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MG MGB Technical - Engine vibration felt at 55 mph

I need some tech advise. My 79 mgb has a vibration in the steering wheel that occurs between 50-60 mph ( not from wheels out of line or out of balance). The PO said this was caused by the engine rpm and the close tolerance of the steering and the catalytic converter in these 79's. Going through the gears with a lot of rpm's past 60, no vibration, but normal slow Sunday driving (the way I drive most of the time, it occurs). I normally drive about 55 mph, so you see this needs to be fixed.
Question?? Could this be fixed by new front motor mounts? And could these motor mounts be changed out without pulling engine out? The current motor mounts look like they are the original ones.

thanks in advance for any helps or ideas you may have. Happy motoring! PS: I had my 79 out today in Penna. a little cool, but fun to drive anyway.
Donald Wheeler

Are you taking the PO's word that it isn't balance? Sure sounds like wheel balance to me. Have you had the balance checked yourself?
Check the L shaped brackets between the mounts and the engine. These are prone to cracking. Motor mounts can be done with engine in-situ, although with some difficulty on the drivers side (LHD) due to steering interferance. Check the archives here for plenty of tips.

Mike
'79B
Mike Janacek

Donald-
Here's a quick way to eliminate the possibility that it's the motor mounts: Note the range of RPM's during which the vibration occurs, and see if the vibration is there in the lower gears while the engine is operating in the same rpm range in which the vibration occured in fourth gear. If it's not, then it might not be the motor mounts. If the vibration is there only in a fourth gear, head up the steepest hill that you can find while in that vibratory RPM range in top gear. This will put a heavy load on the engine, causing the vibration to worsen if the mounts are bad. If the vibration isn't worse under this condition, and it isn't present in the lower gears, the problem is most likely a wheel out of balance or a defective tire.
Steve S.

Thanks Mike and Steve for your advise. The PO had this MGB for 15 years, only put 11k on since restoring. This was his pride and joy. He knew the MG inside and out, so I'm sure he thought he was giving me good advise. I've put new tires on (as close as possible to correct size), put new steering boots on and had re-aligned.

Mike the L shaped brackets you mentioned are cracking.
Steve, I'll try your method the next clear day to see if it vibrates in lower gears, plus try a steep hill.

Quick last question? how do I check to see if I have a "rim" out of line? Could I check this myself in some simple way?
Donald Wheeler

Donald,
Replace the cracked metal mount brackets ASAP. I had one break and it was a bit hard to drive home with a large hunk of wood in there to keep the engine in place.
To replace the rubber mounts, leave the engine in, but the one on the driver's side requires the steering rack to be removed to get at the nut inside the bracket. I spent an hour trying with the rack in with no success and then half an hour, including taking the rack out and in. Ensure you do not lose the shims between the rack and its mounts as these serve to aligne the lower column with the upper one. Good luck,
Greg
Greg Taplin

Donald-
Chaecking for "trueness" of a rim is simple: Hammer a long nail into a short 2 x4 and use a pair of pliers to bend the nail to a 90 degree angle. Jack up the car so the wheel is off the ground and spin it. Hold the 2 X 4 on the ground so that the head of the bent nail is very close to the rim. If the wheel is untrue, then the edge of the rim will contact it intermittently. It's a crude method for sure, but it works!
Steve S.

Steve S. Thanks for the tip on checking out the rims.

I'll try this the next weekend I have warm weather. I know a mechanic that replaced the worn boots in the steering, If the motor mounts need to be replaced I'll have him do it.
Donald Wheeler

This thread was discussed between 17/02/2002 and 22/02/2002

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