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MG MGB Technical - IDLE PROBLEM IN THE DARK?

Hi,
A strange thread title you may think. The question is; With the headlights on, the car idle drops about 150 rpm, if i then dip the clutch the rpm drops another 100 rpm approx. I turned the idle up slightly from 850 to 950 rpm to compensate for this, but when i switch the engine off it sometimes runs on a little. I have been told that this is this normal for older vehicles but i'm not so sure. This is the first time i've driven the car in the dark in the 12 months of ownership. Any advice would be very welcome. Chris.

74 1/2 Rubbernose 'B'.
Chris82

You're putting extra load on the alternator so it's certainly not unexpected. Having said that, if your battery is in good condition and well charged I'm surprised you're seeing a significant drop with headlights - my B only has a dynamo and I don't experience a perceptable drop. 850 rpm is quite a low idle (I've never managed to get mine to idle smoothly that low)and 950 certainly isn't excessive so I wouldn't worry too much. Run on is just one of those things - there are plenty of fixes such as anti-run on valves if you're that bothered about it!

Cheers,
Tim
Tim Jenner

Power from the lighting has to come from somewhere and the alternator steps in here immediately preventing the battery from discharge so the revs will drop. In addition it is normal for the revs to drop when the clutch is depressed due to frictional losses between the release bearing and the cover. All this will pull the 800rpm idling speed down to about 600. Now if you increase the revs at idle to 900/950 the engine will run on when you switch it off and this is normal. If you have it idling at about 800 rpm it won't run on. You are stuck pretty much between a rock and a hard place but I would aim for about 800rpm and let the rest take its course.
Iain MacKintosh

You might try using a higher octane petrol. If the running on is caused by a high compression ratio (some MGB's can be very high many rebuilds) or an accumulation of carbon; this should help. The slightly denser higer octane fuels will also run a tiny bit richer.
You can also try playing around with your timing.
You can also fit antirun on valves. These were used on the American cars so you can buy then specifically made for MGB's if you want. Your hanes manual shows how they are plumbed. Mind you there is nothing complicated about them. These are attached to the intake manifold and stay closed when the ignition is on, open up when it is off. Lots of cars have them, VW's and Range rovers for exmple.
Or you can just let the clutch in (foot on brake of course) when you switch off.
P.N. Sherman

I have the same problems as Chris, although my car will eventually stall if I sit at a stop light without blipping the throttle once in awhile. This is a PITA when sitting at an incline. I have a 73 MGB with HIF4s, which I understand have a tendency to do this. There is no problem when the engine is cooler ie choke just turned off but engine not really warm, but when the engine is at full operating temp (which shows alittle below the N on the gauge) it does not like to idle. It does idle better with the headlights off, but I drive everywhere with the lights on to increase my visibility amongst the ever-larger SUVs and large vehicles that are on roads around here. My car will run on if I set the idle high enough to stop the stalling, even though it has the anti-run-on valve which appears to work (clicks when turning the key off). The heat sheild is in place and should be fine.
Does anyone have any ideas how to resolve this?

Erick
Erick Vesterback

You should do everything you can to the ignition, carbs, and engine to improve it's idle. Start with the plugs, wires, cap, rotor, timing, valve adjustment, and carb adjustments. You probably need to give the carbs attention: jets, needles, throttle shafts and bushings. You might need to give the head some attention.

Run-on when the idle is high is normal. You could try an anti run-on valve (there is just one on cars so equipped), but it would be best if you can get the car to idle well at or below 900 rpm. The anti-run on valve system needs a signal from a modified ignition switch and another signal from an oil pressure unit that was mounted on a T fitting placed in the oil pressure line between the engine block and the oil gauge; it was mounted on the shelf in front of the heater motor.

After doing all of these things (except the anti run-on system) my car idles OK and does not run-on. I will admit that on hot days here in California's Great Central Valley (+100 degrees F) it idles poorly after a run.

Good luck and keep at it.
J Brown

Check the ignition timing - especially at idle.

If it's retarded, the engine will be more susceptable
to display the symptoms that you are having.

Advancing the ignition idle timing will at first speed up the idle rpms - but it also allows the engine to
output a bit more "ommph" - or torque with each
cylinder firing. This torque will help it cope with the low-speed load that an alternator may put upon it.

Of course, after tweeking the timing, you will have to adjust the idle back down to spec.
Daniel Wong

If after doing all the above, you still have an idle issue, try this. Have the alternator checked. There are new ones that put less load on the engine.
I have another car that was showing the signs your does. It got to the point of the turn signal taking too much power so the radio would shut off and on when the wipers ran. If the lights were on, it ran horrible. By the time I checked the alt, I had fowled the plugs. The alt was putting out a max of 8 volts and 26 amps! I recently installed a high output alt and will replace the plugs. This is not an MG that I am talking about but a motor is a motor. Sometimes, its the thing you think of last.
kids1

If you are having trouble with the idle speed dropping while driving (usually IDLING) in the dark while your lights are all on, it could also be an indication that your voltage regulator is not kicking in at idle speed or below. If your battery is old, and the charging circuit is not charging (or in this case, not producing enough juice to cover the loads on the electrical system, it could do as you describe.

I have a 68 GT which used to do this all the time in the dead of of a winter's night with all the electrical stuff on. The early model alternator did not cut in below a 1000 rpms and with a cold battery, halogen headlamps and every other high current heater, wiper, rear defogger thing going, it would drop in rpms about 150-200 rpms, lights would dim, fan slow, and voltage drop to about 9.5 VDC. When at a long stop light it would even die as there wasn't enough juice to keep the ignition coil up to snuff voltagewise.

Replacing that alternator with one that cut in at about 400 rpms aleviated my problem once and for all. In your case I would guess, as I said, you might need to check the voltage regulator on your higher output alternator.
Bob Muenchausen

This thread was discussed between 28/02/2007 and 02/03/2007

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