To make a long story kind of short: I have an STL Miata with a 1990 1.6 engine (Megasquirt, serious Webcams, JE Pistons, etc. ...but stock HLA's that will probably be going away soon so I can raise the rev limit) that has lost compression when the engine got warm. I cleaned up the head while it was off of the block during springtime, installed it, then cold compression this was ~200psi on cylinders 1,3,4 and ~180psi on 2. Leakdown was between 4-6% on all cylinders using a modified HF tester (tested at 50psi). While the head was off I let the pistons sit in Marvel Mystery oil for a month to free up the rings if need be. The engine had sat for 3 years in a nice garage before I picked it up last summer.
Started the thing up today and it ran great for a few minutes with a loping idle around 1000-1200. I set the timing with the usual Tunerstudio instructions. It sure sounded mean compared to the stock 200k-mile engine I was running in STL last year so I was excited! Drove it on the street to clean the cobwebs out of the car, then the idle started gradually going up to 3000. Pull back into garage and it will barely start again. I've fought this before.
First thing I do is check compression while the water temp is 170. It went down to 90psi on cylinders 1 & 2. Didn't even bother checking 3&4 out of frustration. And leakdown on those same 2 cylinders is almost identical to the cold reading above (4-6%).
I guess my questions are: are these discrepancies even possible? If so, what problem do they indicate?
I verified the tools worked with a stock street 1993 Miata. I am also pretty sure I have the leakdown and compression procedures correct after many searches using google and this site. I have tons of experience with R/C car engines and Formula 500 Rotaxes...unfortunately that's not too applicable to the 4-stroke world. This is the first 4-stroke engine I've ever worked on myself.
Thanks in advance for any ideas. I am completely stumped and would like to figure this out before I pay someone else to rebuild the thing for me.