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Old 09-22-2014, 06:39 PM   #1
redprimo OP
Gnarly Adventurer
Joined: Jul 2011
Location: central coast of California
Oddometer: 369
school me in tuning a 38mm Mikuni VM round slide carb

I need some help chasing down some carb issues. The bike is a 1980 Honda xl500 with a fresh top end. The carb is a 38mm Mikuni VM round slide. It floods out if I start with the starter valve open but starts just fine with it closed. I can slowly roll into the carb up to about 1/3 throttle, past that and it cuts out and if I snap it it cuts out.

I found a Mikuni tuning manual and it just makes me want to smack somebody. it explain in great detail the importance of a properly set float height but skips the specs or how how to find the correct height. I think this is likely my issue, or at least one of them, so could some please explain it and type slowly so I can follow.

The idle air mixture screw I can handle but please refresh my memory on the whole clips on the needle thing and which way to move for what effect.

I may need to mess with the jets but from the looks if the carb it was on the bike for some time before I bought in running condition but with a seized piston.
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Old 09-22-2014, 09:04 PM   #2
Retired Roadracer
Joined: Jun 2007
Location: Denver 'Burbs, Colorado, USA.
Oddometer: 5,282
In short:

Your pilot jet controls the lower end mixture.

The needle controls midrange, with both taper and height. Top position on the clip is leanest, bottom is richest.

The main jet, that the needle slides up and down in, controls top end mixture. At top end, all three metering systems are in effect. In midrange, the pilot assists the needle a bit.

A quick-n-dirty way to see how yer doing is to look at the insulation around the spark plug's center electrode. Tan to milk chocolate brown is good. Darker means rich, lighter means lean.

That's the place to start. I'd start on the rich side, and work leaner. You may hole a piston by running too lean. Good luck.
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