compression

[ribcage]

I just rebuilt a 3 cyl 90 hp motor. I had it bored 20 over and got new pistons and rings. I just checked all the cylinders and its on ly reading 90 psi for each cylinder. Does this sound right? The manual says 135 to 140...is it possible something was done wrong. Its not firing correctly correcrly at all. When I turn over the engine I sometimes get a pop like its firing but nothing after that. .just cranks and cranks. Any ideas?

 

[MickLovin]

Re: compression

Firstly I found out that not all gauges are the same, so I bought my own new one to find my compression was fine. Are you familiar with your gauge you got 90psi on, I say this because I used a mate of mine's gauge and it read 75psi with my new gauge it read 150+ psi.
So before you panic, check your gauge with another to make sure it is actually 90psi.
Also you advise that it is not firing properly, are the leads connected in correct spark plug sequence?

 

[Jiggz]

Re: compression

What type of compression gauge are you using, pressed or screw in type? Have you used the same gauge before on this motor and actually read correctly like maybe above 110's? Proper torque and sequence done correctly? Since this is newly rebuilt, has the timing been set correctly? Link and synch done also? Have you tried spraying fuel mix into the carb to see if it will try start? Have you verified you have sparks on all plugs? We can start troubleshooting as soon as you post your responses to the above questions.

 

[Big Fish Billy]

Re: compression

Hopefully you got the correct oversized pistons and rings...there is an end gap measurement for where the ring ends meet in the cylinder, might be worth checking for that spec as well as Jiggz suggestions. Also I have three compression gauges, two out of three are about the same, the other much lower, so check with a buddy's gauge, also make sure the pressure release on the gauge is sealing, if it leaks back after you take your reading, lube it up with some WD-40. Also make sure your piston is pumping 4 times during taking your reading.