80% Leak Down Result and Rebuild Options
 

Hi

My 1978 911SC hasn't really been running very well and I initially thought it was a bad fuel system issue but it turns out I've got a lazy cylinder (or 2).

My leak down results actually showed 2 bad cylinders, one with an 80% result and one with a 40% result.

The car had been sat for about 8 years and only covered about 500km in that time. Strangely I haven't really notice a power issue but I might not be calibrated to the car (I also run a 356 and 930).

I'm going to plod around like this until the new year (car actually won't be on the road any until Nov as its getting some other work).

But what are my options for a rebuild. I'm thinking a stock rebuild but perhaps increase the compression and add a little spice, what are some options?

Thanks in advance.

The most usefull thing with a leakdown test is that it can tell you where it leaks, intake valve, exhaust valve or rings.
I would redo the test if you didn't get that.
I would also give it a good flogging before the test you could just have som sticky rings or carbon on the valve seat if it has been sitting for a long time.

Edit: I assume you have checked valve lash so that valves aren't so tight its holing valves slightly open.

Ditto. Give it an “Italian Tuneup.” Then test it.

Add Sea Foam, commence flogging

Since it's an SC I would check under the valve covers how many head studs that are broken too.

Thanks everyone.

Extra info:

1) No head studs are broken
2) clearances have been adjusted

I'm just checking where the air came out and if the test was done before or after adjustment. I've also asked if they did a compression test as well.

More info incoming

Update

clearances were adjusted and then the leak down test was done.

My mechanic said there was so much air coming out that is was hard to tell where is was going from.

Might be time to do some hard driving and recheck

Might be time to get a new mechanic if he can't detect leakage.

Could be leaking from a few spots and not just one but I get the point.

This is silly. If your head studs are good, there are only three places air can leak, and none of them are near each other. Very easy to tell.

There is not a lot of air that you pressurize the cylinders with, so it should be easy to hear where its leaking.
As you say it could be from several places, but you would hear that too... If it leaks from the valves it may not matter if its from the intake or exhaust, you would be tearing the head of anyway.
Doing this you can add a little oil to the cylinder to help the rings seal better, see if it leaks less.

But I would suggest to go out and drive it like you stole it and re test. The seafoam treatment or equivalent can't hurt!

Get a new mechanic

I must say, that the procedure to listen to three areas is pretty basic Mechanic Skills. A length of hose to the ear. One at intake past the throttle, one at exhaust tip, and one up oil drain plug.