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MG MGB Technical - Electical charging help needed/will NOT idle

Hi guys,
some of you know I have a 77 just will not idle...3 years and still it won't idle right...I've done EVERYTHING to this car mechanically/carbs/ignition...the works. I finally changed the alternator...guess what. The alt was not self when I started the car the alt. did NOT start charging...but the car idled PERFECTLY....for a couple of hours til I figured out the alternator issue!!! WTF!
So I got the alt to start...(ignition light circuit was faulty) I'm back to the original lumpy crappy idle, same as with the old alternator,(which I changed as it did have poor charging)!!! ANY electrical load on the charging circuit makes the idle even worse.
Electrical guys...what am I missing???????
85 900T
73 Saab 96
77 MGB....needs help!

Check the engine/transmission ground strap. Make sure all electrical connections are good going to the coil and distributer (white circuit) and the proper bulb is installed for the ignition dash light.

Ignition dash light circuit is the alternator is getting the kick start supply from a switched fuse box connection. I checked the ground's good. I get 10 volts across the coil when the car is running, and 13.5 volts coil +ve to ground when running. This is really weird. If any connections were bad....why would it idle perfectly on battery alone?


Go to our site @ , and look over the manuals.

In my mind the both the Battery & the Alternator should be connected to the Starter solenoid. The way the theory has been explained to me when trying to hook up an Amp Meter is that:

o There are (2) power sources: Bat & Alt
o Amp meter measures difference between those (2) power sources
o If Bat needs charge then Alt ramps up Juice; otherwise, most of the Power is coming from the Battery.
o If Bat is weak/dud then Bat is a bleeder of Power from everything else.

I hope this Helps ;-)

Advance Auto Wire
MM Masters

A bad engine ground strap will be very obvious as slow cranking and smoking accelerator, heater and choke cables. If you can crank it enough to get it started then there will be more than enough of a ground connection for the alternator and the ignition.

Unless you have a very high electrical load on the alternator there should be very little difference in idle between the alternator connected to it being disconnected - unless it is partially siezed. Does it spin easily wih the fan belt removed?

How is "the alternator is getting the kick start supply from a switched fuse box connection" if the warning light isn't working?

With the alternator charging it is *that* which is carrying the electrical load, not the battery. The alternator operates at a nominal 14.5v and the battery at a nominal 12.8v. So when battery, alternator and load are all connected together the *alternator* is carrying all the electrical load *plus* trickle charging the battery. It is only when the electrical load gets so large, either from a very leaky battery or everything on the car switched on and the alternator reaches its maximum output, that the system voltage will start to fall from 14.5v to 12.8v. And it is only when it falls *below* 12.8v that the battery starts to carry that part of the load that the alternator can't supply, and hence is discharging.

If there are seriously bad connections between the coil and the solenoid than switching things on could possibly affect idling, but it would be *worse* if running on battery alone, less worse when the alternator is charging.

Only 13.5v at the coil does sound to me like bad conenctions or a heavy load on the alternator.
Paul Hunt 2

Thanks Sooo much for your replies. I have got the ignition lamp circuit working...brow/yellow to the alternator trigger connection...the alt is working correctly and I'm getting 14 volts plus at the coil +ve. Still the crappy idle and still worsens with electrical load. It also bogs down some with depressing the clutch. Could it be valve timing? I rebuilt the engine last camshaft and dual chain timing gear. I'm a pretty good hobby mechanic...just more used to Saabs which never seem to have these kind of issues. Just for info...I have a better or worse than with points. When you rev the engine under no load, you can hear minor has always been like this whatever distributor I've had...(has a 45D now). Also I had a weber 38/38..chaged THAT out for twin SUs....made no difference. Vavle clearances set at 13 thou hot...trying them looser (15) makes no difference, just more noise. Head was rebuilt...valves ground in....120psi all 4 pots.

I started writing about the clutch thing but removed it as I thought it might only serve to confuse. Nopw you've mentioned it, I think the relatively small additional load on the engine from pushing the clutch or turning on electrical load is having a dispropotionate effect on idle speed, rather than their being an *electrical* change which is affecting ignition, and hence idle. The clutch thing is something which seems to affect North American cars far more than UK cars, I think I have only heard of one UKer complain of it. As such I think it is a side effect (one of several) of all the North American emissions clutter - if you still have it. The UK only got one aspect of that which was the move of the distributor vacuum source from carb to inlet manifold, and that much later than North America got it.

As an experiment, remove the vacuum pipe from the inlet manifold and plug the port on the manifold, and then see what effect clutch and electrical load have on idle speed. Note that you may well have to screw the idle screws in a bit when first disconnecting vacuum to get the idle speed back up to the correct level. If you have twin SUs make sure you adjust both carbs by the same amount in the same direction.
Paul Hunt 2

Thanks Paul....I have tried this and it makes no difference...I've read elsewhere that when taking vacuum off the manifold you lose vac advance when on high throttle...manifold vacuum drops to zero...throttle plate vacuum is higher???
Maybe this is also to do with low compression ratio of the emissions strangled cars? I have taken ALL the controls off...restored it to twin SUs/a 45D dizzy/pertronix. I tried a weber...and the dizzy with points...all make no change.
I checked and rechecked my valve timing...seems spot on...but I DO have to have my timing advanced a long way off the timing cover marks to get the engine to run at looks like I'm at 25 degrees or so at idle...if I reduce it to 10 on the timing cover...the car stalls from being too retarded.
Set up like this it pulls fairly well...I get 25mpg (US), although it's no sports car and does labour up hills more than I'd like considering the SUs it has. Maybe I'm too used to my 85 Saab turbo!
1 Questions...Mr Masters mentions hooking the alternator directly to the solenoid...but isn't the brown wire from the alt. already hooked up this way even though it disappears into the wiring loom?? My Haynes wiring diagram also indicates their any benefit to fitting another main feed?

