1990 DR 250 S CV Carb problem

Let me first say thanks for your help. I tried searching but no matter what I searched for I found 20 pages of info. I went through the titles but I may have missed something so if I missed it I would be happy to be pointed in the right direction.

I recently bought a 1990 DR 250 S. It has developed a problem. It will start and idle great but when throttle is given it acts like it is very rich and will not rev up. It just misses and runs about like it's half throttle even if it's full throttle. It ran fine before so I don't think its a jetting issue.

I took the air cleaner off and you can see the plastic slide vibrating up and down but never going up all the way. If you are careful and have it at half throttle or more, you can stick a screwdriver in and make the slide go up and it revs right up and runs great.

I thought it might be the diaphram but my local motorcycle shop says it's not. I have worked and worked and don't know where to turn. I would greatly appreciate any help you could offer.

Thank you,


I was trying to find a better way to describe what the slide does - I think saying it "flutters" is better than vibrates. You can see it flutter up and down.

Thanks again,


The rubber diaphragm on top of the slide can tear and leak air. If it is OK, make sure it is seated correctly when re-assembling it.

Another common problem on the old Dr's is the fuel petcock on the tank. It has a rubber diaphragm that will become brittle and leak. When this happens, it will drip raw fuel directly into the carb through the vacuum line and cause a rich condition. Make sure the vacuum line from the carb to the petcock is completely dry.

Thanks for the thoughts - -

I checked the peacock valve on the tank and it seems to be fine.

I didn't see any holes in the diaphragm. When I put it back together I added just a little silicone to make sure it sealed.

This is really weird. When it's cold it runs GREAT however after about 45 minutes right off idle it almost won't run. If you get it past that it revs and runs fine. If you pull the air cleaner off you can see the slide fluttering right off idle.

I talked to my dad and he suggested the intake valve could be tight causing a lack of vacuum when the engine warms up.

So what do you all think?

Thanks again,


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