351M Compression difference?
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Joined: Feb 2001
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351M Compression difference?
What could cause the compression to be much higher than normal on just one cylinder?
144 4 8 120
125 3 7 123
129 2 6 128
115 1 5 128
Front
If the compression is higher in one cylinder, could it cause the
engine to vibrate as though it were missing?
The engine only has 25K miles on it from the last rebuild, and I am pulling what hair I have left, out trying to fix this before it shakes itself apart. It's not critical yet, but I don't want to wait until it is. I can hear that there is something different
but I can't swear that it is comming from cylinder 4, but I think it is.
Thanks for any leads on this!
Bruce
144 4 8 120
125 3 7 123
129 2 6 128
115 1 5 128
Front
If the compression is higher in one cylinder, could it cause the
engine to vibrate as though it were missing?
The engine only has 25K miles on it from the last rebuild, and I am pulling what hair I have left, out trying to fix this before it shakes itself apart. It's not critical yet, but I don't want to wait until it is. I can hear that there is something different
but I can't swear that it is comming from cylinder 4, but I think it is.
Thanks for any leads on this!
Bruce
New User
Joined: Apr 2001
Posts: 10
Likes: 0
351M Compression difference?
IF YOU HAVE A TEST LIGHT, GROUND THE CLIP ON YOUR MOTOR SOMEWHERE AND STICK THE POINT IN TO EACH PLUG WIRE AT THE CAP.RUN THROUGH THE FIRING ORDER ONE HOLE AT A TIME UNTIL YOU FIND THE ONE THAT AFFECTS THE IDLE QUALITY THE LEAST.THATS YOUR DEAD HOLE.IT MIGHT BE THAT THE VALVES WERE NEVER CUT RIGHT TO START WITH OR YOU MIGHT HAVE A WEAK SPRING.I KNOW FROM EXPERIENCE, WITH BOTH MY 4V CLEVELAND, AND THE TRUCK MOTOR, THAT IT'S USUALLY #3 & #6 THAT END UP PLAYING PING PONG WITH A VALVE BACK AND FORTH THROUGH THE INTAKE.MAYBE BY FINDING THE DEAD HOLE YOU'LL KNOW WICH COMPRESSION FIGURE TO TRUST.
Lead Driver
Joined: Nov 2000
Posts: 6,635
Likes: 349
From: Albuquerque
351M Compression difference?
When you did the test did you remove all the spark plugs and block open the carb before starting. It can make a difference.
1977 Ford F-100
400m/c6
280,000 miles
Stock on the outside
modified/rebuilt everything
1977 Ford F-100
400m/c6
280,000 miles
Stock on the outside
modified/rebuilt everything
New User
Joined: Feb 2001
Posts: 20
Likes: 0
351M Compression difference?
Thanks mhmcgyver,
I will definetely try this. I take it that the idea is to bleed off some of the spark to reduce the power derived in the one cylinder that is causeing the problem by developing more power that the rest due to the higher compression. Right?
Bruce
I will definetely try this. I take it that the idea is to bleed off some of the spark to reduce the power derived in the one cylinder that is causeing the problem by developing more power that the rest due to the higher compression. Right?
Bruce
New User
Joined: Feb 2001
Posts: 20
Likes: 0
351M Compression difference?
Thanks for the tip beartracks, no, all I did was remove one spark at a time and screw in the preassure gauge. (cap to distributor lead pulled off) Ran the starter for about 10 sec.s
and moved to the next plug.
Bruce
and moved to the next plug.
Bruce
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