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MG MGA - Revs drop under braking

Anyone know what may cause my engine to lose revs when I brake and have the clutch depressed?
A bit odd but when cold it can stop the engine.
The car runs fine otherwise.

Neil
Neil Purves

By chance do you have a brake booster installed? This in conjunction with clutch drag can dramatically slow mdown an engine . Maybe you have a vacuum leak on the booster.
DARNOC31

Neil. The first thing is you should not be braking with the clutch depressed due to the carbon throw out bearing used in the MGs. Keeping the clutch disengaged for any longer than it takes to put it in gear and start off, or shift gears, will cause rapid wear of the bearing. Depressing the clutch will, also, move the crankshaft forwards (end float), causing a slight drag on the front thrust washer. Depending on how badly worn the thrust washer is, you have a slight possibility of making contact between the moving crankshaft and the block. Always start the car with the transmission in neutral and the clutch released. Then, try to keep the clutch pedal depressed no more than a few seconds at a time. (Seven seconds was the old rule of thumb, which probably is not a bad one.)

Normally, when the engine tries to stall when braking, it is related to either a too high, or too low, fuel level in the float bowls. I would check that as my first trouble shooting area.

Les
Les Bengtson

Neil,

I'm having the same problem in that when I depress the clutch pedal for shifting gears, the engine revs drop considerably and if I'm not quick enough the engine stalls. Now, I have recently replaced the two crank thrust washers but the problem still persists. I have discovered since that the thrust washers come in different thicknesses, and that one has to measure the gap first then install the appropriate thickness washers. I had replaced mine with the standard washers. Has anyone replaced these thrust washers, and in doing so were oversize washers used? If so did this solve anything?

Frank
F Camilleri

Frank. You have to measure the end shake (end float) on the crankshaft, then use the proper washers to ensure it meets the factory recommended .004" to .005". The factory workshop manual details the procedure for taking this measurement.

Les
Les Bengtson

Standard thickness thrust washers are always the norm, unless the thrust surfaces of the crankshaft were "touched off" during re-grinding.

If you have a rusty crankshaft cleaned and reground, the machinist may touch the thrust surfaces with the grinding wheel to remove pits to have a final smooth surface. If this removes more than a few thousandths of an inch of material you can have too much end float, in which case the oversize thrust washers are available. Thrust washers are available in Std, +.005, +.010. By mix and match you can change the stack up in 0.005" increments from Standard to +.020" total.

Install Std thrust washers first. The use a thickness gauge to measure the gap on one side, or a dial indicator to measure end float. Specification is 0.002" to 0.003" end float when new. However, you need to have 0.007" end float before you can do +.005" on one side to reduce end float to 0.002". So when the crankshaft may have excessive end float, the target for using oversize thrust washers is 0.002" to 0.006" end float. If you want to be more precise, you can grind the back side of an oversize thrust washer to make a non-standard oversize part with any in between thickness desired.

The amount of end float and condition of the thrust washers should have no odd effect in engine idle speed (unless the white metal on the thrust washer is all worn away so you have steel on steel contact). When clutch is depressed at slow idle, you may expect about 50 rpm reduction of engine speed to friction on the crankshaft thrust washer and clutch release bearing. If you get greater reduction of engine speed, the most likely cause is incorrect fuel mixture in the carburetors (especially with dual carbs).

At slow idle the throttle valve is open just a crack to admit only a small amount of air, which should be mixing with the correct amount of fuel for correct mixture ratio. The amount of air allowed to pass the throttle valve determines engine idle speed. If you apply a load on the crankshaft to slow it down a little, the throttle valve is still admitting the same amount of air, to fill a smaller displacement per second. Crankshaft torque then increases slightly to offset the increased load, and the idle stabilizes at a slightly slower speed.

Correct fuel mixture is very important, as the engine is sensitive to incorrect mixture. When mixture is wrong (either a little too lean or a lot too rich) you lose torque, so the idle slows, and you have to set the throttle valve slightly more open to have the desired idle speed. Under this condition a small change of engine speed will result in more screwed up fuel mixture, especially if one carb is too rich and the other carb is too lean. When this happens the engine loses torque when it slows down, cannot maintain idle, and it stalls.

Solution is to set the fuel mixture right on both carbs. Loosen the throttle shaft coupling between the carbs so the carbs can open independently. Close one throttle valve while you adjust idle speed and mixture on the other carb, while it is running on only one carb. Then close the second throttle valve and open the first one to adjust idle speed and mixture on the other carb. Then close both throttle valves, and open each one the same amount (1/2 turn on the idle screw) to synchronize air flow, and lock the throttle shafts together. After that turn both idle screws the same amount while adjusting idle speed, to maintain equal air flow.

Finally, for fine tuning of the mixture, you can use the "lift the pin" technique described in the Workshop manual. This only works with idle speed under 1000 rpm, so keep it slow. When a tiny lift of the air piston results in slowing of the engine, it is too lean. When a tiny lift of the air piston results in increasing engine speed it is too rich. When you get it right, a tiny lift of the air piston will result in no change of engine speed, or a very sight increase of speed followed by settling back to the original idle speed. Then when you depress the clutch it will not stall.
Barney Gaylord

Many thanks guys, Les and Barney. As usual Barney's explanation is extremely clear and very detailed. I'm indebted to you both.

Frank
F Camilleri

Neil,
It may be that your clutch bearing (throw out bearing) has no more carbon left on it, and the additional friction of steel on steel is causing the engine revs to drop. Or possibly the clutch plate is not freeing itself completely off the flywheel, ie, it has sticktion.
Maybe you have not changed the clutch thrust bearing in a while?
Peter.
P. Tilbury

This thread was discussed between 17/04/2013 and 19/04/2013

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