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MG MGB Technical - Vibration

HI All

Sorry for all the questions lately. Just getting the little bugs out of my 67 GT after last falls change to an OD transmission.

Car has been driving fantastic. OD works great.

I took it for a couple hundred mile trip this week and today I noted a vibration in the steering wheel when in 4th gear at low speeds (25 -30).

Know that these can be a real bugger to identify the cause. The car had a full rebuld of the front suspension (V8 black bushings), new king pins, wheel bearings, new brake rotors, calipers, hubs and new dayton 72 spoke wires,New motor mounts, transmission mounts and new rubber for the rear end.

OD transmission was fully rebuilt, new clutch, pressure plate, throwout and pilot bearing. Flywheel was refaced and I am reasonably sure I got it back in the same position it came off

Vibration is not like a shudder that something is loose, but a fine steady hum type vibration. It appears to vibrate the dash but I do not feel it in the shifter.

I dont notice it at higher speeds Suggestions on what I might look to check?
BEC Cunha

Bruce,

Don't be like me. I did all kinds of things to rid my '58 Z Magnette of a similar vibration, without thinking of basic stuff first. In my case, I should have checked the tires. Broken steel belts had broken, producing a bulge in the left rear. In your case, that would more likely be a front tire, as you say your rears are new... Unless one is defective.

You didn't mention wheel balancing. Even if it's been done, you might have the guy check it again. I suppose it's possible that one of your new Daytons is out of round too, although they're usually very good. Also, were your wheels balanced on the car? Then, how are your "U" joints? Usually I don't notice "U" joint vibration until around 45 mph at first, but the "humming" sort of vibration you mention fits the diagnosis.

This is a stretch but how about your radiator fan?

I'm out of ideas.

FWIW,
Allen
Allen Bachelder

Another thought is to check the phasing of the uni joints since it sounds as though the drive shaft has been parted from the transmission at some time.

Even if they are as they are meant to be it may be worth rotating the driveshaft 180 degrees if all else fails.

I had something similar to this on a car I owned many years ago and it tooka a long long time to track down but about five minutes and no dollars to fix !

Cheers , Pete.
Peter Thomas

If the hum changes frequency when accelerating of de-accelerating it may just be engine vibration. Either someting from the drive train is touching the body or your new motor and trans mounts are stiffer than the old oil soaked ones you replaced and are transmitting some vibration to the body. Put your hand somewhere other than the wheel and see if you can feel it in the body. At 25 in fourth gear you are getting close to lugging the motor which will cause some vibration.
John H

Bruce,

Don't drive 25 or 30 mph in fourth gear. Use third and overdrive. I wouldn't be in fourth at less than 40 mph.

take care,
dave
Dave Braun

Thanks all

Drive shaft is a potential. It was rebuilt also and While I marked it for the rear diff. Since the OD transmission is new, and I did take the yoke apart, I can't assure I got it back correctly.

Tires are also new and the tire/rims were balanced on a spin balancer using correct cones for mgb wires (I had a set machined by a friend). No actual noise to the vibration. Just a higher frequency fine vibration felt in the wheel. It causes a slight rattle from the dash.

Hard to check as it goes away below 25 and above 40.

Dave. Good point. Still have not gotten into the habit of using 3/OD. I normally do not drive in 4th lower than 40, but feel the vibration while slowing down. It also appears to be there in 3rd.

Wondering if it may be a water pump? I noted on my trip home that the fan belt or alternator was periodically slipping and squealing. I readjusted the belt but it was not all that loose.
BEC Cunha

Mine happily drives in 4th at 30mph (1700 rpm), but not up a mountain obviously.

Try jacking both back wheels securely off the floor and get someone to drive it at the speed that give the vibration. If it has gone away, it is the front end.

If not, check the wheels for buckles as they go round, you will see what is vibrating, be very careful about getting underneath to check the propshaft: leave until last and take the necessary precautions.
Martin Layton

Martin

Excellent sugggestion. As the 67 sits low to the ground as it is, getting the wheels up off the ground and running it should be easy.

That should at least let me know if it is mechanical or coming up from the suspension from the road.
BEC Cunha

This thread was discussed between 18/05/2007 and 19/05/2007

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