Vintage Dirt Bike Q & A

Hello,

I have an MR250 that won't rev out. Well, it won't rev out until the fuel runs low in the carburetor...? When I fill up the carb it starts up and runs slow, missing a lot. If I put it in gear it goes, but I can't get it out of first gear, not enough power. Then if I turn off the gas, after a little running it starts to wind out, bringing the front end off the ground, lots of power.

Bike has a bunch of compression.

When I got the bike it had no condenser in new the coil, so I ordered a new one, installed it, no difference.

Cleaned the carb out when I got it, all jets are clean.

Lowered the float level thinking that I was flooding it out. No difference...

My only thought now is to lower the main needle in the middle of the slide to lean out the main jet.

Any other ideas?

what size main are you running?? I like a 108 in mine when I was using the stock carb. Try moving the needle down (clip up) and see if it makes any difference. Inspect the slide. Part of the problem I ran into with the old Keihin is that the needle jet and jet needle were just so worn out it absolutely couldn't be jetted correctly. Still, I got it running pretty good (although rich), even with worn parts. Is your compression good??

Check out this thread:

viewtopic.php?f=21&t=2691&hilit=mr250

If you want to actually fix this problem, you're going to have to buy and install some jets.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Compression seems great when I kick it over. I don't know how high, I guess I will put the compression gauge on it and see.

I don't know what size jet I am running, will find out when I get home... .

I will try to lower the main needle.

Going to Mid Ohio Swap meet next week and I am bringing it, maybe they have some jets there, what sizes should I get?

Or, if somebody there offers me the right price this problem may be become somebody else's problem....

Found the problem.... Thought about the way it was running, felt like the choke was on. Decided to check the air cleaner... I had cleaned out the air box a long time ago. Took the lid off and there was a briggs and stratton air filter on top. I must have cut that foam thinking it would work as a good filter... Well, it did work, but too well. It was acting as a choke. Took the filter out, it started up and ran great... Must find a stock filter now.

Great !! Best ones are the Honda ones. About $35. Or you can get a uni 4025St, but you'll have to order it. (and probably have to order the honda one too, at the dealer, but they are pre-oiled and very very well made these days!!)
Good luck !!