|
Sponsored Links (Register now to hide all advertisements) |
|
|
Thread Tools | Display Modes |
09-23-2017, 08:38 PM | #1 |
Senior Member
Join Date: Jun 2014
Location: Williamsburg, VA
Posts: 1,627
|
Compression Test Question
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? |
09-23-2017, 08:52 PM | #2 |
Senior Member
Join Date: Dec 2014
Location: Santa Maria, CA
Posts: 985
|
Re: Compression Test Question
"Figured it could be a valve sticking or not seating properly."
My thought as well.
__________________
Alan 1929 Special Coupe 1941 Pick-Up 1955 Victoria |
Sponsored Links (Register now to hide all advertisements) |
|
09-24-2017, 08:00 AM | #3 |
Member
Join Date: Nov 2016
Location: Kenyon MN/Gold Canyon AZ
Posts: 64
|
Re: Compression Test Question
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.
__________________
Regards - Randy 1933 Tudor |
09-30-2017, 05:00 PM | #4 |
Senior Member
Join Date: Jun 2014
Location: Williamsburg, VA
Posts: 1,627
|
Re: Compression Test Question
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? |
09-30-2017, 07:50 PM | #5 |
Senior Member
Join Date: Sep 2015
Location: Clarkdale, AZ
Posts: 121
|
Re: Compression Test Question
You'll never regret the PowerMaster.
__________________
53 F100, 8CM flathead ------------------------------- |
10-08-2017, 07:41 PM | #6 |
Senior Member
Join Date: Jun 2014
Location: Williamsburg, VA
Posts: 1,627
|
Re: Compression Test Question
Sponsored Links (Register now to hide all advertisements)
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. |
|
|
Sponsored Links (Register now to hide all advertisements) |
|