RM400 Stock jetting/Problems

DRH

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Feb 14, 2007
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#1
My 1979 RM400 is just about finished and having a little problem with carburetion. I upgraded to a later mikuni with a larger 1/4 fuel inlet to get rid of the pee hole sized inlet that caused these bikes grief. First thing I noticed about the carb was I had to lower the fuel lever durastically to get it to quit slobbering out of the overflow. I would only leak while it was running i'm guessing because of the vibration? The float valve is brand new. I know the stock main jet is a #270 which is what I have, Slide needle #6jf6 on clip position #4, stock nozzle. However it seems to run extremely lean on the pilot circuit. The pilot jet I have in there is a #50 and i'm not sure if that's the right one. Could one of guys steer me in the right direction?
 
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Oct 21, 2006
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#2
bigger pilot-
adjust the float level-
4th clip ?- from top or from the bottom-
its usually best in the 2nd down from the top-,if your on the verry bottom you need a bigger main-
 
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#3
Lots of questions here. Is the carb bore the same as the original one? Are the jets, needle, spray nozzle, and slide the same sizes as the original one?
OriginalDirt
 

DRH

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#4
The carb bore is the same size (36mm). The fuel inlet is different and the slide has a smaller cutaway (2.0 instead of 2.5). This would richen the bike at idle, correct? Other than that the nozzle, main jet, and as far as I know the pilot jet is stock. It is so lean on the pilot circuit that it races while sitting idle. When you ride the bike, it runs great as long as you're in the gas, and the throttle response is great, but when you chop it, it wants to race for a few seconds and act like it's running out of gas. I adjusted the air screw all the way in which made it better, but i'm out of adjustment so I guess this means bigger on the pilot?
 
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#5
Your pilot is in the ballpark, I used to run a 45 in my 1980. It sound more like there is an air leak, something out of whack in the pilot circuit or the altered float level is screwing you up.

Check out the intake manifold. The one on the 79 I just bought looked fine until I took it off and found it was cracked as hell inside.

Some mikunis used an air jet, if it has an air jet it might be too large and letting too much air into the pilot circuit.

You shouldn't have had to change the float level. On a mikuni vm, the float arm will be fairly level with the gasket surface when the float level is in range. If the carb is used you might have a bad float. Get the float set up and working right before you try messing with anything else.