MGB Shakes at about 3000 RPM 1980 MGB

[MGBIOWA]

I was wondering what may cause my B to shake at about 300o RPM. If I give it gas or let off or push the clutch in it stops. I was wondering if it maybe a throw out bearing? It has been doing it for over a year and would like to fix it. Thanks for any input!!!

 

[SilentUnicorn]

Welcome to the Forum.
your problem sounds rather unique. Throw out bearings usually just rattle when they are bad. Do you have any more info on this shake? does the steering wheel shake? how many miles on the B? Tires in good shape? does it shake when stopped? or only when rolling? have you checked the universals? any other clues that may help us out?

Mark /ubbthreads/images/graemlins/driving.gif

 

[Bruce Bowker]

[ QUOTE ]
I was wondering what may cause my B to shake at about 300o RPM. If I give it gas or let off or push the clutch in it stops. I was wondering if it maybe a throw out bearing?

[/ QUOTE ]

Shaking or vibration on a rotating object such as and engine or any part that turns is due to something out of balance.

Since this stops when the clutch is depressed I would certainly say it is something to do with the clutch. But, when you depress the clutch are you keeping the rpm of the engine still at 3000 or does this also drop? If so then try depressing the clutch but keeping the rpm at the shake level. If it stops I strongly lean toward something in the clutch and you should have this looked at as it will not get better and can cause more damage.

If it were tires out of balance it would continue to shake regardless of clutch as long as that speed is maintained.

Bruce

 

[adam90009]

When my b is at 3K the shifter will rattle but i jsut give it some gas to take care of that.

 

[screenprinter]

Weak engine or transmission mounts?

Out-Of-Balance drive shaft from casual U-joint replacement?

Crown Wheel and pinion in the rear end worn?

All possibilities, but I tend to agree with Bruce about the clutch being the most probable culprit -

Maybe a worn caged bearing in the butt of the transmission input shaft? My GT6 had a problem shifting from 1st to 2nd - The gearbox would vibrate around 2500 to 3500 rpm and when I'd shift it would grind a bit before it would slip into gear - When I had the Gearbox rebuilt the mechanic told me that the head of the mainshaft which plugged into the little cheapa## caged roller bearing in the butt of the input shaft was chewed up and was allowing the input shaft/mainshaft to change position or bow slightly under load causing the gears to grind when shifted and to vibrate - This guy was the best darn mechanic I've ever met - He looked like an old tobacco farmer, had a couple joints missing on a couple of fingers, but when you looked him in the eye he had micrometer scales, not pupils and could build anything with some scrap steel and a welder - he's now building high output 4 cylinder racing engines for SCCA types here in North Carolina.

He built up the head of the mainshaft with weld, and machined it down to push-fit the inner race of a small Timken roller bearing - machined out the butt of the input shaft so the outer race of the Timken bearing was an interference fit - pressed the roller bearing into the input shaft , assembled the mainshaft to the bearing in the input shaft with Loctite and rebuilt the rest of the gearbox - Now the head of the mainshaft wasn't being used as the inner race of the caged bearing and things were a lot smoother -

One other remote possibility (And I mean really REMOTE) Is that you have a badly mounted rear shock - either the the structure it's mounted to is cracked, bolts loose, something - I owned a 66 Corvette years ago which would really vibrate and dance around when it hit a certain point on the power curve - Drove me nuts - it turned out to be a cracked shock mount in the rear suspension. Like I said, a REMOTE possibility, but something that's simple to eliminate from the equasion.
Anyway, best of luck - sorry I can be more precise!.

 

[Mark Beiser]

Does it do it when the car is sitting still, or moving, or both?
If it is only when the car is moving, does it do it in any gear when the engine gets to 3,000 rpm, or only one gear, and what gear is it?


My scary u-joint failure story:

I had a front u-joint give out on my '68 MGB-GT while I was going nearly 100 mph downhill on a 7% mountain grade in Arizona on I-17 once. The front of the driveshaft dug into the road surface and felt like it nearly tore the rear end off the car, and caused the car to become partially airborne. Not good when one side of the highway is a substantial dropoff into a canyon, and the other side is a solid wall of rock. I got the car under control, it never had all 4 wheels off the ground, and kept going straight. From then on drivetrain vibrations make me REALLY nervous! /ubbthreads/images/graemlins/eek.gif

The amazing thing about it was that there was no serious damage to the driveshaft or flanges. 4-5 hours later, I was driving the car again, nearly 3 of the hours were spent hitchhiking to and from a NAPA for new u-joints with a driveshaft in my hands...
I drove like a grandma until I got home and triple checked everything from the transmission back.