Bad vibration in 300 Six cylinder

[RiverRat4168]

Hello All,

New to site, searched for my problem but don't seem to get around in the site to well. I was hoping someone can point me in the right direction.
1968 F100
problem: vibration at low idle/worse when put in gear. Whole truck vibrates.

Truck has a 300 6 cylinder/C4 trans. Had engine rebuilt by local machine shop, installed engine and truck vibrates badly at idle in gear. truck also vibrates at cursing speeds of 38-45 mph (oscillation) type vibration while driving.

If you raise rpm in park or neutral, engine gets smoother, if you load it at idle (put in gear) it gets worse.

Ran engine without belts on alt, fan and PS pump, no improvement.

I was told that the engine is "internally balanced "so dropped trans, changed flex plate (had weights on it and ring gear damage). Changed torque converter while I was at it. Reinstalled into truck and no improvement. . Thought about doing a compression check to look for a cylinder that isn't carrying its fair share of the load.
Engine and trans mount looked good. Thinking harmonic balance next but hate to keep changing parts with no improvement. Any help is greatly appreciated.

[Ren]

I went through similar trials with a 352 awhile back. Alot of your text reads like what I did, tried, checked, and went through.
I had the harmonic balancer rebuilt by Damper Dr. in Redding, Ca. Things improved considerably.
No real complaints with the end results or service, although they spraypainted it with a crappy, black finish that came up with my thumbnail. Saved me from having to buy a Made in China replacement. Or a stupid-expensive 'racing' balancer.

[HIO Silver]

When I have parts machined I'll put witness marks to make sure I get my own parts back.... and not serepticious, undisclosed replacements.

The only thing changed or modded was the engine, right? Deduction would have lead me in that direction first - not the TC or flex plate which were red herrings. I'd be looking at the balancer... maybe it was pulled from the shelf and is not yours.

.... And that's why I prefer to do my own engine assembly.

[crazyhorse]

Mr RIver Rat , to the best site period.

Hope you like it here.

Pull up a chair and stay awhile.

Have you tried pulling spark plug wire , install, remove another spark plug wire, until you have removed all spark plug wires while motor is idling?
See if that makes any difference .
Check for intake/exhaust leak.
When it is dark, with motor is idling, raise hood and look for any "spark" across the motor.
I wish I could be of more help.
It is aggravating hunting problems you can't find.

Good luck & keep us posted.

[RiverRat4168]

Thanks all, I should mention that I never ran the engine before I had it overhauled, CL special and it was purchased to replace the warn 240. I wanted this engine for its simple and dependable design. I will try all the suggestions and report back.

[jzjames]

vibration; motor mounts? drive shaft? Poor running?
Think outside the box and cover your bases.

[RiverRat4168]

No components touching frame other than engine/transmission mount. All mounts look good, no visible damage.

Pulled one plug wire at a time, change in running condition every time, not plug or wire.

Jacked up on trans tail slightly, no change.

I will do a leak-down test this week end and I have a new damper on the way and will install next week when it comes in. on the way

[tnab1970]

Does transmission seem to work correctly? could it be a BAD vacuum leak? some the C-4 have a vacuum modulator valve that can be bad causing a vacuum leak and bad shifting.

[RiverRat4168]

Trans shifts fine, did a vacuum test on modulator, holds vacuum. Thanks.

[RiverRat4168]

I could cut of vacuum to trans and see if vibration stops.

[tnab1970]

I think trans checks out. what about ignition issues? Points, condenser etc. all good? Sometimes ignition issues come in at load, like putting in gear.

[RiverRat4168]

I changed ignition a while back to electronic, no difference except it starts a little faster now. HOWEVER...I did note that when I put a timing light on the damper it doesn't seem to be steady( seems to be all over the place). Another observation, when I installed distributor old and new, when I get it seated all the way down-it seems to be slightly past the #1 cylinder. Can this run and be a tooth off? I would think it couldn't run or be "dialed in" if it was one tooth off.

[Ranchero50]

You just rotate the housing so it times correctly with a light. If you are concerned about the timing moving, strobe it while you rev it and see if there is a coincidence between the vibration and the timing shake.

Runs rough in gear at idle, smooths out with revs leads me to vacuum leaks. Check everything, especially the carb base gasket.

[RiverRat4168]

Ok, got a different timing light, set time again after re torquing manifolds, got a little on each bolt, vacuum gauge reads 18". Did a compression-lowest was 178, highest was 189.

Last week I used a MAP gas bottle to check for leaks around the carb and intake, never had an increase in revs.

New damper will be in Wed.

[RiverRat4168]

I will blow cigar smoke in pcv hose and see if I have an intake vacuum leak. I will say that engine pulls a good vacuum when I hold my hand over oil filler hole on valve cover.