ok i have a 81 12a that i built a few years ago. did a compression test on it with a mazda tester .... got good #'s back 90's on all... the thing is when the motors cold it fires up... when its hot you have to roll start it to get it to run... not far or fast may be 6 mph and pop the clutch verrom shes running good.... the odd thing is i am pushing compression OUT the front rotor intake ports. I pulled the intake off and put tape over the ports on the rear it sucks the tape in(normal...vacume) when you spin it with the key... on the front rotor ports it pushs the tape off.... blows it off the motor... and its pushing HARD... what would cause this? and y would i still have good compression?
Thanks. David


and im gonna open the motor soon to figure this out 4 sure

 

Wow...That is odd...

~T.J.

 

i know

 

Well 90 aint all that good. And thats cold comp which is always higher than hot comp, so your probably looking around 80 hot which is marginal. Possibly worn apex seals (or bad rotor housings) are leaking enough compression to pump it back thru the intake, and also causing low compression. sorry

 

carl... i think your backwards... you get lowwer #'s on a cold motor and higher on a hot motor. my tester might be off a little but no more than 2 psi max. i get very consistant #'s on motor... a fresh(10-15 miles) ill get 85-90 psi thats a mazda motor... i get about the same on my rebuilds. i just did a fd with 48,700 org miles and it was bouncing 85.3 hot