Compression Test Question

drolston

drolston

Well-known member
About 500 miles on a 284" rebuild and I started noticing a slight 1" dither on the vacuum gauge at idle, hot or cold. Figured it could be a valve sticking or not seating properly. So I ran a compression test, cold, throttle open. Offenhauser 425 heads, Schneider 248f cam. Six cylinders were in the 165 to 175 psi range, one was at 150 and one at 155. Plugs all look the same, dusty gray.

The disparity is hot horrible, but enough that I am a little worried. I have added 4 oz of MMO to a full fuel tank and will drizzle some through the carb tomorrow, to see if the vacuum gauge at idle settles down.

No other symptoms; the engine runs great. How big of a problem do I have? Any other suggestions?
 
Might be a ring sticking a bit.
I would not put Marvel Mystery Oil directly through the carb myself as it may foul your spark plugs. I would put a couple squirts of SeaFoam or two stroke oil in through each spark plug hole. Crank the engine over a dozen times to distribute the oil and then let it sit overnight. Typically Neither the SeaFoam or Two stroke oil will foul the plugs as both are formulated to burn off without leaving carbon deposits.
Check the compression the following day to see where you're at.
If the compression came up, the leak was in the rings. If it did not come up, the leak is in the valves.
 
Ran some MMO through the center carb and put some miles on with MMO in the gas. The vacuum gauge has settled down at idle and # 5 & 6 compression is up in the 170's. So, MMO does it again!

Compression in the 170's is evidently a bit much for my stock starter (converted to 12V). Often it will start to crank but stall before making a full revolution. With repeated attempts it will crank, and once it gets going is cranks briskly. I am going to add a ground cable from the battery to the starter case to see if that helps.

Any other ideas besides switching to the $245 Powermaster?
 
A 32" cable turned out to be the perfect length to run from the ground terminal of my battery to the outboard mounting through-bolt on the starter. That totally solved my slow-no crank problem. Can't get the finger off of the starter button fast enough.

I have a new heavy gauge cable from the battery ground terminal to the firewall, and a good braded strap from the firewall to the right rear intake manifold bolt. I guess the newly painted oil pan and gaskets were putting a lot of electrical resistance in the ground path from the intake manifold bolt to the starter case.
 
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