Missfire
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Joined: Jan 2008
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From: Chesapeake Virginia
Missfire
My 350/300 has developed a bad "miss" that I think I've isolated to a specific cylinder. I've check for spark (and replaced plug and wire) and have spark at that cylinder. There don't seem to be any vacuum or other obvious issues.
I think the next step is to check compression. That may tell me if I have a valve or cam problem. Can anyone tell me what the compression should be?
I'm also open to any other diagnostic suggestions you may have, so bring it on!
I think the next step is to check compression. That may tell me if I have a valve or cam problem. Can anyone tell me what the compression should be?
I'm also open to any other diagnostic suggestions you may have, so bring it on!
Le Mans Master
Joined: Jan 2006
Posts: 6,499
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From: Butler Pa
Hard to say what the compression should be. Too many variables. Check all of them and compare to each other. They should all be within 10%. If not, you may have found the problem.
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From: Chesapeake Virginia
Thanks for your reply. I checked and compression was fine, but I did find a few severely fouled plugs that indicated a leak in the intake valve seals. I'm planing on replacing them soon using a method a mechanic friend of mine suggested - Using the hose and male screw fitting from the compression gauge, pressurize the cylinder using an air compressor to keep the valves seated and from falling into the cylinder. Then remove the rocker & spring assembly and replace the seal. He says it's easy - any thoughts?
Safety Car
Joined: Aug 2000
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From: San Antonio TX
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From: Chesapeake Virginia
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Le Mans Master
Joined: Oct 2007
Posts: 7,410
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From: Tennessee/Rhode Island
I had plugs fouling and blowing some blue smoke on my L-82 about 15 years ago and replaced the OEM valve stems seals with umbrella seals only and solved the problem. If had to do it again, I would definitely change both the valve stems seals with OEM seals AND the umbrella seals for insurance. Bad valve stems seals will definitely foul your plugs as the only problem.
Pro
Joined: Jul 2000
Posts: 590
Likes: 1
From: Suffolk New York
Thanks for your reply. I checked and compression was fine, but I did find a few severely fouled plugs that indicated a leak in the intake valve seals. I'm planing on replacing them soon using a method a mechanic friend of mine suggested - Using the hose and male screw fitting from the compression gauge, pressurize the cylinder using an air compressor to keep the valves seated and from falling into the cylinder. Then remove the rocker & spring assembly and replace the seal. He says it's easy - any thoughts?
I also fed a rope into the cylinder through the spark plug hole, so that when I compressed the springs the valves hit the rope and let me pop the keepers out.
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
