Compression test readings...

[kfosburg]

1971 911E, stroked to 2.4, with webers:

I performed a compression test on all cylinders yesterday and am not sure how to interpret the results. Test condition: All plugs removed, charged battery, basically cold engine, throttles closed. All 6 cylinders were 100PSI +- 3 PSI. Consistent is good, but this seems pretty low. I double checked with another compression guage and it was exactly the same.

I know warming it up will increase it as well as open throttles, but does this indicate that I possibly have a valve timing issue?

The car runs OK and pulls to the redline, but it is not too smooth. I've been doing a lot of carb tuning to attempt to diagnose and fix, but haven't been able to close the gap which is why I did the compression test.

The engine was totally rebuilt and only has about 100 miles on it since. However, the car did sit for about 5 years without running!!! (Sad, but true...)

Suggestions?

Thanks,

Karl

[TerryH]

...you can add that the rings are probably not seated yet too.

[Jdub]

I'd be sure to clean all your jets, replace the gas filter, and get fresh gas through the whole system if you haven't already. Old gas will play havoc with carbs.

If you can be sure of the plug, timing, and mixture, then drive the car to break in the 'rings and get the fresh gas through. Gummy buildup from old gas sitting in float bowls, etc. used to cause me no end of headaches with Amal etc. carbs on my Brit bikes.

Sorry not to be of more help,
John

[kfosburg]

Yes... sitting around was not good for the fuel system! I ended up replacing everything from and including the tank, lines, pump and filters! I have cleaned and rebuilt the carbs several times already and each time find a little more gunk in them despite changing all the fuel system.

How long would rings take to seat in? And, how much would that contribute to compression readings? What type of readings should I ultimately be looking for? I'm not sure which pistons are installed, but I'm pretty sure they are E, not T or S.

Thanks,

Karl

[Grady Clay]

When you measure cranking compression with the throttles closed, you are measuring a combination of carb idle settings and the mechanical compression condition. Try again with the throttles wide open.

It is a good idea to regularly measure cranking compression and cylinder leakage to form a baseline.

Best,
Grady

[addictionMS]

put a battery charger on it as well, more juice is good. If you have a high torque low speed starter that will affect the numbers as well.

Jim