Inliners International Forums General Discussion Engines 250 engine vibration

I have a '37 Chevrolet pickup with a 250 inch engine, Turbo 350 trans, 3.42 gear. I drove it this way four three years. Last year I replaced the 3.42 with a 3.08 and then over the winter I installed a rebuilt balancer due to mine having pulley wobble. All this summer I have been fighting a vibration at 65 mph. It comes from the Lokar shifter mounted directly on the tail shaft of the transmission. The rest of the truck doesn't shake or vibrate. It could have been there the whole four years, but I never new it with the 3.42 gear and the rpm difference.

I have rebalanced all the tires, double checked the driveline angles, checked converter for missing weights, check flywheel (new), replaced the fan, fuel pump, fuel filters, ignition modual (HEI), plug wires, plugs, two intakes, five different carburators, two different exhaust manifolds, and fuel grades.

I now believe its internal, or the new balancer. I don't recall having the problem last year. This spring, I assumed it was carburator icing up, but when it did it again in July (100 degree day) I new I had a problem. This is a show truck so I only drive it 1000 miles or less a year. I can drive through it, sometimes, say get up 70, then next time I would have to drop down to 60 or even 50 to get it to smooth out. At idle in park, the engine is dead smooth, at 2400 to 2500 it shakes, which is at 63 to 66 mph. From a dead stop, it will run right up to 4000 rpm in gear with out a vibration till you let off and coast down to the speed limit. While coasting down, I notice the vibration in the shifter. If I throw it in neutral, it goes away right away.

I guess what I am asking is, can a worn cam gear cause any problems at higher speeds? Can a balancer cause a vibration if it runs true?
What could cause a internal vibration?

I ask about the cam gears because they are the only used parts in the engine. I broke the new gear, pulling it off a different camshaft, and it was a holiday weekend and the only way to finish up was in stall a used set. This winter I plan to replace them if I pull the engine out for other repairs.

One thing I still need to do is recheck the driveshaft for balance. It was built new by a driveline shop so I don't expect it to be bad, plus it shows no sign of missing weight. The whole truck is basically new with only 4800 miles. Am I stuck at 63 mph??? Joe

I'd think about checking the Ujoints also take a Look at the tranny Mount.(maybe broke????) Also I would check the FlyWheel BOLTS. But it still sounds as if It could be a issue with the convertor as well?? If the Plates/Veins in side it have shifted off Balance. I had this happen to me once In one of my Vans.

I am leaning towards the converter as well, I have built to many engines and can not think of anything that could vibrate with out coming apart! The cam gears are worn and do make noise, but even with that, the most that could be going on is the timing jumping a degree or two, which shouldn't hurt it. Sometimes just putting it in writing you answer your own questions.... Thanks for the ideas, Joe.

Dear Joe;

On my 74 350 someone had installed the wrong flywheel before I bought it etc.

Some had "external weights" because some SBC engines are 'internaly ballanced' & some need that type of flywheel, as they are not.

If your flywheel has weights spot welded to it, knock them off with an air chisle & see if the vibration goes too.

Good luck.

If you are not sure about the new balancer, you might try putting a timing light on the motor and run it up to 2500 rpm where you feel the vibration and see if the balancer has an unusual motion or wobble.

When you mentioned pulley wobble, was just a bent pulley bolted to the balancer or an OEM style balancer with the belt grooves cast in it? If it is the OEM style balancer and it wobbled, you may have worn/damaged the snout of the crankshaft. This would allow your new balancer to move on the crank and cause a vibration.

For less serious items, make sure the bolt holding the balancer to the crank is tight and that the flexplate bolts are properly torqued. Another clue to a loose flexplate is when you shut the motor off, is there a tinny clunk when the motor stops?

Good luck on finding a simple solution.

could be a broken crank,at #1 rod i had that on a 327 one time .

The balncer has the grooves machined in the outer ring. The new one looks just the same. I have tried the timing light all ready and the new balancer is dead on straight. This motor is dead quiet, no knocks or strange noises other than the gear noise when the oil warms up. It runs to good to internal, with all the moving parts in a converter, and the weight of a converter, it wouldn't take much to vibrate a small engine like this.

I have been drag racing for years and a truck mechanic for 15 years so finding problems is what I do, this on the other hand has me stumped! What I need is a set of heavy rollers like a chassis dyno, then I could find it!! Joe

Dear Joe;

Are you 'running' the same flex plate you did before??

Yes, thats the thing, I haven't changed anything but the ring and pinion and balancer since I got the truck on the road in 2001. This just slowly showed up. Joe

J;

Well, It's none of those parts (being wrong)then.

Any chance the flex plate has come loose?? I had that happen once, somehow the wrong lock washers were used.

Also; it may have cracked from stress over time and nothing was done wrong.

i should have said cracked crank. the one i had ideled good ran ok till about 2500 then it started to shake stopped at about 3500. carried good oil pressure no knocks. i hope that is not your problem.

I think if a crank snout could shake an engine it would also be hammering out the front seal. Just my $.02 Doug

What kind of rear end? you might want to check your backlash settings, they may have changed.

