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MG MGA - strange idle phenomenon

I have notice a strange and bothersome phenomenon with my 1957 MGA. The entire engine was recently rebuilt so I know it is in great running order.

After the car has been running awhile and the engine is hot, the idle is at about 900 RPM. It is stable at this RPM until I push in the clutch and it immediately drops to 300 RPM and stalls unless I push on the accelerator - or let the clutch back out.

HOW is the clutch related to the idle speed?? The car is still in neutral, the only difference is that the clutch pedal is pushed down.

Any suggestions?

Many thanks,
Bill Chandler
Ann Arbor, MI USA

W Chandler

Bill

Barney has a bit about it. http://mgaguru.com/mgtech/care/cf108.htm

Steve
Steve Gyles

You could also have excessive clearance on the thrust surfaces of the center bearing, allowing the crankshaft to be pushed forward by the clutch pressure plate. Cheers - Dave
D W DuBois

Whatever force you need to apply to the clutch it is being opposed by the front crankshaft thrust bearing - is it progressive, the further you push the clutch in the slower the engine goes? Too much clearance and something else will be taking this thrust - crank flange against bearing cap, etc. Has it been doing it since the engine has been overhauled?
Do you have all standard MGA clutch components ie fork, thrust bearing, clutch cover, clutch plate and flywheel?
Have you tried giving it a few drag starts - 4500 rpm, feather the clutch and throttle to try and limit wheel spin.
Mike
Mike Ellsmore

My wife has just suggested that the phrase "Strange Idle Phenomenon" kind of sums me up! :-)

Colyn
c firth

lol
mog

Thank you all for your comments - very helpful.

Mike can you clarify about "Drag Starts"? I assume you mean taking off with the engine reed to about 4500 rpm. What is this supposed to do?

This problem is clearly worse when the engine is very HOT - i.e. after long drive on hot day. The car runs at about 190F, but when turned off the temp goes up over 200F. This is when the engine will stall with clutch depression.
When the car is cold the revs drop from about 1000 to 500 with clutch depression and it does not stall.

Thanks,
Bill
W Chandler

Yes see here: http://mgaguru.com/mgtech/care/cf108.htm

Most of the time this phenomenon is caused by the carburetors, one being too rich and the other being too lean. When carbs are adjusted properly, a slight reduction of engine speed results in a slight increase of torque, so it will not slow down much, and it will not die. (Reason being, constant fuel and air flow going in is constant energy, so reduction of speed with a small load means increase of torque).

Bottom line is, it needs a standard tune up.
Barney Gaylord

Barney - Thank you.
Is this tune up something I can do myself? Any web guides for this?
Bill
W Chandler

Easy DIY job. See here: http://mgaguru.com/mgtech/care/cf120.htm
Barney Gaylord

Sorry there I was of on the wrong track (again)! We have had several people here recently with clutch problems resulting from mixed mga/Mgb clutch bits and clutch issues with T9 conversions I thought "oh no here's another one"
Mike
Mike Ellsmore

This thread was discussed between 17/04/2015 and 20/04/2015

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