PJ. I have been following this tread as I have time. Unfortunately, not very much right now.

I agree with what Paul has posted so far. From your additional comments, I wonder two things. First, has the harmonic balancer begun to delaminate--the inner and out portions are held together by a rubber ring which bonds them together. If, as has happened, the ring losses its bond, the outer ring can rotate. This leads to the timing marks being out of correct location and might account for your 25 degs of advance it idle necessary to keep the engine running.

The second thing is some form of vacuum leak on the induction system--manifold to head gasket, the other gaskets and the spacers. Checking this out, using carb cleaner sprayed around the joints or moving an un-lit propane torch near the joints, should demonstrate if you have a leak.

Other thoughts, as I write, are the relatively low compression you demonstrate. On a rebuilt engine, I would expect more than 120 psi cylinder compression. The low compression engines I have rebuilt for my daughter's and my cars are showing 150 psi+ compression even after several years of use.

It would be interesting to put a dial indicator on the rocker arms and find out how far the rocker arms are moving. They should all be moving about the same amount. If you have one, or more, flattened cam lobes, this could be part of the problem.

Les Bengtson

I had exactly the same problem earlier this year after i bought a new 45d distributor. After fitting I could'nt get the car to tick over at all. After many hours tuning, and re tuning and a great deal of hair loss i put the old vacuum unit on the new dizzy and hey presto, it works fine. I bought the unit on ebay and although it was the correct number i was told by the MGOC that this dizzy was used on many other cars at the time but with different vacuum units.Got to be worth a try. Chris.

Again...thanks for your MG wisdom...
Paul; yer know it does run better with the vac advance pickup off the manifold.
Mr. Masters; I did add neavy guage wiring directly from the alt to the solenoid....this also improved the idle...I'm at 18 hg vacuum on idle finally...but still the load causes some lumpiness.
Les; I think you are right here...when I pulled the motor last year, (it's only done 40K ), the cam was indeed crapped out but the bottom end sounded great, oil pressure is 50 psi hot idle and the performance did not indicate impending's not burning oil and there is no pressure or blow by in the I left the rings/pistons/bearings. But I think you are right on the money here...chances are it's rings are tired also and I really should pull it again...put high compression pistons in it and 'complete' the job.
The real reason I didn't was I was under pressure to finish mt Saab turbo engine....before the cold weather kicked's all priorities.
Chris82...the 45D came from an mg specialist here in the US....brand new....but I know 'new' doesn't mean good....I have the old 45DE dizzy...but that's set up all wrong for a now non emission car...the continuum goes on.
Thanks again chaps.

As you say the alternator is already connected to the solenoid directly, but adding an extra heavy gauge wire won't hurt anything, and will cause result in less volt-drop under heavy electrical load. But if it really *does* change the idle speed then I'd suspect something significantly wrong with the wiring or connections somewhere, and only then if there was a heavy electrical load.

The only difference between carb vacuum and manifold vacuum is that at idle manifold gives maximum vacuum advance (which is why you have to measure and retime it disconnected) and carb gives zero. As you go off idle manifold strarts to drop slowly and carb vacuum rises very rapidly to meet it. After that all the way up to full throttle the two are virtually the same.

On important thing to remember is that you only finally set the idle speed *when the timing is correct*. If you are adjusting the timing to get a particular idle speed, it is almost certainly going to be wrong. Over advanced will give a higher idle - for a given carb idle screw setting - than correct timing, or conversely if you reduce the advance from too high to correct the idle speed will fall. You then adjust the idle screws to get the correct idle, as part of all the other carb adjustments, which must only be done *after* ignition is correct.
Paul Hunt 2

thanks again Paul...I have a vacuum guage on the car so I have been tuning it for best idle vacuum and going from there. I think Les has it right that my pulley may have 'travelled' I cannot really set the timing other than by trial and error. I'm thinking more error than trial!!
Yesderday I finally scored 18 on the vac guage at idle and there is now only a slight drop off idle with load...even dipping the clutch it's better than it was...also seems more powerful. We'll see how it does...probably a bunch of symptoms resulting in this issue. V8 conversion next perhaps?

PJ. Please, as you have time (I fully understand about being busy right now), what you find. I should be interested in what your experiments suggest. I should also be interested in knowing what you find when you test for air/vacuum leaks. Had a major problem with this myself one time and could not find the cause. A professional mechanic friend started pinching vacuum hoses and, when he pinched the one to the air pump (emissions testing required here) and the engine ran very well. Lesson learned--professional help is worth every cent spent and a second set of eyes always helps.

Les Bengtson

Interesting problem. I have owned two MG's in the past few years, a 77 & 80. I read the threads now because I enjoy the input of Paul, Les and others on problems.
Since you have determined that you have a steady idle without the alternator charging, have you considered separating the ignition circuit from the rest of the electrical system? Using a fully charged second battery one could disconnect the car's electrical system at the coil and hook up an external system using the second battery. If the idle is still erratic, the problem is not in the ignition circuit, but somewhere else in the engine. This would give you a chance to test other theories, i.e. change the electrical load by turning on the headlights etc.

This thread was discussed between 09/12/2006 and 13/12/2006

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