My friends I was wondering if I could plead for some advise and help. Heres my story, three years ago, I bought a running, yet total basket case 1985 Polaris Trail Boss 250 from a friend and wanted to rebuild it from the ground up. The major problem with the bike is that the crank shaft had been altrered from the previous owner, with little to no experience, and the lobes were out of alignment by .055 thousandths. As you can immagine this blew all three crank case bearings, and the piston skirt, however ( and this is unbelievable ) it did no damage to the casings, or the cylinder! I had the shaft re-alligned, and replaced the piston and rings. Im not new to a good old rebuild but this one is boggling my mind. I finally finnished the motor, and all the bad *** flat and gloss black paint on the bike, and dropped the powerplant in this past weekend. We have tons of good compression, spark, and fuel. She ran for a good 3 mins on choke, and once taken off, she quit out. I took the carb apart and cleaned for the third time, and yet nothing. This bike will not run to save my life and I dont know why. There arent alot of electrics on it ( as you guys that are into old timers would konw ) So I dont think its an electronic issue, we just cant figure it out! Any advise would be greatly appreciated! Thank you in advance!!!!!!

 

Could be something as simple as a cracked/broken intake tube on the carb.

How does the exhaust look? Any critters making a home in there?

 

As you said you have the three major components that make an engine work;compression, spark, and fuel.
My first thought is maybe not in the correct proportions, may be a carb issue or leak near the manifold.
Compression is what it is, either you have it or you don't, same with the spark. That is generally speaking and not getting into a details of each. Yes lack of either can cause problems but your post implys both are great.

 

Oh one last thing just thought of. Had a very similar issue with my Honda 200X ended up being the oring on the main throttle need, was partial there and torn, it was sitting up near the spring. I'm referring to where the throttle cable screws into the neck of the carb. It ran on choke barely and wouldn't draw in fuel very well = no running engine.

 

Compression,fire,fuel,and air needed to run. Check for what posters mentioned above,plus when you rebuilt you did use new gaskets and crank seals and not reuse?? If no or leaking low end compression(base gasket,crank seals) will not be able to run even if top end compression is good. Just fishing?? OPT

 

i would do a leak down tst if you can to make sure all the seals(even though thier new)aren't leaking as well. is the plug dry after you try to start it after it's been running?

 

Well....Im an idiot. As a matter of fact I am beyond embarassed to say the LEAST to even post this but....I do owe all of you a great deal of gratitude and thanks, so I will tell you my findings. I....I made the damn fuel mix too rich. Yep....I did it, and I cant believe it. I took the tank off, and dumped the carb. I lit my good old torch and stuck it over the spark plug opening on the cylinder, and the flame I got, well....my bic lighter couldve done a better job than what ignited. And following was a nice cloud of white smoke. I sprayed a little ether in the carb and one pull she was purring like a kitten. Too much oil, so Im going to lean her out with a 50:1 mix instead of 40:1....once again, thank you to all of you that responded. I greatly appreciate it guys.

 

Your welcome and glad to hear that you got it running.