Vintage Dirt Bike Q & A

Okay so the new carb arrived and the bike runs better. Took it for about a 4 mile test run. The issue is still showing up though. When I start it I have to turn the gas off until it starts to run out and will rev. Once it starts to rev up I can keep it running as long as I don't let it get below 3000 rpm. If I do it bogs again and I need to turn the gas off. At that point it will idle as long as I have the throttle wide open but it will only idle at 1500 rpm. If I let off the gas it will die. It will start right back up but only to 1500 rpm with the throttle wide open. I shut the gas off wait for it to run out after about 2 minutes and it will start to rev again and run great as long as I keep it rapped up. Any ideas. Possible reed valve? Air filter? I did install a Uni 4 inch sock filter.

Clogged exhaust? Take off the pipe. Look for crusty carbon plugging up the header section. Check the spark arrestor for a nasty oily mess, and maybe even mud wasps.

Reeds? Check them next.

We've got to believe the carb is fine now. The only odd carb thing that I can think of would be if you adjusted the float level the same way as on the old carb and that maybe they both had too high of a fuel level. That's a stretch, right?

I've seen clogged exhaust cause trouble on the 50cc scooter mopend type bikes that idle around all day with oil injection systems over feeding. I've also seen it on a weed eater that was running too much castor based two stroke oil in the left over premix it was using.

Good luck!
Jon

Sorry I can't keep track anymore. What bike??? What carb?? Do you know the baseline settings on the new carb???

His bike is a mt250 not sure year. The carb is the used carb from my 75 mt250 I went to a 36mm mikuni. My bike started and reved up just fine with the carb on it before I took it off. I was not able to actually drive the bike as the tranny was toast. I asumed the carb w as working fine as I could hit the redline sitting still. I dont think the carb is the problem anymore but I could be wrong. There is no reeds on your motor btw and to see if its the air filter just take it off and see if it runs better just dont do it in the dust. It really sounds like your runnin rich cause your having to force it to go lean by cutting off the gas. Make sure your getting enough air cause that carb did nothing like this on my bike.

1975 MT 250. I'm sure its not the carb. It is acting the same with both carbs and the carb I just bought is in terrific shape. Thanks a ton btw. The exaust thing sounds right because when I first got this running I was really mixing heavy. Now where the pipe connects to the head it pushes a ton of oil out that connection. In fact all my right shoes are covered in black nasty oil spray. Can these pipes be cleaned and how? Everyone here has been a ton of help thanks so much.

it was the air filter. I lost the air screw the same day the filter arrived so I just figured the air screw was the problem. Put on the new carb and same issue. Removed the filter and it runs like crazy. Now can anyone recommend a hi flow filter I can clamp on. I have no air box and the 4 inch uni even dry without oil doesn't give enough flow. Which way on the air screw for less fuel and more air?

I am not sure what oher filter you could use maybe a k and n or somthing like that. I also had to take the airbox of of my mt250 and was gonna run a uni filter, already bought the four inch with a pre filter. My filter is gonna be positioned under the seat where the box used to be so I could use a k and n if this dosnt work which dj has got me worried now. O well I will see what happens when I get around to finishing my bike. Good luck on finding somthing that works dj your almost there.

Thats the same place I put my filter. I removed what was left of the old airbox and put it on. There is a ton of room maybe just a longer filter to allow more air and more surface area? If I find one that works I'll let you know.