Hi everyone, I joined this forum not long ago and this is my first post. I was hoping it wouldnt be for help, but unfortunately that is not the case.

The problem. I purchased a 1991 250x for a real good price. Was running a bit rough when I bought it, as well as the throttle was not very responsive when you laid on it. Also it was having a tough time staying started. Now it will kick over after quite a few attempts, but will not stay running for anything. From what I have read, I can replace the carburetor with a newer one from a 300ex. I have found one of these off of a 2004 300ex. I also found a complete wiring harness including CDI box, coil, regulator fairly cheap as well. Would this be a good place to start and will this all work on my 1991 250x? I also read on this forum that these are notorious for needing the top end rebuilt. I also have access to a 2004 300ex cylinder complete with piston , rings, clips etc? Will this work and be a inexpensive swap? Also there is a 2004 300ex head with rocker cover assembly. What does everyone think on buying these parts to swap onto my motor? will it work? Also, if Im going the wrong direction with this, please point me in the right direction and I dont mind constructive criticism!

Thanks,
Corey

 

Hi. I have rebuilt the top end of a 250x about 6 months ago and can offer alot of info. The cylinder is the same, the rocker box is the same except for the decompression lever"wich you don't need anyway because the 250 isn't hard to kick". The piston however is the same diameter but has a higher piston pin hieght, so it wont work. Everything off a 300ex will work perfect except the piston. The 300ex was a takeover from the 250x. The wiring harness is different also. If you rebuild the whole top end not incuding piston it will cost a little over $700 from a dealer. If you buy good used parts off ebay for a newer 300ex it will same you alot of money, trust me. My other 250x I converted it with a 350x engine, big upgrade. It sounds like your electrical is fine, just work on the engine. These are really good running quads for the size. If you need any help, pm me. RUSS

 

I don't think the 300 cylinder will work. I am sure that they have different stokes, therefore you will have piston/head clearance issues. The top end rebuilding might be it, but I would check the compression before tearing it down. If you haven't done the gas tank dump, and start with fresh try that. I would also try a new plug, because even it sparks it might not be enough.

 

The cylinder is the same, the stroke is different, thats why the piston won't work " the pin hieght is higher"! I also assume that the engine needs rebuilt, as most do when they have enough time on them and were run low on oil. Not saying this is the case but I assume it has been narrowed down.

 

Thanks for the information guys! I just bought a 2001 300ex carb for $23.50 plus shipping and guaranteed to be in good working condition. It also comes with a new thumb throttle control as well as the cable. Im going to try putting that on and also pulling the tank clean thing also. Ill also pick up a new spark plug as well. Anyone know what spark plug I should be using in it. Im also curious what I will need to buy from ebay to do a top end rebuild and how tough it is.

Thanks,
Corey

 

Corey, let us know how it helps if it does. The carbs are pretty much the same, but a newer one is better. A top end rebuild would consist of whatever is bad, so you'll have to inspect it. But probably cam. rockers, valves, timing chain, rings, wrist pin, valve springs, valve cotters. The head will need to be machined for new valves, and if so tell them to watch how far they go "very little if poss" and the cylinder will need honed if not bored. This is a little compicated but don't worry. If you need a rebuild we will help. By the way if you think the gas tank is giving you probs then try putting a rubber line on the carb with a funnel and feed fuel that way. But the tank being bad would only cause a fuel loss if the screen is clogged, so it would'nt really run bad when running, it would just spit and sputter when it's running out of gas. Try these things that we are suggesting and see if they help. RUSS

 

Would it be a good, workable idea to just purchase a newer head. Say one from a 2001 or 2004 and use that as well as hone the cylinder and replace the piston , etc?

Thanks again and Ill let everyone know how things work out with the new carb!

Core

 

Yes if the head is in a-l shape. Just the valves are $25.00 a piece and there is 4 of them and the springs are around $90.00 and the cotters "or valve locks" are about $3.00 a piece and there is 8 of those. A great and honest ebay seller that I have bought several things from is, well let me look, but I think his user name is trmanx or something like that. He sella only 250x/300ex parts and they are really good parts if he says they are.

 

I pm'ed ya. Give me a call if ya want. RUSS