Engine vibration sanity check

AnthonyG

AnthonyG

Well-known member
Did a fresh engine, FW, PP, clutch over the winter. Engine is an 8BA with Mallory electronic. worked a bit with a 500cfm Edelbrock 4 Barrel. Engine started right up and idled smooth but had a pretty car rattling vibration between 1000 and 1500 rpm then smooths out at higher rpm. Driving for the last couple of months to see if it would smooth out. Accelerates great, pulls hills great, smooths out at 50 to 60 MPH. But the range it vibrates at is all through gears until the higher rpm is attained ( the s/b fun range ). Been driving for 500 miles thinking if it was plugs from all the starting in garage and not really cruising would burn off and smooth out. Nope! I have sent the dist. out to check it for the internal timing curve and it's fine, am in the process of getting the smallest jets for the primary's on the carb thinking it's a rich fuel condition causing. In the mean time would like a mechanical sanity check.

If it runs smooth at idle to 1000 RPM and then again smooth above 1600 RPM, could there be any other mechanical balance causing it's issues besides timing or carb? Beings it's smooth at idle and then again higher RPM I didn't think so but thought I check with you guys?
 
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Take the fan belt off and see if it is the fan out of balance. Push the clutch in to see if the disk is the problem. Past that, it looks like a balace job on the clutch assy.
 
Clutch assy balanced together and indexed with punch marks on PP and FW.
Does it not seem odd that the vibration is only in the mid range rev point. 1000 to 1500 then it goes away and is smooth?
 
Was the flywheel/pp assembly balanced together with all the other engine componets?

If the flywheel is balanced seperate from the crankshaft it has to be mounted to the crankshaft with the same center as it was balanced with

On my A I have a vibration going through 800 or so, moves the car in a circle ----the crankshaft grinder didn't center the crank to it's true center before it was ground , it has close to .004 runout on the flywheel flange

First run it with the belts off

Another thing I learned is that too much oil in the crankcase can make it seem there is a balance problem ---it had 4 xtra quarts, the crankshaft sloshing through the oil made it seem like the just replaced transmission had a balance problem
 
Hi Kurt, PP / FW not balanced together with the engine. I had the problem when I first got the engine from the builder thinking it was a FW and PP not match balanced. I complained to him and he sent me a match balanced FW / PP which is in the car now.

If I had a clutch assy balance issue wouldn't it be prevalent at idle or just get worse as the rpm increases? This is smooth at idle and above 1500 rmp only vibrating at approx 1000 to 1500rpm???
 
Is it even possible to balance the FW / PP while on the engine. If so wouldn't you have to have the engine out of the car at a shop specialized to do it. Is that something the Flatty engine builders do or should do. Haven't seen any threads about it?
 
Are you running an alternator in a horseshoe mount? I had the exact same issue, and it turned out the horseshoe wasn't stiff enough. The alternator would pick up fore-aft vibes and resonate with them. It was at the same RPM's you are describing. I reinforced the horseshoe to make it stiffer and no more problems.
 
Ross, I'll take the belt off and run it for a short burst to check. Hope that's it. It was starting to drive me crazy ( still may be ). Out of curiosity how did you reinforce it? Any Pic's?
 
It actually didn't make it better to pull my belt, it made it worse! The alternator wags fore-aft and the belt actually dampened it a little.

Pics of the reinforcement (before cleanup) below
 

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First off, you don't say that the crank assembly was balanced initially.
Assuming it was:
There are 3 possibilities.
1- The crank assembly was NOT balanced initially. Even if someone said it was.
2- The person that did balance the crank assembly screwed up when he did the math for the bob weight.
3- Like others have said, you could have a serious balance problem with the clutch/flywheel assembly. You said the assembly was balanced and punch marked so that may not be the problem. However, it is highly possible that the crank flange has enough run out either radial or lateral to cause the problem. The crank could be bent also.
The fact that the vibration stays in a certain rpm range no matter what you try, indicates
an internal balance problem.

A tech note, for what it is worth, I have balanced many of these engines and the vibration is occurring at the second harmonic of the assembly. It won't go away till you tear it down and find the problem.
 
Thx Ross F1 and Pete. While I can't say with absolute certainty, my experience with balance issues on engines / FW / crank, etc, where we're dealing with items which are not dynamic on the spin centerline ( everything is torqued and can't flop around as it spins) is that a radial out of balance situation causing a vibration gets worse with increased RPM and doesn't go away. I think what Ross said makes sense as varying rpm can cause a vibration to come and go on a flexing bracket holding a devise ( alternator, power steering etc.), spinning under the power of a v-pulley and belt. I think what Ross F1 describes in #9 & 11# makes sense because if the horse shoe bracket is moving / flexing with the pressure and spining from the belt it is a dynamic situation which could cause a radial vibration to come and go at different rpms due to the movement and pressure. I will remove the bracket and alternator and run a short test to verify and report.
 
If the "vibration" exhibited the identical behavior before and after changing the flywheel and PP I would be very surprised if the problem is within those components.

Finding the solution should be simple---but, sometimes it isn't. Does the same situation occur with trans in a gear and clutch depressed to floor?
 
JWL, sounds like your thinking the only the clutch plate might be out of balance. That occurred to me early on but it happens with it in or out of gear clutch in or out.

On vacation now and will try pulling the alternator and running it to see if Ross F1's suggestion might be the prob. when I get home. He said he had the same issue and it turned out to be the weakness / flexibility in the horseshoe alternator mount. His looked to be steel, mine is aluminum so it's even (tensile strength wise ) more flexible. Also think how extended that bracket is when adjusted to get the proper tension on the belt can be an issue. Mine ended up almost at the tallest part of the adjustment making the moment arm a little longer which makes the scenario even more plausible I think.
 
Tony, your comment on being up high prodded my memory. I went to a belt 1" shorter to help keep it low. Note my truck actually has the Car twin-narrow belt system.
 
I had a similar problem in a Chrysler Cordoba years ago. I finally tried driving it into the vibration speed and put it in neutral at speed and let the RPM's drop to idle. The vibration went away. I changed the torque converter and the flex plate and it cured it. I also had the same type of problem on a Flat-O torque converter on my 46 Ford. Changed the flex plate and the torque converter and it cured it, this after having it balanced by a reputable shop. Not all shops are good at balancing.
 
I had the same problem as Ross, I had an aluminum y yoke mount for alt. changed to steel and it cured it.
 
Reporting back: Ross F1 hit it on the head. Removed Alternator and Fan ( same assy ) and no vibration. Next question, while I see Ross's fix by welding a fabricated flange on the bracket does anyone know of a commercially available bracket with additional support. Or an additional support feature on the rear of the alternator? Just looking for something which will both perform and look good and the motor is pretty slicked out and want to keep the look.
 

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I can see why yours is so bad, that is way up high, and with a fan to boot. Do you still have your stock generator mount? They have a stiffening rib that curves back from the mounting face. Many alternators will fit in the bracket, if you drill a hole for the locating dowel. I was tempted to try that. It would be tough to arrange a brace on the rear of the alt to the intake manifold, but that is likely the best solution.

Electric fan?
 
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