Low compression cylinder question
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Joined: Apr 2013
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Low compression cylinder question
Hello all, purchased my 66 a few months ago and diving in to several issues I've run into.
The truck has a '73 351W that's running rough at idle like one spark plug is weak/unplugged.
I have already installed new plugs, wires, coil, cap and rotor.
Can't set/check the timing cause there's no timing indicator tab. Still trying to figure out what it should have. It doesn't ping and has decent power, so I think timing is pretty close.
I checked compression and got between 127 and 140 on all cylinders except number one, which was 62.
I added about a tablespoon of oil to the cylinder, tested again and it came up to 90.
I've read several post about stuck compression rings as a possibility and a burnt valve, worn cam lobe, bent valve, or bent push rods being a cause.
The engine doesn't smoke at all, leading me to think it's not the rings.
Anyone had any similar experiences and suggestions for further testing?
My initial thought was pulling the heads for valve job, then replacing the cam and lifters, but hate to do that and have bad rings haunt me later on.
The truck has a '73 351W that's running rough at idle like one spark plug is weak/unplugged.
I have already installed new plugs, wires, coil, cap and rotor.
Can't set/check the timing cause there's no timing indicator tab. Still trying to figure out what it should have. It doesn't ping and has decent power, so I think timing is pretty close.
I checked compression and got between 127 and 140 on all cylinders except number one, which was 62.
I added about a tablespoon of oil to the cylinder, tested again and it came up to 90.
I've read several post about stuck compression rings as a possibility and a burnt valve, worn cam lobe, bent valve, or bent push rods being a cause.
The engine doesn't smoke at all, leading me to think it's not the rings.
Anyone had any similar experiences and suggestions for further testing?
My initial thought was pulling the heads for valve job, then replacing the cam and lifters, but hate to do that and have bad rings haunt me later on.
Lead Gopher
Joined: Aug 2012
Posts: 10,041
Likes: 1,593
From: Seattle
Probably the next test without any major disassembly to do would be a leak-down test. With a leak-down test you put compressed air into each cylinder one at a time and rotate through the four cycles and it can help isolate what is wrong. If air comes up through the carb or out the exhaust pipe when at TDC then valves are suspect. If air comes out the oil fill tube or vent then valve seals are likely suspect. Anyway, that's what I would try next if it were me.
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mdlave
1973 - 1979 F-100 & Larger F-Series Trucks
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May 28, 2013 03:50 PM
704 dentside man
335 Series- 5.8/351M, 6.6/400, 351 Cleveland
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gwp36530
1961 - 1966 F-100 & Larger F-Series Trucks
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Apr 23, 2004 01:45 PM
