Chicagoland MG Club
Is Your MGB Idling Rough? Check This Out!
From the time I purchased my '79 MGB (about two years ago), it did not have what I would consider a smooth idle compared to other cars. Having owned a '65 Spitfire along with a '58 TR3A, I just assumed that all British cars idled a little rough. Every once in a while I would adjust the fast idle adjustment on the carburetor, but it didn't seem to help much. This summer my MG started idling a little rougher than before. Once again I played with the carburetor, but with little improvement. By this fall, my MG was idling really rough, shaking the whole car. When I gave it a little gas, the engine would smooth right out. I was adjusting the carburetor and gunning the engine a little when I noticed the engine lifting as I gunned it. You guessed it, the motor mount was broken. The first thing you will discover when replacing motor mounts is that the books don't say much about that area.
After blocking up the front end of the car and placing a jack under the motor, I found that the steering assembly from the universal joint on down had to be removed before I could take off the old motor mount on the driver's side. After popping off the ball joints on the tie rod ends and removing the bolts that held the steering rack, I broke the vacuum hose going from the manifold to the brake servo unit while trying to pull the steering shaft out of the universal joint. At the time I didn't think much about it except that I would have to repair the line. After removing the steering assembly I jacked up the engine and removed the old motor mounts. Installation of the new motor mounts and putting the steering assembly back together again was quite straightforward. I then tried to repair the vacuum line that I broke, but discovered that it was very hard and brittle. Upon checking the other vacuum lines, I found them to be in the same condition, along with some cracks. I replaced all of the vacuum lines and started the MG to see how it ran. The idle speed needed to be turned down a little, but I couldn't believe how smooth it was running. By looking at the parts I removed, (the one motor mount was in two pieces) and the vacuum hoses. I determined that the rough idle was caused by those items and had nothing to do with the carburetor.
Vacuum leaks are often the cause of rough idling and poor running, particularly on the pollution-equipped cars of the '70s. The engine mount failure was probably aggravated by his rough running engine. Careful checks of vacuum hoses should be a part of regular preventative maintenance.
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