1977 YZ250 need help....

[Homeless Dave]

Me and a friend picked up a 1977 YZ250 (for free!) a while back, is decently complete, but missing side panels. It does run, but it will only run for a little while, then it starts to bog and most often dies. It seems as though it is running out of fuel, but does not do what any other 2 stroke i have dealt with does, lean out ant rev way up. It does not want to idle no matter what we do and it will not stopping trying to die after a few a few minutes of riding. It still has the factory Mikuni carb on it and i would like to keep it original. It would be nice of it was rideable, but since it keeps dying it is not more than around the block once or twice..... Also, if anyone might have the wiring diagram, that wold be much appreciated, because it has no kill switch, so the rider has to stall it out when then need to shut it off. We dont know much history about it, but we can tell it must have gone ice racing back in the day because it had studded tires when we got it. The last time it was running, it must have leaned out a lot. I was not the one riding it, but i was following on a quad, it went to full revs. It was pinned in 3rd gear directly into my garage...wiped out a trike and did some damage to the wall, but thankfully my friend bailed off before he hit anything, so he was not hurt. Anyways, any other tips about this thing would be helpful, there is not a lot of info around on those aircooled YZs.

[ironchop]

Leaning out and death rev is usually from an air leak somewhere. If it only does it after warming up, i would check main seals and cylinder intake boot and base gasket

Could also be that your float has an obstruction in the needle seat, in the fuel line, fuel filter, or fuel tank and it's not letting enough fuel fill the bowl before you run it all out riding it. After it shuts off, maybe the fuel trickle fills up the bowl again as it sits so it starts after sitting awhile. I had an old dirtbike that would rev high just when you ran it completely out of gas (more air to fuel vapor ratio than it was supposed to have with a full bowl). Your results may vary.

I would do a leakdown test before I started it again because running it when it's doing that is no bueno. If you pass the leakdown test, try cleaning the carb really well especially the jet passages and the fuel inlet into the bowl through the float needle and check the fuel line and filter. Also look for trash in the gas tank

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[Homeless Dave]

I have used ether to check for air leaks, and it made no difference in the way it was running, so i don't think that is it. If you throttle it just right, sometimes you can keep it running and it will keep going fine. We completely removed and cleaned the tank and petcock, so there is no issues there because we have checked the flow and there is no problems. The carburetor that are on these uses 2 floats that are on poles, they push on a metal fork that is attached to the needle. Could float level possibly wrong? i dont know what the measurements should be, that goes back to not having much information about these vintage YZ available. I want this bike to run well because it is a decently rare bike (just over 6000 produced). If i get it to run properly, i would ride it a lot! From what little we had been able to ride it, i can tell ya that she rips! pretty impressed me for an aircooled 250. Personally i am a 4 stroke guy, i prefer the smooth pull of them for normal riding as well as their reliability, but i like the power of a 2 stroke when racing and they are easy to rebuild.

[ironchop]

... The carburetor that are on these uses 2 floats that are on poles, they push on a metal fork that is attached to the needle. Could float level possibly wrong?....

I would imagine that the floats could be set too low and the bowl doesn't fill all the way with fuel...so then the bowl runs dry or the fuel bouncing around in the carb makes the issue worse.

However I would think that, if this were the case, as soon as it dies, it should restart in a few seconds as the bowl refills. Did it restart quickly and then you made another couple of laps and it died again? Or did you have to wait awhile before it restart?





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[Homeless Dave]

It would start right away, sometimes if it wasnt run at a high rpm, it would stay running (barely) and then pick back up, so im really leaning towards a float level issue.

[shortline10]

Those old style mikuni carburetors with the individual floats are tricky to set the float level , I have seen the floats installed upside down by mistake many a time , causing the floats to keep the float valve almost closed . The prong on the float is installed the short side down .

[El Camexican]

Has the carb been opened and cleaned? (Sorry if I missed it)

Check that the throttle cable is in perfect condition and your friend to pull the clutch in next time something like that happens.

[Homeless Dave]

Carb was cleaned, and before the cable was good. I asked him about what happened when he crashed, as it turns out, he tires off the fuel tap......so he caused the accident. Pulling clutch might have slowed the bike, but without a kill switch it probably would have blown some parts out of the block.