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MG MGB Technical - clutch/engine running problem

I'm experiencing a strange clutch/engine running problem in a 74B with a newly rebuilt engine and new clutch. When the car is idling with the gears in neutral, putting slight pressure on the clutch pedal causes the idle RPM's to drop. Holding the pedal down causes a still greater drop, to the point when the engine stalls. Likewise, after highway driving (>3000 RPM), stepping on the clutch as to downshift also causes a large drop with the engine stalling. What is happening here? Any help is greatly appreciated.
WL Leong 1

WL--

My car has always done similar to a degree. I believe that its normal for some rpm drop off due to load of throw out bearing. What is your hot idle rpm? How many rpm does it drop when pedal fully depressed? How many miles on the new engine? Hot idle should be +/- 750 rpm. You may be too low.
Paul Hanley

Paul is right, don't have your idle speed too low. I like mine up around 1000 rpm.

But another thing you can check is whether the crankshaft is moving forward when you push the pedal. Get a friend to push the clutch pedal while you closely watch the front crank pulley. If you can see it move forwards, you have a problem with the crankshaft thrust washers. These little half-circle thrust bearings go against the centre main bearing cap and should only allow a couple of thousands of an inch fore-and-aft freedom. Any more movement than this and the bearings all start binding, leading to the drop in rpm. If this is your problem, take the car back to the engine rebuilder and get him to put it right.

Mike
Mike Howlett

"What is happening here?" The throw out bearing, being a carbon block, steals a few horsepower from an idling motor due to friction.

While some claim pushing the clutch doesn't affect idle, my '74 B has always dropped a little, even after rebuild. I keep the idle at 1000 rpm or so, and at worst the idle will drop to about 700 when the clutch pedal is held down.

But - if your motor is starting at 1000 rpm and dropping enough idle to actually stall, then I'd check things that affect your state of tune. Mine did this recently, which led me to find a carb problem. When your car was rebuilt were the carburetors overhauled? What old parts were reused - camshaft, distributor, rocker gear, spark plug wires...?

As to your other problem - car stalling after a highway run: While there may be another cause (i.e. state of tune) a weak idle after a fast run is a typical symptom of float level too low. It may be that your float is just a hair low, which only poses a problem under the additional duress of the throw out bushing stealing a little power.

Matt Kulka

I have idle set at 650 rpm and notice no drop in rpm when I push the clutch. I would suggest that any drop is far from ideal and must be caused by something not being within spec. I would check timing, points, plugs, carbs and vacuum leaks etc. Sounds to me that if the friction from the clutch causes the engine to stall then you haven't got it setup to optimum spec.

Matt
Matthew Crabb

Same here, Matthew, and I have never noticed it on any other car I have owned either. And after a run my idle tends to be a little higher than normal, settling down over a minute or two. Can't recall any other nationality complaining of the clutch/revs problem but it seems a relatively common thing with cars in North America. Something to do with their emissions kit, perhaps? I've also chucked the thrust washers thing into the ring in the past, only to have it roundly pooh-poohed by someone saying any engine that worn would have put a con-rod out through the crankcase. But OTOH I think variations in float level are far less significant than people think.
Paul Hunt

I have always depressed the clutch, with the tranny in neutral, when starting the engine as a saftey measure. In a recent thread someone wrote that the car cranks easier with the clutch engaged. I tried it and it definitely cranked easier. So obviously the throwout bearing puts a drag on the engine; however, it doesn't reduce my engine's idle by much, if at all.
Fred H

ok, i fudged a little. in practice when i tune the car it will idle at 500--factory spec. 500 rpm is a bit tricky to achieve and it seems just looking at the clutch pedal will cause a drop in rpm! in practice, when idle is set to 500, a full depression of the clutch will drop rpm about 25 to 50 rpm--not too smooth an idle so, i set it a smidge higher to 550/600. i only bring this up because i have no emission controls (1964) and find it hard to believe this only happens in north america unless we get different petro. or it has something to do with 3 main engine perhaps? just my two cents

paul
Paul Hanley

Oooh, I've got one that does it. When this engine was in my GT (which has a roller throw-out race but I doubt that is anything to do with it) the clutch made no difference to the idle. However now that it's in our roadster the tune has been drastically affected, which I must assume is to do with the difference in fuel pump (both the same part, but different instances). The engine runs much richer in this car and as yet I have not bothered to sort it out, as I really am that lazy. Clutching drops the idle from ~1000 (where it is running on all 4) to maybe 550, where it is stumbling and so on.

But, another thing to note is that my oil pressure isn't what it once was so maybe all my bearings are shot and the thrusts are in fact worn. Who knows? Who wants to come and push my clutch pedal while I watch the pulley?

Cheers,
--
Olly
Oliver Stephenson

This thread was discussed between 01/12/2003 and 04/12/2003

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