Hey All, Maybe you can help me, I run a small engine repair business in western mass, and recently took in an 89 polaris trail boss 250 in trade for a compound bow. The machine hadn’t run in what id guess was a year or 2, and was in bad need of a top end I proceeded to dismantle the engine purchased a top end kit .40 over and took the jug to a very reputable auto machine shop to be bored and honed, put the engine back together did a thorough carb cleaning, and tried to fire it up, nothing, ok so I took the spark out, perfect spark, strong, spanning almost a half inch, so I thought it might be a timing problem, removed the flywheel and adjusted the timing plate to factory specs, put it back together tried to fire it up again, and again nothing, not even so much as a sputter. I took the fly wheel off several other times ,trying the timing in multiple positions, and every time i turned the motor over the machine wouldn’t even so much as burp, so i began to test the other components of the ignition system. The exciter coil measured in at right around 120.2 ohms of resistance, which according to factory specs is good, the stator measured at 1.2 ohms, and the ignition coil also at about 1.2 ohms, which I believe is high for these coils but I would still think the engine would do something. The only other thing is the CDI unit, but I have good spark and it’s not often that a CDI unit can go bad and still give spark tell me what you think.

 

If you have a leak down tester check that the crank case holds 7psi with no drop. If it doesn't pull the drive clutch and see if the crank seal has popped out. Usually this is the main one that has problems,but sitting for two years both crank seals could be bad.

 

Dont have a leak down tester but I just pulled the clutch anyways the and removed the flywheel and magneto seals on both side look to be seated correctly.

 

I just looked a little closer at the crank seal on the clutch side while cleaning some dirt off I did notice it was leaking some fuel that is sitting in the crankcase

 

Crank seals are what most people overlook when doing just a top end. If it's been sitting with gas in the crank case for up to two years, seals can distort and wont hold bottom end crank pressure for the fuel mixture.The seals usually will show signs of leaking when this happens.You have to have a good top end and bottom end compression for the air/fuel/oil mixture to shoot up the transfer ports to fire.. If you're sure the top end is done correctly,spark is good as you say,stator plate timing is correct,plus it won't even try to kick off with starting fluid,then I would suspect the seals.

 

Ok Ill order some new seals hopefully that will solve the problem.

 

Just installed the new seals tried to fire it up and still haveing the same problems, Any more advice?

 

What is your compression reading after you installed the new piston? You said you have spark and I suppose you have a good fuel flow.

 

Only about 60 psi on both wet and dry test, A bit low Although I am quite confident in the machine shops allowed tolerance, and the cross hatching on the hone looked good. The machine shop I used has been in business for 30+ years building motors for various racing vehicles, they Have high end cnc boring and honing machines.

 

That's sounds like the problem though,if you're sure that there are no leaks on the cylinder base gasket or head gasket! Compression on a new bore should be around 135 to 140 psi or more. The manual states between .0015 to .002 clearance between the bottom of the piston skirt and the bottom of the piston on a new bore.(Later models,95 and up, show to have an even tighter setup. (.0011 to .0021 ) I always tried to get a tight bore closer to .0015 finish. I've found a lot of machine shops that have allowed too much clearance when customers ended up bringing cylinders or complete engines in to me to work on that had problems. Sounds like you need to pull the top end down,pull the rings(check in case of a broken one) and check the piston to the cylinder clearance.