My 1987 Trail Boss 250 recently became un-startable. After eliminating some of the "obvious stuff" it appeared that maybe bad crank seals were to blame. But now I'm not so sure because the motor seems to pass leak-down tests. It holds 6 psi for 6 minutes when filled with air, and spraying the air filled engine with soapwater solution reveals indicated no leaks anywhere.

I seems not to require rebuild, but maybe I'm overlooking something(?)

Over the past weeks I have done the following:

* Drained gas and replaced with fresh gas
* Cleaned carb (it looked fine when I was "inside")
* Verified spark OK and replaced plug
* Replaced dry*rotted intake boot with new
* Tested compression * ~110 PSI
* Tested leak-down - easily holds 6 psi for 6 minutes

Motor had started and run fine for past year, but became progressively harder in recent weeks - prior to not starting at all. During the period of difficult-but-possible starting, it re*started fine when warm and ran great once started.

Hope nobody flames me for hijacking my own thread. I think the old one became stale and the nice folks who had responded may have stopped paying attention.

My ATV is my plow vehicle - hoping to resolve before the snow flies in PA.

 

Should hold 6 pounds a lot longer than 6 minutes. The usual seal that goes out first (if this is the problem) is the seal behind the clutch. I just rebuilt a 300 top end,ran good for about 20 minutes and died! Still had good compression and fire . I just went after the crank seal as I've had this happen before,seal was in two pieces. Replaced seal and ran good. Probably what took original piston out in the first place (lean seizure on exhaust side).Double check with soapy water around intake and base gasket and exhaust plug to eliminate them as the problem when pressurized. If no bubbles,then suspect seal(s)

 

The motor probably holds 6 psi for longer than 6 minutes. I only measured for 6 minutes because the info I'd read about homemade leakdown testing seemed to indicate that 6 psi for 6 minutes was a good standard. Sounds like maybe not. Is there a better standard?

When measured 3-4 hours later, psi had dropped from 6 to 4. I sprayed every possible place on engine (and the plugged orifices) when testing for leaks, so intake and base gaskets check out AFAICT. I have replacement crank seals lying in the garage. I guess I could just put them in and see what happens. Just didn't want to do that without confirming the problem

Is 110 psi good compression?

 

what about the crank bearings? did you check for any wiggle on the crankshaft. if the bearings are bad then the crank will slop around while spinning. a brand new seal can not keep up with the wiggling shaft.

110 psi is pretty good. general rule on those motors is anything less than 100psi needs a topend rebuild. a fresh bore should read anywhere from 115 to 120psi. have you check the exhaust too see if its clogged up?

 

The theory of a good seal unable to keep up with a wiggling crankshaft may be a good one.

Tests for play have confused me a bit. I can feel play when pulling on the crankshaft in the direction by which one might pull off either the drive clutch or the magneto. But I was wondering whether some play in that direction might be normal (Clymer manual speaks of "crankshaft end play" and some shimming to be performed when installing new crank bearings). There seems to be no play "up and down" play. But maybe *any* play is bad and I'm confusing the text of the Clymer manual(?)

Oh, and yes I did check the exhaust for cloggage.

 

ok my next stop would be the piston. have you taken the intake boot/ exhuast manifold off and looked at the skirts? they might have broken off and for some reason not caused any binding.

will they thing fire off when using starting fluid? usually if you have good compression and even the weakest hint of spark the starting fluid will bust off a motor.

 

Since I had to block of intake and exhaust ports to perform the leakdown test, I took a few looks at the piston while I was there. Nothing appeared broken or even scored. Also looks comparable to an allegedly functional spare motor I picked up yesterday.

I tried starting fluid on it a once or twice. It seemed not to help, but I'll admit I sprayed into the air cleaner and not the cylinder, and I may not have used enough. I was a bit hesitant because I'd read in a few places that this is bad to do for a two stroke. I thought of dumping a small amount of premix directly into the cylinder instead, but someone here advised me against that.

 

the good thing about using starting fluid is that for the most part it will fire even a flooded engine. you can use it on a 2 stroke and as long as your not using it to keep the engine running it shouldn't hurt anything. you could try spraying it inthe air breath w/ the filter removed while cranking oiver the engine as it would be sucked in w/ the engines vacuum. if it doesn't at fire then something is wrong. imo

 

Minor update. Since I acquired a spare motor a few days ago, I had the opportunity to compare two motors side-by-side. It appeared that the crank seals on the motor which had been troubling me were in OK condition, but seemed to have "worked themselves outward" a bit from their expected position in the cases. The flywheel-side seal was particularly lopsided in it position, as well.

I used a homemade PVC "seal driver" to re-seat them, the re-installed the motor. After a tiny shot of starting fluid directly into the cylinder, the motor fired right up, but raced like crazy. I think I may have mis-routed the throttle cable, though...

 

I forgot to mention that I tested the "fixed" motor without the drive clutch attached. Would it make any sense to think that the drive clutch is heavy enough to slow down the crank, and that a motor without the drive clutch could be expected to idle faster (given equal carb adjustment, etc.) (?)

Motor without drive clutch is running/idling great after significant adjustment in idle speed (at the carb).