Low compression but good leak down possible?

[Dave Kost]

I have 911 1970E with MFI that feels a bit anemic. The engine was rebuilt 5000 miles ago and eight years ago. Had a leak down and compression tests done and the numbers were

#1, #2, #3 mid 90's PSI
#4, #5, #6 mid 100's PSI

leakdown # all below 3%

Is this possible?

Leaks no oil. PO has moved so, no help there.

Can cam timing effect compression?
Pulled head studs?

any suggestions?

[yelcab1]

Yes, it is possible if the compression test was done incorrectly.
Cam timing does affect compression.

[HawgRyder]

Cam timing at cranking speed can have a big effect on compression.
The more overlap there is (both intake and exhaust open at the same time) the lower the cranking numbers.
Cams like 906 and S can have larger overlap than CIS so they have higher compression ratios (static) but sometimes lower numbers on the guage.
In some engines...the numbers are just barely ehough to get the engine to run...they idle like they are about to stall...but when you hit the gas...POWER!!!
Bob

[Dave Kost]

Ok i understand...

How to you check cam timing if you don't know what cams you have?

[Dave Kost]

A few more facts

The tach bounces around between shifting gears. What can cause this?

It never runs hot and has good oil pressure - 1 bar at idle and 4 to 5 bar at 4000 rpm.

Power is down. ??

Comments welcomed

[Porshaah]

Originally Posted by Dave Kost

#1, #2, #3 mid 90's PSI
#4, #5, #6 mid 100's PSI

leakdown # all below 3%

Can you provide precise numbers for each cylinder?

[Dave Kost]

Sorry cant do the specific numbers. The tests were done right before is was put to sleep late last fall. I may have put 100 mike on it since then. I had a mechanic do them before going into storage as a quick check.

Thanks for the reply.

Any thoughts on the bouncing tach? Did a search on it and came up with speed sensor, maybe?

[Flat6pac]

Try the compression check with the throttles open.
90# you barely have ignition.
Bruce

[Ronnie's.930]

A bouncing tach is often caused by a faulty voltage regulator (too high or low voltage - often too high). There is quite a bit of info about this in the 911 forum.

[Steve@Rennsport]

Bad/dirty points can also make the tach bounce.

[matt demaria]

I am in the middle of one right now. Compression should be about 145. Deviation of 20. Cam timing is easy and quick to check. A degree on the crank pulley is 1 mm. Use a tape measure. Porsche timing specs are listed at 1mm total lift.
Most of the issues I have seen with the MFI have been with the pump going out of calibration or the setup was wrong. Too much gas gets delivered to the cylinders and borewashes the cylinders. This burnishes the cylinder walls and polishes out the crosshatching. The crosshatching is used to retain small pockets of oil to help the rings seal as it travels down the bore. You can add a little oil to the cylinder and perform another compression test to see if the value kicks up a lot and you can borescope the cylinder and look for the crosshatching.

[Dave Kost]

I quick up date.

The points were closed on the distributor, so I don't know how it was running at all. So I decided to pull all the plugs, # 1 and 3 were all wet and I don't think they were firing at all. The little block on the points was missing. Weird. The points had a plastic top, not the metal with the brown nub that rides on the shaft of the distributor. I also noticed that the fuel line going to the MFI pump from the bottom of the filter assembly had a kink in it ( just out of the bottom were it elbows to miss the frame on the engine bay)

Going to do a compression test, valve adjustment and set timing, while I wait for new parts.

Probably need to check head stud too. I really don't want to mess with the MFI because it was running good before.

Anything else I need to do?

[Dave Kost]

up date

The distributor rotor wasn't advancing much so I decided to pull it and send it to a local porsche guy with a sun machine for inspection.

Question?

Can I do a compression test without the distributor installed?

[docrodg]

Yes, you can do it without the distributor installed. You just have to remember to set back to #1 TDC when you go to reinstall the disty.

[Dave Kost]

An up date.... finally got to work on my E this weekend.

Compression test was good ~132- 143 psi. Installed the rebuilt distributor and recurved for an E and still had the bad miss. Could not get a reading for setting the timing, the timing light read rpm but the light didn't flash. Tested the coil and intermittently it Ohmed out OK. Cleaned the female coil distributor copper fitting. Have that green oxidation. Fired up and ran good. Tach didn't bounce and the power is back.

So, it was a combination points, distributor and coil.

Happy ending!