What should the compression be for a 88 trail boss 250? I'm thinking it should not be less than 100, but what would it be with a new piston and rings???? I gotta check mine again, I think I got 100, but its nearly impossible to start, I rebuilt the carb and it made no difference. Plugs still get soaked. I'm ready to rebuild the top end, but I'd like to be sure that it is the problem. It is getting spark, seems to be getting fuel, all is left is compression. grrrr. after about an hour of switing and drying plugs I can get it started and it runs great till I shut it off and then it is hard to start again!!!!!

 

should be bare minimum 100 psi. tip top shape should be around 115 - 120psi. sounds like you might have an air leak of you have good compression. get at your flyhweel and see if you can jiggle the flywheel up and down. if so then you need new bearings/seals on the crank

 

hmmmm, thanks, sounds like a plan. how difficult is it to replace those bearings and seals? I've done top end work on 2 strokes, never had to tear apart a bottom end yet. I got the Haynes manual, that's pretty good at walking your through stuff.

 

Also take the pipe loose and check for raw gas in it.I had this happen more than once.100psi it should start and run

 

its not very hard to replace the crank bearings. one thing i like about these motors. all you need is a heat gun and a rubber mallet to get everything apart.

 

<div class="FTQUOTE"><begin quote>Originally posted by: ftwflh

Also take the pipe loose and check for raw gas in it.I had this happen more than once.100psi it should start and run</end quote></div>

What Pipe do you mean? there was a lot of fuel collecting in the airbox, I replaced the needle and seat when I did the carb, then just cleaned all the jets and body and reassembled with a new bowl gasket. OR are you talking about the exhaust pipe? Do you mean trying to see if there is a puddle of fuel in the cylinder. If thats possible, that seems like it could be the problem, I'll look.
I'm going out now to do a 2nd compression test with a different tester, I have reason to believe the 1st tester was bad. I'll get back with the results. Thanks

 

With this piston port engine when you have a float problem like you did when you had gas in the airbox it will also flood into the exhaust pipe not letting the engine breath.Take the exhaust off and see if it has raw fuel in it.Also you may want to get a complte carb rebuild kit from sledpartsguy on EBAY he has kits that include a needle valve and seat along with gaskets and jets for about half of what the dealer wants for just a needle and seat.

 

I checked the compression again. It was around 120, seemed fine, it went down slowly on the gauge, I don't see how it could do this as the testers have a one way valve in them. That is why I thought the 1st tester I rented was bad, I went to a different autozone which had a brand new unit and made sure it was in tight...got a great reading but it slowly dropped from 120 to about 90ish. could this be a sign of an internal seal leaking?? I dunno, seems to me if the cylinder stopped at the bottom of the stroke the pressure would go out the exhaust anyway.

I took the Exhaust OFF, it was wet in the opening. I tried to crank it over with no exhaust. Nothing. This bike is getting me frustrated. I just bought it, It fired right up when I went to buy it, ran great, shut it off, started it up again. Now it's really getting on my nerves. I've never had to give up and take a bike to a dealer and ask them to fix it, I'm just about ready too.

Anyone live near eastern PA and want to take a look at it for me. I work long hours and don't have a lot of time to work on it. I'll pay ya to get it running. [img]i/expressions/face-icon-small-confused.gif[/img]

 

Put the pipe back on and try it again
If it doesnt start take the exhaust and the intake off and plug them off real tight do a check but listen for air leaking around the crank seals

 

the compression pressure bleeding off is normal the rings do not create an *air tight* seal in the cylinder. just makes it close enough to get the compression needed. that why if you have a leaking carb it will fill the crankcase up with gas. it runs in ontop of the piston then it works it way through the rings into the bottom.
if you know for a fact that the motor is getting spark. you have three of the basis covered. you know its getting fuel you know its gettin compression. the last thing is air. its getting it just way too much of it. i really think the seals are shot on it. and probably the bearings too.
have you tried the wriggle test on the flywheel yet?