The MGA With An Attitude
ENGINE DIES When Clutch is Depressed - CF-108

On 29 Dec 2005 Patrick Mulholland wrote:
>"When engine is idling (1,000 RPM) and I depress the clutch, engine starts to labor and the revs dramatically reduce (even if neutral gear is selected) and eventually engine will stall unless I give it more fuel or release the clutch."

Pressing the clutch pedal applies end load on the crankshaft. This load is taken between the release bearing and the center thrust washers on the crankshaft, both of which exhibit a small amount of friction. A small slow down in engine speed (50 to 100 rpm drop) is normal. When stronger clutch springs were used for the MGA MK-II (and superseded for all replacement parts), tune-up instructions in the workshop manual were changed to include having a helper hold the clutch pedal depressed while setting the idle speed.

If the engine is in good tune, a small load at idle speed will slow the idle slightly and increase torque to compensate for the load. If the engine is slightly our of tune, especially if running a little lean, a slight slow down in idle speed will reduce torque, and the engine will die. The cure is a tune up to get the fuel mixture and ignition timing right. If the problem persists, do a compression check. An engine with very low compression can be quite sensitive to small loads at idle speed. Also set the idle speed with clutch depressed. Then releasing the clutch pedal causes a slight increase in idle speed.

Here's the theory on how and why it works. The carburetor has just a small opening of the throttle plate when idling. This creates a strong vacuum in the intake manifold so a little bit of fuel/air mixture gets through to fill the engine displacement to keep the engine turning at low speed when the only load on the engine is from internal friction.

When you apply a small load to the engine while idling (the clutch thrust friction), the engine will slow slightly. With no change of throttle setting, there will be very little change of air intake flow (still a strong vacuum). You then have the same volume of combustion gases but less engine displacement per second. This makes slightly higher pressure in the cylinders to provide increased torque to handle the increased load. Result is, more load makes slower idle, but it doesn't stall unless a significantly large load is applied.

Problem comes if the engine is out of tune, not running well, and you have increased the air flow (throttle opening) somewhat to keep up the desired idle speed. This commonly involves incorrect fuel mixture, possibly too rich in one carb and too lean in the other carb. In this case a slight increase of load and a small slow down of engine speed upsets the already incorrect fuel mixture with poor burning, and the engine does not increase torque as much as it should. Then it slows down more to create more torque. With worse condition out of tune the engine might actually stall from such bad running with only a slight bit of load.

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