One of my 350s started vibrating and making some real nasty sounds only when I geared up to third gear and held it there at a certain RPM..I was convinced the ring gear was loose or something...Anyhow it turned out to be the clutch fan bearings...the thing would vibrate the whole truck enough to make me not hold that RPM very long.....Al

Right now I am leaning towards a new converter. I have a new one being shipped out in a few days. I am not sure this is the problem, but by changing it out I will eliminate at least one part!

I had my dad ride along Sunday afternoon for a while and the both of us are sure we don't know anymore now then before the ride!

Heres what I do know for sure, at full throttle the vibration starts around 2500 and never quits. When pulling a hill, the harder I step down the worse it gets, once I let off it takes it a bit to settle down. Down hill or on level, it runs fine at any speed.

So what does this tell me? It takes a pretty good load on the engine to set off the vibration.
The converter, u-joints, ring & pinion, and combustion chambers (more heat, higher pressures). A rpm related vibration wouldn't care if it was loaded or not, tires, driveshaft, balancer, flywheel, etc.... Something is changing when worked hard. Joe

J;

Your probably right, but check that flex plate really good anyway.

Are the U-joints aligned properly with each other?

I checked rear axle and related parts, everything checked OK as far as I could tell. There is no noise in the axle. The driveshaft is a one piece and has new u-joints. I checked it for out of round using a indicator. I found it was out by .030" at the front. It is at the shop now getting checked out. They like them to be with in .010" or less. I also plan to change out the front and rear bushings in the transmission when I pull it for the converter change. If the driveshaft cures or even helps a little, I still plan the converter change. I never did like this one, it seems way to tight, it helps fuel milage, but really jumps into gear even at 650 rpm. I will keep you posted, and thaks for the ideas, Joe

Well heres the update, The driveshaft was out of round and off balance. The shop redid both ends and balanced it. It didn't help my problem though. This weeked I replaced the torque converter with a new balanced stock unit. It to didn't help the problem. We spun the distributor,HEI, on a machine, checked wires and cap and found no problem. I install a new points unit I had, no change, new plugs, no change..

I have two shows to go to in October which are close to home, after that I plan to pull the engine out if I can't come up with a solution by then. I don't really know what to look for once its out, in runs so good with no load on it. Last tank of gas it got 19 mph with three carburetors!

The truck should really be smooth when I get this solved! Joe Hand

I have finnally solved the vibration problem in my 250. I worked all fall and into the winter tring to solve this. I pulled the engine at Christmas time and looked it over. I installed Rhoads Lifters and a different cam with new gears. I had the intake/exhaust machined flat, replaced the broken shifter mount, replaced the tail shaft bushing, added a torque strap from the head to the frame, rejetted the carburaters, replaced one rear axle, and none of it worked. It still had the vibration at 65 mph!

I was out of ideas, the only thing left was the head. In the first post I didn't say any thing about changing heads, figured no way it could cause a problem!
I had changed it two years ago when I thought it was leaking water, turned out the block was cracked at #1 head bolt. Anyway, the head was modified with expoy to simulate lump ports. The exhaust valves were inlarged and ports cleaned up. My dad has a flow bench so we new what kind of improvements we made. The intake flowed the same up to .350" then gained from there up to .500". The exhaust out flowed the stock port from all the way to .500" .

Over the last few days I changed the head back to the stock '68 250 head. The truck runs great, not quite as responsive at low rpm, but the vibration is gone! It will run any speed now, I was out today running 70 mph with no problem. We have no idea what is wrong with the other head, we plan to reflow. We know the intake to exhaust ratio is off slightly, but something else has to be wrong. At least using epoxy, I can chip it out and still have a good head to work with. I do miss the small intake ports, it idled better and was really quick right off idle.

Anyway thanks for all your suggestions and ideas. Joe

Very interesting thread. Joe, is it possible that the combustion chambers don't have matched volumes? Seems like it would have to be off by a fair amount. Just a wild guess.

We did nothing to the combustion chamber. We added a lot of epoxy to the intake ports, probably closed off 1/3 or more of them. We were counting on the high air volocity through small ports, which did work, but something else was wrong. Maybe to small! I will let you know what we find out when I get back over to the flow bench. Joe

i have an '81 corvette with the exact same problem. i have changed all 6 u-joints twice, balanced tires, etc etc etc...to no avail. the engine is stock as well as the trans.

at 55-65 the vibration comes on, especially upon accelleration, throw it in neutral...it coasts smooth as silk. bring the engine up to same RPM's while in neutral...no vibration.

i'm assuming it has to be in the transmission....but before i bring it to a shop, i was hoping to at least know what to look for.

if it is the transmission, what could have gone wrong?

I tried so many things on mine I lost track of them all. Check the tail shaft bushing on the transmission, if its loose the drive shaft will bind when under a heavy pull. Mine was a vibration but not due to a balance problem, it was more of running/tune up type of problem, it just seemed like a balance vibration.

A torque converter can get out of balance if it throws a weight off, but you should feel it at the same rpm in gear or out. A clutch should do the same. I found if I drove in low gear, I could get the vibration at the same rpm, only not as bad, It was really noticable pulling a long hill where the engine was working hard.

Check the drive shaft really good for missing weights or damage. Grab the front u-joint and really pull on the tail shaft bushing, also make sure the yoke is in the bushing all the way, it should be if its a factory correct driveshaft and the ride height has not been altered. Joe