Idle Mixture Tuning

Submitted by Chris "Beetlejuice" Bruce

Info about rich/lean running:

 Typical lean conditions:
 - poor acceleration - feels flat
 - Engine doesn't respond when throttle is snapped open - picks up speed as
   throttle is closed
 - Engine runs hot, knocks, pings and overheats (end result- hole in
   piston)
 - Engine surges or "hunts" when cruising at part throttle
 - Popping/ spitting through carb when throttle is opened, or popping and
   spitting through pipe on deceleration with a closed throttle (classic lean
   pilot circuit symptoms)
 - Engine runs better in warm weather, worse in cool
 - Performance gets worse when the air filter is removed

 Typical Rich Conditions
 - Acceleration is flat, uneven
 - Engine will "8 stroke" as it loads up and skips combustion cycles
 - Throttle needs to be opened continuously to maintain acceleration
 - Engine works better when cold
 - Black smoke from the tail pipe
 - Poor fuel economy
 - Engine performance improves when air cleaner is removed
 - If the pilot screw is overly rich, idle is rough and the engine won't
   return to idle without blipping the throttle
 - Black sooty plugs, sooty exhaust pipe

 Here's some tips, old wives' tales and cute remarks:
 - If the bike pulls hard, idles, doesn't smoke, detonate or punch holes
   through the piston - ride it
 - If the bike pulls and the plugs are not blistering white - leave it
   alone
 - If you install a new exhaust system or air filters, go up two sizes on
   the main right of the bat - then evaluate
 - If you encounter mid-range detonation, try raising the needle one clip

Tuning the leanness out

The backfiring indicates air/fuel mixture or pilot jet leaness. I'd start, as you correctly stated with the idling mixture as it's the easiest and cheapest. Remember there's another carb under your tank which you'll also have to do.

Manufacturers specs are normally one and a half turns out from the slightly seated position - don't force the screw it's got a taper on the screw you're turning as well as in the carb.

Start the engine and get it to working temp. Light a cigarette or whatever you smoke as you'll be busy from here on. You'll be letting the engine run for a while so do it outside where there's air circulation or put a fan on the engine and put the beers within reach.

Adjust the idling speed to the spec or to where you can hear it's running at a constant speed (the idling speed is adjusted by hand by turning the brass screw with the spring on it sticking out of the back of the rear carb).

Now turn the pilot screw out one quarter from the manufacturers spec. If the engine speed drops, return the screw to where it was and turn it in one quarter. If the engine speed drops, you're OK (and I'm OK). Drink the beer. Don't go for a test ride after the beer.

If the engine speed increases as you turn the screw out, keep going until you reach the point of max RPM. If it takes more than 3 turns for the engine speed to level off, the pilot jet is too large. Install a leaner one. This will probably not be your scenario.

If the engine speed increases as you turn the screw in, keep going until it levels off. If the pilot screw ends up less than half a turn open, the pilot jet is too small.

Trick : When you've found the peak idle speed, turn the mixture screw in one quarter turn. If it doesn't change the idle speed leave it there. If it does drop, return it to the last setting.

I haven't tried this, I have someone who did it for me and I was so happy with the results I'll pay him again. As I mentioned, this is from the April 99 issue of American Iron magazine. I'm still on stock jets and needle - so this can be tuned out.