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MG MGB Technical - Low idle problem

Been trying to solve this problem on a 79B for a while. Engine idles smooth @ 1000 rpm until I switch on the headlights with the radiator fans (stock twins) running, and idle starts to drop. I have converted to the Bosch alt., replaced light switch in the steering column cowl, converted main beam to a relay, and replaced the pos battery terminal cable to starter solenoid when I found the terminal cracked and loose. No effect. Had also converted to 10 inch puller fan, and converted back. Engine off, battery reads 12.8 volts, 12.8 at the main brown wire to the fuse box and 12.8 at both brown wires at the plastic connector in the wiring loom under the hood. When engine warms up, digital voltmeter in the cigar lighter reads 14 volts at idle of 1000 rpm. Fans come on, voltage to 13.8, light switch to first (parking lights), no effect, second position, idle starts to drop, bottoms out at 450 rpm, and run rough, voltage drops to 11.7. Highlights are sylvania halogens, were there since I bought the car. Wiring doesn't seem to be hacked, I bought the B because it appeared to have good PO's. Runs a weber and disconnected the anti run on valve connection to the manifold to eliminate possibility of a vaccum leak. Cleaned all ground wire connectors at the front of the car. Converted from opus ignition with crane cam electronic unit to a Lucas euro spec dizzy and sport coil, and the recommended wiring connection, no effect on problem. Can't drive the car except on sunny days, and when I do, she runs strong. Any ideas????
Thanks.
Bob Johnson

A couple of ideas, although I suspect Paul Hunt will
know the answer. I have a 79 also.

Is the ground strap to the transmission in good
condition with a good connection -- this would be my
first guess.

Second guess -- loose fan/alternator belt.

The sports coil needs 12V, not ballasted 6V. Although
you stated that you changed the wiring, are you
powering the sports coil directly from the brown-white
ignition circuit?

Is the battery in good condition? The battery
voltage should be at least 12V -- you measured
11.7 V at 450 RPM, which means its discharging at
that low RPM. Its also possible that the new Bosch
alternator is defective out of the box.
Ronald

OK - alternators (esp. high output ones) absorb engine power.

At 1000 rpm on a closed throttle your engine is probably producing less than 5 bhp so the power used by the alternator is a significant percentage of this and I would expect the idle speed to drop some - maybe 50/100 rpm.

The fact yours drops more than this points to a mixture or timing problem.

I would be inclined to try dropping the idle speed to 750/800 rpm and sorting the mixture and timing to get it running smoothly at this speed and then see if the alternator coming on power made as much effect.
Chris Betson

Concur. Making sure all the connections are clean and tight will only make the situation 'worse', given that it will allow the alt to deliver a higher current hence require more effort to spin it and so slow the engine even more!

I wonder if this is similar to the complaint that pushing the clutch pedal down slows the idle, which I have never had but seems to be common to North Anmerican spec cars, i.e. is something to do with the fuel, ignition and emissions systems on those cars.
Paul Hunt

This thread was discussed between 18/12/2002 and 20/12/2002

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