i'm a new member and i need help! i have a 1993 trail boss 350L 2 stroke and it won't start. I've been working on and rebuilding various brand dirt bikes and ATV's for several years now mainly as a passionate hobby. I was riding my polaris in and out of the woods hauling firewood and it was starting fine each time I got on it. However the last time I got on it to go back to the woods.... it suddenly would only crank and not start. I'm getting spark at the plug and my compression is 125 psi. I can put a little fuel in the plug hole and it will fire a couple of times. I thought the reeds may have gone bad...and they looked questionable so i changed them...but that didn't help. It seemed like no fuel was being pulled into the cylinder and I noticed as i was cranking it that there was virtually no suction on my hand on the input side of the carburator. I pulled the carb and cleaned everything and reinstalled but got the same results. Next I pulled the head and the piston seemed to have a lot of side to side play in it and i noticed scaring on the cylinder walls. I thought i was losing some vacuum due to bad rings or something. By the way the previous owner said he had changed the piston & rings. I took the cylinder, piston & rings to my cylinder shop expert and he found the piston was stamped wrong from the factory and it was too small for the cylinder. I had it bored, honed and new piston & rings and gasket kit. I reinstalled everything only to get the same results. I was told there were seals inside the crank case that could be bad and causing loss of vacuum in the cylinder to pull the fuel in? I was also told to check the exhaust pipes and muffler for blockage which may cause air flow / vacuum issues? I haven't done this yet....but does anyone have any other idea's or experience with this problem? It's got to be something that happened all of a sudden because it was starting fine during the day I was riding it. What about flywheel timing? I'm running out of ideas and I HOPE I don't have to pull the motor & change seals..

 

Welcome! Crank seals are probably the main culprit! I replaced em every time I did an engine job,top end or complete rebuild. Most people over look the seals as a cause of low compression also. You have to have good top end compression(piston/rings) plus bottom end compression also which is the crank seals. The piston as it goes up creates the suction,compresses the fuel/air mixture under the piston as it travels downwards,the seals hold this mixture in the crankcase and then it travels up through the transfer ports to the top of the piston. There spark ignites it and the sequence starts again.You don't have to pull the engine,but have to have a front clutch puller to get to the left seal along with a steering wheel / harmonic balance puller to pull the flywheel,then the remove the stator to get to the right case seal.You can inspect the flywheel woodruff key also when you remove it in case the key may have sheared,altering ignition timing. OPT

 

thanks a bunch! i appreciate the help. It's good to know i can replace the seals without pulling the motor.....which I was dreading to have to do. My gut instinct was telling me it was likely the lower seals but i wanted to try the simple things first obviously. The top end needed to be done anyway so I'm glad this happened to make me do it. One thing i noticed and forgot to mention in my first post, was the sound of the top end cranking has been different.....& what I mean is, you know how it sounds when you pull the plug out and crank and its just moving without any compression. Well it was similar to that. I know when I put my finger over the plug hole the compression when the piston is going up will push my finger off but on the down stroke I think it should also have some suction which i'm getting very little of. So it makes sense if the crank seals are gone, why i'm losing that suction to pull the fuel in.

I'll try it and let you know how it turns out.