I just picked up a desert racer for a great deal. Problem is that the top end is wasted, 70 psi compression test. I got it to run, but could not get it to idle. This machine has been ported by Duncan, has paul turner pipe, moto tsarni reeds and the carb bored to 37mm. I spent a good deal of time cleaning the carb (it wasn't that bad) and even tried my good stock carb from my 88. I can get it to fire up and run ( badly) but not idle. Is this all in my top end, or is there another problem I am missing?

 

It could be in the top end but before you go tearing down the engine, and this is assuming that it is a two stroke, make sure your crank is sealed. With the engine running spray some carburetor cleaner around the crankcase. Top,bottom,front,and back. If the motor starts to rev higher then you have a leak. This will cause low compression readings. Don't get carried away with the carb. cleaner near the carburetor, intake tract, or the airfilter or it will rev making you THINK you have a leak. If to no avail. tear her down. KNOWSALOT

 

It is a 250R, and it has been neglected by its former owner. I can't get it to run well enough to do the spray deal. I have to keep the rpm's pretty high and eventually it just dies. I did inspect the case gasket area and it looks OK. The intake from the air cleaner to the carb was full of sand, so the top end has got to be wasted. The filter that was on it looked like a sand dune. I thought a leak around the crank case would cause erratic idle or high idle. My problem is it won't idle at all, it just dies. Could my symptoms be caused by the bad top end?

 

Well with that description I would say a definite yes! You are gonna have to start there anyway to find any other problems. By the way I haven't heard the "looked kie a sand dune" description before. I like that, I'll have to use that in the shop next week! I'll be sure to give credit where credit is due. Oh, a crankcase leak will cause the idle characteristics you gave, but I have seen a few that had a very low idle and not wanting to rev symptoms also. Hope you get out on the cheap. KNOWSALOT.

 

I would really be surprised if the engine had any internal problems. Whoever built it had it done right. I was told by former owner that the motor was sent out to Duncan in '95. I pulled the magneto cover and it was brand new just like Christmas inside, so the crank seal is good on that side. It is supposed to have a "race clutch" in it, I will find out what that is tomorrow.
This guy would have no reason to lie to me as I just happened to see it burried in his garage as I was driving by. I kind of backed up and yelled out the window if he wanted me to take it off his hands and he said sure. I checked it out a little and when he gave me the price I took off for the bank!
Thanks for the assist, I will update you after I get it apart tomorrow.

 

Sounds like a plan! {Also sounds like you got one of deals in a lifetime too!} KNOWSALOT

 

I got it all apart. The piston is very worn and the cylinder may need to go out two overbores because of a scratch in it. It is on the standard size piston, but it is not a stocker, pretty sure it is forged. It has STD 614PS F43F5 CD machined on the top of the piston. It also has an area machined out of the front between the rings. The cylinder is completely ported and polished with the intake port machined out to look like an upside down mushroom. There has not been a whole lot of material removed, but it has obviously been done professionally. Any idea by who?
Also, the rt side crank seal is leaking, bearing is OK. I will probably replace both sides. All in all, not too bad, but I would rather be riding.

 

how much did you pay for it?

 

Probably too much. It does have a lot of stuff though. PEP tripple rates up front ( probably will have them rebuilt ), PEP modified rear shock, all the motor work, new clutch, ( and the clutch hard parts look brand new, as does everything else inside) Full skids, Hi- flight Desert seat that I don't like, steering stabilizer, etc. By this weekend, I will have it back together and hauling the mail. It hasn't been abused as much as the guy never cleaned the air filter. He was an old guy who just cruized around and never did any maintainence. I Paid 3 grand for my 88 R that was near mint condition. This machine was not that good of a deal.

 

The numbers you gave are Wiseco numbers, but none that appear in the catalog I have. I show the '86 useing a 526PS piston that uses 2598CD rings. The '87-'89 use 562PS that uses the same rings as the '86. It sounds like a piston that was custom made to spec. for a race shop. Curtis Sparks uses custom pistons to their specs. A call to Wiseco can tell you who it was built for. Therefore maybe letting you know who done the porting. Hope this helps. KNOWSALOT.