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MG MGB Technical - 1974 MGBGT Clutch and Headlight Problems

I have a 1974 MGBGT that hasn't been on the road for at least four years. I have had the car for more than a year, working on it as time and money permits. I thought I had the mechanicals pretty well sorted out and had planned to take it for a short drive today. I fired it up with the transmission in neutral and pushed in the clutch. Engine idle speed dropped by about 300 RPM's. The transmission shifts smoothly, no vibrations, no grinding of gears, etc. Every thing seems normal except that the clutch pedal reguires a lot more pedal pressure than my '71 roadster and the idle RPM's drop concerns me.

I turned on the headlights with the engine idling. On the low beam, engine RPM's dropped by about 100. Hit the high beams and engine RPM's drop by about 300. I havn't had a chance to check the alternator output but it seems to be charging OK. Battery is new and fully charged.

Any ideas would be greatly appreciated.

Mack
M Sneed

Mack,

Until you get some better-informed answers, perhaps mine will tide you over. All MGs will idle slower when the clutch is pushed in. It's the drag of the throw-out bearing. ' Confess I've never watched the tach when this happens so I can't say how much is normal, but if your car has gone four years since it was last driven, there is a chance that some rust on the engaging surface for the T/O bearing is creating additional resistance. I don't recall the term for that engaging surface, but know it broke off on my GT many years ago. As I recall, the clutch still worked - sort of, but the engagement point on the pedal travel was pretty high and certainly it would have chewed up the T/O bearing very quickly had I driven it much of any distance that way. If it's just rust, this should become much less of a problem with a little driving. It's possible that your car has a heavy duty clutch - in which case you'll just have to live with the heavy pedal and the drag until you replace it.

I take it, from your description, that there is no problem shifting into reverse or first. That pretty-well rules out any clutch hydraulic problems. Since you have another B, you know what the clutch is supposed to feel like. Besides that, if there was a hydraulic problem, you wouldn't notice the drag from simply pushing the pedal in while in neutral, but you would feel it once you put the car in gear. And of course you would note that the engagement point on the clutch-pedal travel was way too close to the floor.

Regarding your headlights, again I might suspect that the problem is four years of disuse. I would engage in some cautious test-driving, but keep in mind that replacing the alternator on a 34 year-old car that had not been driven in four years would not be an unforeseeable requirement. By the way, you can still get rebuilt alternators exchange from Advance Auto. ' Can't remember the price, but recall it's pretty reasonable - and there's a lifetime warranty as long as you own the car. No, I don't work for, or have any connection to, Advance Auto!

Then there's another thought that strikes me. Have you now actually driven this car? Is the engine pulling strongly? I'm thinking that maybe the engine is just so far out of tune that it struggles under these "light" loads (forgive the bad pun). If you haven't done a thorough tune-up, do it before you assess the other problems you've mentioned. What is the idle speed before these drops occur? Maybe it's too low. I know the norm is around 800-900 rpm, but I never set mine that low - rather more like 1000-1100.

Otherwise, I'm inclined to take a car like yours out and drive it until something breaks. Then you'll know what the problem really is. If there's something wrong with your clutch that really requires a new clutch, you won't hurt it by driving it; the repair procedure will be the same.

I'm not much for "fussing" over my MGs. "What's that funny noise?" If something breaks, I'll know. If nothing does, it's not important. If I'm taking the car on a long trip (which I frequently do), I'm inclined toward the shot-gun approach. Under these circumstances, that means to arbitrarily replace the alternator. But I have to ask: does the idle speed fluctuate when you turn on the heater fan? Blow the horn? Do the turn signals stall out when the engine is idling?

Just some random thoughts...

FWIW,
Allen


Allen Bachelder

Allen,

Thanks for the response and it appears you are correct in your assessment. I still haven't had the car on the road but I have driven it up and down my driveway a few times (I have a fairly long driveway) and the problems have almost disappeared. Even the headlight problem seens to have cured itself. I think I just need to take it out and blow the soot out of it.

The car has had a complete tuneup. New points, plugs, rebuild the carbs, etc. The underhood plague says the time should be 11 degrees at 1500 RPMs. I set it at about 15 degrees because it seemed to run better at that number. Per color tune and the lift pins, the carbs are spot on and idle is smooth at 900 RPMS. I haven't done anything to the clutch hydrolics yet. There are no leaks; and other than the RPM drop, every thing seemed to be operating OK. However, I suspect that it could use a good flush and new fluid.

Again, thanks for the response. I was probably premature in asking the question.

Mack
M Sneed

Not all MGBs drop idle speed when the clutch is pushed in, neither of mine do, either on the tach or by ear. It is something I've read about from time to time, but from memory it is nearly if not always related to North American spec cars, possibly something to do with the lower torque and HP that afflicted them more and more. Clutch hydraulics are unlikely to change this as the drop in revs are being caused by mechanical friction and load, which will happen no matter how the release bearing is pushed against the cover plate. There are periodical arguments about whether it is better to start cold engine with the clutch pushed down or released to give the lowest load on the starter. If operating your clutch with the engine running causes *any* dop in engine speed then obviously clutch down is giving more load so start it clutch up. If there is no change in idle revs then the load must be the same, so it doesn't matter, although it is slightly safer to start with it down just in case it has been left in gear.

Mine do drop very slightly with the headlights, but more by ear than tach. This is normal, as under all normal circumstances it is the alternator and hence the engine that is powering the electrics, and the more current drawn from the alternator the greater the load on the engine and hence the lower revs for the same throttle opening. If anything that indicates the alternator is working as it should, not that it is faulty. However if there is a partial short on the headlight wiring somewhere (or a wiring fault that means both main and dipped beams are on at the same time) that will draw more current than normal and hence slow the engine more.
Paul Hunt 2

Paul,

Do your cars have roller-bearing T/O bearings? I always appreciate your excellent and knowledgeable advice on BBS, but not only have all my MGBs had this characteristic (some 20 or so), but also my Magnettes, and the '59 Morris Minor I bought new nearly 50 years ago. I've always thought that was just part of package.

My vague recollection is that the Morris Minor Driver's Manual said not to start the car with the clutch pedal depressed, but I'm not sure. Certainly the US EPA could have caused some of this on post-1967 cars.

I'm not taking issue with you - just wondering what could cause such a disparity in personal experiences.

Cheers,
Allen
Allen Bachelder

As it happens both mine do have a roller bearing, the V8 as standard and the roadster in a fit of foolishness when I replaced the clutch. But I never noticed it before changing the clutch, or on any of the many other BL cars I have owned. 100rpm I may not really have noticed, but 300 I certainly would have given the 600-700 idle speed.
Paul Hunt 2

This thread was discussed between 07/04/2008 and 09/04/2008

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