What I have is a Polaris 250 two stroke. They don't have reed valves and are pretty much stupid simple engines. I have rebuilt over 20 without incident, till this SOB. So I rebuilt the top end, I put a New Wiseco piston in it with a bored out .50 over. Everything went together nicely. It would not start, with starter fluid, no go. Swapped out carbs, no go. Tore the flywheel off, keyway is fine, changed the cdi out, put a new coil on, spark is nice and blue, still no start. Soooo, I bought a compression gauge and I only have 80 psi of cylinder pressure. Polaris calls for 110 to 130 on that model. So I tore it down and found nothing wrong. I put it together with a stock bore and another New Wiseco piston. Torqued all the nuts and bolts to factory specs. Checked and only had 80 psi. So I tore it back down AGAIN. I changed the head and head gasket. (I got a lot of Polaris parts on stock). I added oil to the cylinder and got it to 95 psi. So I thought I will try to start it and I got it to start. It would rev up and I kept it running for about a half hour. I adjusted the air screw on the carb, set the idle, idled fine no loping, no erratical behavior, fine. Shut it off, wont start. I hit the button, starts for a second and dies and wont start. Let it sit overnight and go out hit the button, fires right off for a second and dies, wont start again, even with starting fluid. I was going to change the crank seals and even threw a set in the freezer, but that's a waste of time, it idles too nice, if they were out, it would do all kind of crazy stuff and that doesn't effect the compression anyway. My next guess is I will swap out the exhaust, I think it could be plugged up. Guess after that is I may disconnect the rev limiter, I ve seen them do some weird stuff on Polaris. If that doesn't work, I will be taking it down again and this time checking the cylinder base surface on the block for planeness. I am at a loss and if I didn't have so much time in this thing already, I d probably just part it out. I am at wits end. I have extra motors, but what could be the issue? I hate to swap motors out at this point, but I am not happy and spent way too much time on this. Any Thoughts? or Ideas?
Two Stroke Engine Trouble Shoot.
- Thread starter Panzerman
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Low compression with the new piston and the jug bored over?
Hate to say it...are you certain that the new piston has the same deck height as the old one? If it's closer to the wrist pin (lower), then that would explain the low compression numbers...
Hate to say it...are you certain that the new piston has the same deck height as the old one? If it's closer to the wrist pin (lower), then that would explain the low compression numbers...
I went through an old Yamaha DT-360 years ago. Couldn't get it to run right. I tried everything. Took it to my Uncle and he kicked it over a few times and said pull the pipe. Yep clogged beyond belief. Ran great after a hot tank soak. He was a lead wrench at the local Yamaha dealer. Your low compression doesn't fit in though.
I don't know anything about your particular model engine, but I would think it almost has to be something with the rings/piston/bore. Have you verified that the bore is round & correct size, the finish matches the ring mfr's specs, etc?
Astro brings up a possibility I hadn't thought of, with the deck height...
Astro brings up a possibility I hadn't thought of, with the deck height...
I would be checking the squish (piece of solder between the piston and the squish band of the cylinder), it'll tell you if you have a clearance issue (piston not coming all the way up in the cylinder) It could be that you have a short connecting rod or tall crank deck but I'm not sure where it would have come from unless they had a variant of the motor in production for a short time.
Ok, update. The piston deck height should be good. I placed a Wiseco, a Namura and factory stock piston all together and they match, wrist pin and heights. I then put a exhaust on, I cut open and removed the guts and welded back up, for just such a occasion, still did the same thing. I went into the wiring and eliminated the rev limiter and "Presto" starts right up and runs fine. Sooo its the rev limiter. Polaris used to make the rev limiter a easy to disconnect once you found the right wire. They are much more complicated on the new models and are hardwired to the key. To eliminate them, you have to split the wires and then your key wont kill the engine, it will kill the electrical power, but not the engine, the kill switch still will. Its a two to one, to stop quick easy fixes. I think the low compression threw me, but I also read, that you cant really get accurate readings on rebuilds before break in, with the cylinder hone and rings not seated, I can believe that. Regardless it starts effortlessly now and seems to run excellent. So the compression must be within specs. I may check it after my break-in starts and heating and cooling cycles and see if it improved to 110+. Thanks a lot for all the ideas, I am just glad to be done with it. I have to wonder how many good Polaris ATVs went to early graves over those stupid rev limiters.
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