Carb problem non KDX

[vlxk98]

I picked up a '78 1/2 RM250C2 a few months ago to do some Vintage MX rides on. It was in very rough condition but did run.

Went through the motor, crank bearings and seals, a few other items in the lower end and a fresh bore with Wiseco piston.

The PO had the float set very low and the pilot jet had been replaced with a #25 rather than the #45 listed as OE. Main jet and needle jet are correct. I set the float to factory specs and replaced the pilot with a #45.

When I tried to start the bike it would not even try to fire. Long story short, I was able to get it to start by pushing it but it ran very poorly. Lots of smoke, spooge dripping from the pipe/muffler joint within 1-2 minutes. Very sluggish on rev up. I had to hold the throttle at 1/3 or so to keep it running. After a couple of heat cycles since the cylinder bore is new, I tried again and had to push it to start it. I was able to start it with the kicker once it warmed a bit. It still ran very poorly. Just for giggles I closed the fuel valve, as it ran low on fuel it cleaned up and ran pretty good, nice quick rev, sounded better. I did a little dance with the fuel valve to keep it running better for a while and then pulled the carb and rechecked the float level, spot on factory specs. I lowered the float about 2mm to see what would happen, no change, runs horrible until the fuel level drops in the bowl then it cleans up and runs pretty good.

My next guess is needle valve. Pulled the carb again and checked the needle. There is no wear ring on the tip of the needle. I left the bowl off and hooked it up to the fuel tank, with fuel running through I can lift the float lever and stop the flow of fuel. I would think that even very light pressure from my finger would be more than the floats raising the lever?

What do you guys think?

Thanks for looking and I appreciate any help!

Kenny

[canamfan]

You say it has a fresh bore. Does it have good compression? Low compression equals hard to start as in pushing to get it started. I don't know if it would run that rich though, seems if your carb float needle wasn't sealing you would have gas coming out the overflow and you don't. You might also check to make sure the timing isn't way off. Sounds like the carb should work but you might check a parts schematic to make sure theres not an o-ring missing somewhere, some of the older mikunis might have an 0-ring that the newer ones doesn't have.

[kawagumby]

Sounds like the 45 pilot you put in might be part of the problem. Try going smaller and see what happens. That 1/3 throttle where it cleans up is about where the pilot circuit signs off.

[Julien D]

Sounds like the 45 pilot you put in might be part of the problem. Try going smaller and see what happens. That 1/3 throttle where it cleans up is about where the pilot circuit signs off.

^ this.

[Jaguar]

Have you double checked that the choke isn't somehow engaged?
put a compression tester on it to see what cranking psi it produces.
How is the spark strength and the timing?
take the pipe off and press the header around your mouth so that it seals and then blow hard into it. If you feel much pressure then there is some kind of restriction that may affect the needed jetting.

[fuzzy]

The slide could be different, requiring a leaner pilot jet?? Never assume the carb is original.

[Jim B]

Just for giggles I closed the fuel valve, as it ran low on fuel it cleaned up and ran pretty good, nice quick rev, sounded better. I did a little dance with the fuel valve to keep it running better for a while and then pulled the carb and rechecked the float level, spot on factory specs. I lowered the float about 2mm to see what would happen, no change, runs horrible until the fuel level drops in the bowl then it cleans up and runs pretty good.

My next guess is needle valve. Pulled the carb again and checked the needle. There is no wear ring on the tip of the needle.

Maybe the rubber of the needle has gotten brittle over the last 38 years. (Crazy, I know. ) Maybe the float doesn't float as well as it should. As someone else stated, it might not even be the correct carb for the bike. (Going from a stock 45 to a 25 to get it to run right sounds fishy.) Are you sure the carb is perfectly clean? Perhaps there's a blocked air or fuel passage somewhere? It does sound like the carb, since you can get it to run better by making changes to it.