Still have a small miss. Have been through most of the basic stuff and tried another compression check. All was 115 to 120 ‘till number 7 – got 60.

Checked compression four times total on number 7. It would pump to 60 real quick when it would get that high (no further), then 15, then 15, then 60…never got over 60.

I’m gonna get new heads anyway, but am just curious about the 2 different measurements.

 

sounds like either that or you have some carbon build up in the seat area...

 

could be, but these heads were rebuilt about 1.5 years ago...

 

If your only gonna do the heads, consider a leak-down test to rule out a ring problem.

 

Duly noted …thanks

but why the different readings?

 

It can take a few revs. to get a max reading...

 

thats why i think it could be some carbon beaking up on the valve seat area.. have you tried runnin some seafoam thru it???

 

Unless ur running a big overlap cam that compression is low. But u need to do the squirt (or 2) of oil in the cyl trick and check again. This is what isolates the rings or vlvs. Higher compression after oil added to cyl means bad rings. If not then bad vlvs.
Yes a leak-down will tell more at least with the better cyls. I don't recall but u what something like 85% or better and all cyls within 10%. If leakdown on better cyls is low then maybe heads are not the fix - rering or rebore. If L/D ok then IMHO tighter chambers (on new heads) needed to raise compression.

Ya know i had a small miss at idle before the cam swap. Pulled the stock cam and found #4exh lobe wiped nearly round. But not ur case as lower lift of wiped lobe increases comp.

Sorry to hear the bad news and hope new heads can be the fix. cardo0

 

How do you do a leak down?

 

Stole this from somewhere-

Question: What is a "leak down" test?

Answer: A leak down or "cylinder leakage" test is similar to a compression test in that it tells you how well your engine's cylinders are sealing. But instead of measuring pressure, it measures pressure loss.

A leak down test requires the removal of all the spark plugs. The crankshaft is then turned so that each piston is at top dead center (both valves closed) when each cylinder is tested. Most people start with cylinder number one and follow the engine's firing order.

A threaded coupling attached to a leakage gauge is screwed into a spark plug hole. Compressed air (80 to 90 psi) is then fed into the cylinder.

An engine in great condition should generally show only 5 to 10% leakage. An engine that's still in pretty good condition may show up to 20% leakage. But more than 30% leakage indicates trouble.

The neat thing about a leakage test (as opposed to a compression test) is that it's faster and easier to figure out where the pressure is going. If you hear air coming out of the tailpipe, it indicates a leaky exhaust valve. Air coming out of the throttle body or carburetor would point to a leaky intake valve. Air coming out of the breather vent or PCV valve fitting would tell you the rings and/or cylinders are worn.

A leakage test can also be used in conjunction with a compression test to diagnose other kinds of problems.

A cylinder that has poor compression, but minimal leakage, usually has a valvetrain problem such as a worn cam lobe, broken valve spring, collapsed lifter, bent push rod, etc.

If all the cylinders have low compression, but show minimal leakage, the most likely cause is incorrect valve timing. The timing belt or chain may be off a notch or two.

If compression is good and leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery (bad injector) or ignition problem (fouled spark plug or bad plug wire).


http://store.summitracing.com/defaul...=egnsearch.asp

 

Thanks!

 

Could be as simple as over tightened rocker arms.

 

... especially if the "twisty-turny" method of rocker adjustment was used.

 

wtf .."twisty-turny"...

....not unless they adjusted themselves... I haven't adjusted them for a very long time...

 

Tom covered pretty much how its done and what the results say. Instructions say it should be done at operating temp but good luck removing plugs and threading in the air fitting near hot header pipes. A cold L/D is much more conservative anyways as the cyls would seal better when hot. Aircraft engines live or die and thier mechanics swear by the leakdown test. Seems kind'a neglected by shade tree mechs but with our aging C3s its time is now. IMHO its best use is to determine when to rebore and ring a mtr. Too often i read of owners overhauling a mtr at 100k when the sb Chevy bottom end is good for nearly 350k mi with reasonable care (and moderate compression).
Maybe u can borrow or rent one but sorry i don't know how to convert a compression gauge for L/D. Somehow u can use the compressor's regulator and an additional vlv. But really once you use it u will want to have ur own ($70 Summit) as it's a great tool to monitor the condition of ur mtr over time and abuse.
Good luck. cardo0