Bad engine vibration
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Junior User
Joined: Feb 2005
Posts: 66
Likes: 2
From: Surrey, BC
Bad engine vibration
My 66 F100 has a 1977 300 I-6 (C6 tranny connected to it). When I bought the truck it wasn't running (or running very well), but I thought it was due to the cold weather and a carb that was missing linkage etc. I put on an Offy intake and Holley 390 carb right away and found that the engine would shake like crazy at low rpm and under load (in neutral or park, it wasn't too bad). I new I had some intake leaks, but after fixing those the vibration is still there (although slightly better). I plan to do a compression check when I get back from a work trip, but in the mean time I'm trying to think of any other tests I can do to narrow this vibration down - and figure out what it could be. Would a bad exhaust or intake valve cause those symptoms?
Senior User
Joined: Sep 2003
Posts: 497
Likes: 0
From: Houston TX
Need to check the timing and the whole
ignition system to make sure it's not a
miss. A slipped timing chain can cause
a problem like that...Also, if the carb is not
set up right...Does it seem rich or lean?
Rich will send black sooty exhaust out...
A bad valve will just cause a miss in that
one cyl...Would feel about the same as a
ignition miss...Leaking valves will usually
cause a flutter using a vacuum gauge.
Make sure no hidden vacuum leaks like
blown tranny modulators, etc...That will also
often send tranny fluid back to the engine,
will will smoke like crazy...
I'd first verify the timing, ignition, plugs, wires,
etc...A scope would help...After that is known
good, I'd be looking at that carb and for any
other leaks... MK
ignition system to make sure it's not a
miss. A slipped timing chain can cause
a problem like that...Also, if the carb is not
set up right...Does it seem rich or lean?
Rich will send black sooty exhaust out...
A bad valve will just cause a miss in that
one cyl...Would feel about the same as a
ignition miss...Leaking valves will usually
cause a flutter using a vacuum gauge.
Make sure no hidden vacuum leaks like
blown tranny modulators, etc...That will also
often send tranny fluid back to the engine,
will will smoke like crazy...
I'd first verify the timing, ignition, plugs, wires,
etc...A scope would help...After that is known
good, I'd be looking at that carb and for any
other leaks... MK
Thread Starter
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Junior User
Joined: Feb 2005
Posts: 66
Likes: 2
From: Surrey, BC
Thanks for the suggestions. I had checked the ignition side with new plugs, wires, points, cap and rotor and checked the timing with a light. The carb was a brand new carb and I had adjusted it the best I could but I have not done a vacuum check yet. I'll look at that next.
Fleet Mechanic
Joined: Jul 2004
Posts: 1,988
Likes: 14
From: Travelers Rest SC
I once bought an old truck w/ a 302 in it. It shook so bad you couldn't see where you were going! It turned out that someone had put a 6 cylinder, non-externally balanced flywheel on the 302, which uses an externally balanced flywheel. I got a proper 302 flywheel, and it smoothed right out (I don't know what it did for the main bearings, though!) It's possible that someone put a 302 flywheel on your 300 (They bolt right on). I know it's a long shot, but you never know what's been done to an older truck. After you've tried everything else, look at the flywheel, just for fun.
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Junior User
Joined: Feb 2005
Posts: 66
Likes: 2
From: Surrey, BC
Just to give a little update. Did a compression check and all six cylinders are within 10% of each other and registering around 105 psi so I don't think there is leaky valves or rings. The timing was checked at the start. I don't have a vacuum gauge, but there aren't that many vacuum lines and none seem to be leaking. The line to the tranny modulator doesn't send any fluid back to the engine. The engine doesn't smoke at all when started, even after I got back from a month long work trip, it started right away without any smoke.
Now, I did have a puddle of trans fluid, but I think that's due to a leaky torque converter rear seal.
What else can I check on the engine? Could there be a transmission problem that gives this bad vibration / shaking of the drivetrain?
Now, I did have a puddle of trans fluid, but I think that's due to a leaky torque converter rear seal.
What else can I check on the engine? Could there be a transmission problem that gives this bad vibration / shaking of the drivetrain?
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