Hello Forum, trying to get my buddies 92 Polaris trail boss 250 2 stroke to run. When I got it it would not start at all, took the carb apart and cleaned it and it fired right up, turned the bike off turned it back on no problems, thought that had did the trick. The next day I go out to start it and take it back to my buddies house and nothing, would just crank over and over. Would also drip gas out of the exhaust. Pulled the carb again, double cleaned it, put it back on and it started up, ran it for about 30 minutes no problems. Again the next day would not start, Took the carb apart again, but this time it did not start back up. Replaced the plug, I get a nice blue spark coming from it. Checked compression I have about 105-107 lbs. I've tried starting fluid through the carb, and directly into the cylinder as well as a cap full of gas, no luck. I've also tried the air/fuel screw out 1 - 2 1/2 turns...did not help Not sure what else to check. Every other bike I have ever had started with gas directly into the cylinder if I had spark. All I can think is the plug is just getting drowned with fuel. Any advice on what to try next?

 

105-107 psi on compression sounds like the major problem. Should be a minimum of 120psi or so for it run right. Gas coming out the exhaust is another by product of low compression as it simply can't burn it and it goes right out the exhaust.Not a fan of doing top ends only(can cost more in the long run),BUT if this the way you're headed,after a new bore and piston kit at least replace the crank seals and a cheap carb kit too..Be sure to check any up and down play on the rod and also for any noise/roughness from the crank bearings when you take the top end off. If there is ANY up and down play on the rod,plan on a complete rebuild..

 

OK Well I took the cylinder apart, ALOT of gas was in the exhaust, pretty much completely filled, as well as down into the crank. Is there supposed to be gas down in the crank? Ordering a new top end kit tonight.

 

Gas will accumulate in the crank case and exhaust when compression is low,as it can't burn the gas/oil mixture. Don't forget to install a new carb kit, check out the rod and crank bearings along with replacing the crank seals as they don't like gas either and can distort and leak. It's a crap shoot on doing just a top end only on this 21 year old atv..

 

So I finally got the engine back together with the new top end kit and she started right up, sounds a lot stronger than it did before, throttle response is much better, I wont really know until tomorrow if this worked or not because it seemed that after it sat overnight it wouldn't start the next day but I have high hopes for this. I turned it off and on about 10 times over the course of about 2 hours and it did not hesitate at all. Thanks Old Polaris Tech! I'll update tomorrow if she starts up again.

My next concern is I've never owned a 2 stroke that automatically mixed the oil for me and maybe its just my old ways of thinking but I just don't feel comfortable with it doing it. So.... would it be wise of me to disconnect the oil feed line and just mix the oil/gas into the gas tank? Or could I just drain the oil out of the oil tank, leave everything hooked up as it is and just mix the gas in the tank? Or should I just let the machine do its job... I've only went through about half a tank but I don't feel like the oil level has dropped much if at all.

 

You wont use much oil.That was the concern of a lot of people that the oil pumps weren't working even when new. On the average from input I got over the years,most 250 engines will go through 4 to 5 tanks of fuel to 2 quarts of 2 stroke oil.I'd always premix the first tank of fuel on a new top end to aid in break in. You can always install a new clear line from the pump to the base of the cylinder,bleed the pump,crank it up and at an idle hold the oil pump lever all the way up. You should see the line start to fill up making it's way to the cylinder.If you don't then I'd be worried and premix the fuel..

 

So the next day, starts up like a dream didn't even need to choke it. So contrary to what others have posted 105 psi is definitely not enough to start the trail boss 250 consistently. I've read through alot of posts on this and everyone says the same thing, that anything over 100 should be good enough, for anyone searching this in the future it is not.

I'll check the oil line to see if its producing anything, thanks again.

 

A little over 100psi isn't good enough as you've found out!! Most manuals say 115 to 120 psi minimum on compression,BUT to me that is still a tad low for them to run properly..

 

Just bought a 1992 trail boss 2 stroke guy before me took the oil tank off and plugged the carburetor. He said he used mixed gas straight in the tank. Any thoughts or concerns ? My gut tells me to put it all back the way it was.