mechanic123 Posted June 16, 2023 Report Share Posted June 16, 2023 Spoiler I have a 1971 Bultaco Matador 250. Checked compression and got 90psi max. Replaced piston rings without rehoning cylinder and got 150 psi. Wouldn't start so I messed with timing and got it to start. It reved really high because I didn't have the carburetor tuned. I shut it off within 10 seconds of running. Went to start it again and it wouldn't start. Checked compression again and now it's at 120 psi and won't even attempt to start. Tore the head and cylinder off and no scoring in cylinder and rings are still intact. Any ideas why compression dropped? Any help is greatly appreciated. TIA. Quote Link to comment Share on other sites More sharing options...
suzuki250 Posted June 16, 2023 Report Share Posted June 16, 2023 (edited) If it's over-revved, I would check the woodruff key on the ignition. They have a habit of shearing! 120psi is about right for a two-stroke 👍 Edited June 16, 2023 by suzuki250 1 Quote Link to comment Share on other sites More sharing options...
mechanic123 Posted June 16, 2023 Author Report Share Posted June 16, 2023 1 hour ago, suzuki250 said: If it's over-revved, I would check the woodruff key on the ignition. They have a habit of shearing! 120psi is about right for a two-stroke 👍 Thanks for the idea. I checked the key and it is not sheared. Any idea why compression went down though? I thought 250 2 strokes were supposed to have atleast 150 to start? Quote Link to comment Share on other sites More sharing options...
feetupfun Posted June 16, 2023 Report Share Posted June 16, 2023 Measuring the peak pressure on a two stroke will always give variable results. Without changing anything else, the amount of oil at the ring/bore interface at the time of the test will make a significant difference to the result. Fitting new rings without honing the bore will make the results of that sort of test even more variable. 1 Quote Link to comment Share on other sites More sharing options...
mechanic123 Posted June 16, 2023 Author Report Share Posted June 16, 2023 I'm getting a constant reading of 120psi. Would that be enough to get it to start? Quote Link to comment Share on other sites More sharing options...
Glayne Posted June 17, 2023 Report Share Posted June 17, 2023 17 hours ago, mechanic123 said: I'm getting a constant reading of 120psi. Would that be enough to get it to start? I would think so. Quote Link to comment Share on other sites More sharing options...
mechanic123 Posted June 17, 2023 Author Report Share Posted June 17, 2023 Everything I see online is 250 2 strokes need around 150 to start though? Quote Link to comment Share on other sites More sharing options...
stevem75 Posted June 18, 2023 Report Share Posted June 18, 2023 You mentioned when it started it revved really high. Have you replaced crank seals ? And made sure there’s no air leaks around the carburetor ? Your engine should run with 120 compression. It takes three things to make engines run. Spark (you have it) fuel and compression. It’s one thing to have 120psi secondary compression but primary crankcase compression is always overlooked and is equally important. Make sure those cases are sealed. 1 Quote Link to comment Share on other sites More sharing options...
Glayne Posted June 18, 2023 Report Share Posted June 18, 2023 I've seen 2 strokes run under 100 compression. It was under debate how well. It was for sale and I didn't buy it. Quote Link to comment Share on other sites More sharing options...
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