the air *beep* and idle speed *beep*.
However, a change to one engine tuning component often affects or is affected by other components. All of these, the exhaust, air cleaner, ignition, cams, etc., must work together if you are to get the best performance from your Harley engine.
The current series of Harley-Davidson engines share many design and therefore operational features going back at least to the original Evo Big Twin of 1984. We and others have amassed a great deal of information about how various tuning components work, work together, do not work together or do not work well at all.
There is no one single combination of parts that is right for all riding styles. The performance requirements of a touring rider usually differ from those of the LTL (Light to Light) performance rider. A combination of performance components that delivers outstanding power ("Torque") in the middle range (2,000 to 4,000 rpm) seldom does as well above 5,000 rpm. The drag racer usually could care less about what happens at 2,000; his/her concern starts above 4,000 or so. These divergent requirements are best served by somewhat different combinations of parts. Except, of course, the Mikuni --- they both need that!
It is very difficult to put together successful performance parts combinations without considerable testing and knowledge about how these parts work and interact with one another. One can, for instance, fit a camshaft design that favors top-end power and combine it with an exhaust that restricts top-end power. Such a combination delivers poor performance at both ends of an engine''s rpm range. Unfortunately, this sort of mis-match occurs frequently.
Mikuni cannot take the responsibility of specifying all the possible performance parts combinations that work together. However, we may be able to help you avoid the general problem of "wrong" combinations with the information found in the web pages assigned to the links at the top of this page.
Tune ups, maintenance, and general help on your sleds
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