Since cash is a problem right now, I'm wondering if I can tune down the mikuni carberuetor rather than purchase a new carb... After reviewing the 20 page tuning manual i found online, the mikuni is fairly adjustable... or is the 2 to 4 mm too big a difference...
frankly, i've never been able to tune a mikuni right. they always seem to run rich. you can try andfool with jetting, accelerator pump, and idle/air adjustments and see......
j
Ajax.......I think I can explain this better if i start with the basics. carb throat size has nothing to do with fuel, in and of itself. The best way to think of a carb is by picturing a narrow spot in a creek. As water goes through a narrower opening, it gains velocity. (Air moves as a liquid, so this really is not that far off. ). The only time fuel is forced through a carb is when the accelerator pump is actuated. The rest of the time fuel is drawn up through the jets by a low pressure result from air velocity rushing past the top of the jet. The higher the velocity, the more fuel drawn. Now if that throat is larger, and there is less velocity, you have to use a larger jet to get fuel into a decreased low pressure area. In lamens terms, if you use too large of a carb you have to use LARGER jets to get your mixture right. Most people do the opposite, and get into trouble.
The downside to this is that intake velocity really helps your engine. Under ideal conditions when your piston hits bottom, the intake velocity is so great that the air going through the carb at that instant will be forced into the cylinder before the intake valve closes. This starts compression before the piston moves. less velocity equals less momentun and less air being forced into the cylinder. This does hurt throttle response
so you will be able to jet your carb to work, but you will never get optimum performance or throttle response
I let my mind wander.......It never came back
Thank you for an indepth discription to this topic. You have imparted knowledge to all of us watching so valuable. Are there any other side effects or problems that can occur? I am wondering if there are any tale tale signs of engine failure caused by this.
as long as the mixture is right, no engine failure will result.
I even run mine a little rich on the bottom jet just to be safe.
I let my mind wander.......It never came back
Heritic and junior are exactly right. I want to share my testimonial with you:
I was running a mikuni 42 on my 80" evo with a mild (andrews 27) cam and it ran very rich. I had the smallest jets and largest needle they make for it and it still ran rich. I wasn't fouling plugs, so I put off going to a smaller carb. Also, I don't mind running a bit rich since it helps keep the engine cool. Putting off the job cost me a burned exhaust valve when some of the carbon build-up in the combustion chamber (there was lots of it!) would come off and pitt the seats and valves, eventually causing the failure in the form of a 3/16" hole in the front exhust valve. I knew you could burn a valve running too lean, but it appears the opposite is also true. I went back to the stock carb after fixing the valve and the bike runs smoother, but the throttle response is not as crisp...
Oh, and I went from about 32-33 mpg to 38-40 mpg.
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