Recently rebuilt the top end - piston was seized and the engine had never been rebuilt. I had the cylinder bored out, put in a new piston, rings, and top end gasket kit. This was my first top end rebuild ever so I'm pretty new to this.

Got everything all finished up last Friday evening. My compression tested around 110. Got everything put back together and it started right up. Saturday morning, it didn't want to start at first so I waited a while, tried again and it started.. I began to figure out that when the engine was warm, it would start on a single crank. Immediately turn on and purr. Yesterday morning, it wouldn't start no matter what I tried. With the first couple of turn overs, the engine sounded like it was about to crank and then nothing. So I pulled the plug, cleaned it and checked compression. I was getting between 110 and 115. Put the plug back in and it pretty much started right away. I set the carb so it idled on the lower side of things and so it didn't grind when shifting gears.

Fast forward to today. Fully cold engine - I tried to fire it up - on the first couple of turn overs, the engine sounded like it was cranking and then nothing. So I tried everything I had tried before: checked fuel, adjusted the carb a bit and pulled out the spark plug to clean it. Wouldn't turn over with electric start or pull start. I ended up grabbing the compression tester and checked it - now it's reading like 30. What in the world???


UPDATE: After I wrote the above - I immediately started pulling it apart. So, I pulled the head off - gasket was where it was supposed to be. Top of the piston looked fine and no scoring - bore still looked and felt very smooth. I haven't pulled the cylinder out yet so I only got a good look at the top of the cylinder. My next thought was piston rings - I can see the rear of the piston rings when looking through the exhaust manifold hole - they both look seated correctly from that angle. I'll have to pull the rest of the cylinder to get a better look at the rest of the rings.



One odd thing was when I pulled the exhaust manifold off, there was a lot of grungy oil in the first joint where the exhaust manfiold meets the exhaust pipe. It was maybe a couple tablespoons of dark cloudy oil. I've been premixing like I said and that's obviously a blue hue so not sure what this was.



The only other thing is that I did check the piston ring gaps before I put the engine back together and they were definitely on the high side of the spec. You guys may rip me a new one but one of the rings was even a bit above the .018 max gap; closer to .020.



Still seems wierd that I had decent compression and then nothing...I'll be working on pulling the rest of the cylinder in the near future. Thought maybe something could be gleaned from the above.

Between yesterday and today, it went from 110+ to 30??? I'm pretty upset, not really sure what to do at this point...Also, I barely ran it yesterday. I did pull off the intake manifold (not sure if that's okay to do or not) while turning it over and I can see the piston moving nicely in there...