My rebuild isn't quite ready and my girl is dying on me, wont start up. I need help diagnosing problem

1) Getting fuel
2) New plugs, wires, measured at least 40k volts spark discharge
3) Valve timing is good
4) No exhaust restrictions

I know my compression is shitty, a month ago i had each 90-120-110-90 psi hot.

Since the engine wont fire up, i can only test the compression cold, im getting 60-120-60-60, cold.

Obviously the compression is suspect but does anyone have any case-in-point experience that this is definitely too low to even fire up? If not, i need suggestions on what to check because this engine needs to last another month until my rebuilt one is ready. thanks guys.

 

60 psi is pretty damn low and the engine will likely not fire especially if it is cold outside.

 

yeah thats pretty shitty compression, can u even hear some combustion taking place?

 

I remember reading that 80psi is about as low as you can go before the engine starts to struggle.

I assume the compression test was performed with an open throttle?

 

I've done a lot of compression tests on a lot of different makes and models of cars. I have never seen a published specification but I've seen cars with less than 60 psi start OK. I've also seen cars with 85 psi compression that wouldn't start. It all depends on the design of the engine. I've worked on a lot of cars from the 20's and 30's and it wouldn't be unusual for one to have less than 75 psi in a perfect engine.

The temperature outside and the volatility of the fuel also makes a difference. Part of the reason the air and fuel are compressed in the first place is to raise their temperature high enough to get them to burn. Try spraying some ether into the intake and see if it will try to fire off. It may just be volatile enough to get some heat into the cylinders and allow you to start the engine.

 

how old are the piston rings? anything done to the valves?

 

I just rebuilt mine, one cylinder had 50psi and was def not firing. I wouldnt expect that thing to start.

 

do a wet test by squirting a teaspoon of oil into that cylinder. SAE 30W is fine.

 

dump a table spoon or 2 of oil in the cylinders..put plugs back in and see if car starts. usually around 90psi is when a car wont start, of course there are other variables, like temps,valve overlap etc