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MG MGB Technical - Lumpy idle gets worse with electrical load

Hi again...78 MGB with Weber/Peco/Eurospec Dizzy/Sports coil/platinum plugs etc etc...everything is new and the car runs very well except for a lumpy idle and a rich smell of unburnt fuel at idle. When warmed up it idles worse and in traffic when the fans come on it almost stalls. Also adding any other elctrical load like headlamps worsens the idle. Alternator output is 14.5V and the battery is recent. I am stumped...maybe something electrically going bad/gone bad...?ignition relay? I've no idea....help?
P J KELLY

Always start with a tune-up. Your problem could be one or more of several.
Query; what is new to the system, all of it?
Follow-up; did it run like this before the change?
Then; what else was disturbed for the change?

Now you have a list of possible places for a problem.

You can add: faulty ground and/or battery connection, kaka in fuel, bad fuel, plugged filter.

I suspect you have both a mis-adjusted weber and an ignition fault that is giving you an intermittant mis-fire. AFTER the tune-up proceedure (remember, last thing is the carb) you should have good spark and the timing should be spot-on. Do the electrical contacts, especially at the starter lug, fuse box and battery (break connection, scrape and re-fit with petroleum jelly).

Now, the alternator will rob horsepower when it is under load (charging or running accessories). The "smoke" don't come free, after all!
The power to run the alternator will drag the rpm of the engine down a little. If you were on the cusp of stalling already it will only get worse. soft motor mounts will exacerbate the issue (make the shake appear worse) and the shaking will upset the mixture in the Weber. All this will upset the timing because the dizzy is bouncing around with the varying engine speed too (and vaccum signal if vaccum equipped).

Whew.

Now that you know everything else is OK, advance the dizzy a little and turn the idle up! Webers are notorious for not liking low rpm and the increased advance will help the equally notorious off-idle bog.

Take the (probably excessively rich) weber and give it a proper tuning via the book or at a shop familiar with them. Learn how to "read the plugs" for the upper registers and invest in a gunson's Colortune for the idle circuit.

Happy motoring!

Mike!

mike!

Hi.

I would start by checking the battery connections (including the ground) and the engine to bulkhead strap, although I would expect cold starting to be difficult if one of these was the problem.

The alternator won't support a high load at tickover, and the battery is expected to 'fill the gap'.

Try measuring the voltage at the coil at tickover, and the voltage at the battery terminals.

Is your coil a ballasted type ?, I think your car has a ballast wire in the loom.

HTH... Don
Don

Thanks guys...the ballast was removed...it has a Lucas sport coil/new silicon wires/new dizzy etc etc. It ran pretty badly before all the emission control crap was taken off. The alternator gives 14.5V at idle and given the rich smell I am suspecting a break down of spark somehow....can this be the ig switch or any other electrical supply part that is getting hot and losing connectivity???
P J KELLY

PJ. How is the coil wired currently? You mention removing the ballast resistor. This was part of the Opus system, not part of the coil system. If you are still using the original wire that went to the coil, you are running your coil at reduced voltage because of the resistor wire in the circuit.

It sounds like you have some basic, pre-existing engine problem which has not been resolved by all of the parts replacement.

You need to do some tests and see what might be causing the problem, then do additional tests based on what you find out. The RB cars had cams which are noted for being "soft". These can often show significant cam lobe wear when inspected. Thus, while setting the rocker arm clearances is a basic check, inspecting the actual valve lift is seldom done. In your case, it might be a good idea to check out the valve lift to make sure all the valves are opening properly. Same thing with all of the other aspects of the engine, including the exhaust system. A plugged exhaust system can show many of the symptoms you describe. Les
Les Bengtson

I think it's all because of your carb settings. A rich mixture will give a lumpy uneven idle as well as cause all sorts of other expensive problems. I never tuned a Weber but I have done Holleys which are basically the same. Buy a Weber book and see what they have to say. It should also give you some tuning hints regarding jet selection. You need to get the idle mixture correct before moving on to the other jets for mid and high running as well as the accelerator pump settings.

Idle speed can drop depending on the load on your alternator. I can see it happen in my Miata when I turn the lights on and off but the FI and other electronic gizmos keep it at the same idle speed.

Do a Google search.
Mike MaGee

Pete,

My 77 is set up almost exactly like your 78, only difference is in the brand of exhaust. I've had the same difficulty getting the Weber to run lean that you are experiencing. After almost two years of trial and error testing my car is close to right, but not spot-on.

I'd like to give a big ten-four to Les for his comment about the plugged exhaust. That problem held me back for more than a year before it was diagnosed. An indication is a strong blow-back which can be tested with the oil filler cap removed.

Now to your current problem. The unburnt fuel smell is probably richness in the idle circuit. At speed you probably have enough air flow to mix the gas and dry-out the manifold. If you've been running rich for more than an hour, your plugs may be foulded, compounding the problem. Get a spare set of plugs and follow the advice given in a thread a couple of weeks ago to gap them at .035+. Then crank the mixture screw in until the plugs turn light brown to white. You may have to advance the timing to get a smooth idle.

Another thing to check is the choke mechanism. Mine (the electric version) was a little soft straight out of the box and didn't allow the choke butterfly to open fully. This restricts air flow. The electric choke can be tightened by removing the three retaining screws and turning the white cover clockwise (I think)one or two places.

Doug
D. Cook

Thanks you again for all your comments. Peco exhaust is new, not blocked, camshaft and all valve gear is new, head job done. It runs very well all the way up the range except at idle. According to the instructions on setting up the 12 volt sport coil, you remove the ballast resistor and wire directly to coil +ve. Jumping a wire from the fuse box also makes no difference to idle quality. My Weber has been leaned out once...this improved things markedly...but the hotter the car gets...esp. in New York's current 90 degree weather, the crankier the idle becomes.
Les, I will triple check coil wiring again and you were right about the old cam which was, at 38000 miles, the worst worn out cam I have ever seen.
Doug...I found the 1 step leaner jet kit helped things...and reduced fuel consumption. Am getting about 25mpg...but the way I drive I know it can do a lot better than this. Will keep you posted.
P J KELLY

PJ, there is excessive resistance in the system somewhere. Heat aggravates resistance and when the fan begins to run less voltage pressure is available for the ignition. It's possible that the excessive consumer is the fan.
Charles Edwards

The alternator will carry most if not all of the factory electrical load at idle unless there is something wrong in the charging circuit. Any increase in electrical load will put more load on the alternator which puts more load on the engine which is why it slows to some extent. If the state of tune is below par it may have more of an effect, and yours sounds in need of some adjustment, mixture sounds very rich. Resistance due to bad connections will tend to *reduce* current, and so reduce load on the engine. It would have to be very bad indeed before the coil voltage got so low as to affect the engine speed, would be more likley to start misfiring or cut out altogether than simply reduce the idle speed, and the turn signals would stop flashing long before that. The headlights are on a different branch of the brown to most other things on most years, so you would have to go back to the starter solenoid before you find a common point i.e. where a bad connection causes the ignition voltage to drop when the headlights are turned on. Measure the voltage on the coil +ve with fan and headlights both on and both off, a volt or so change is normal, several volts would not be.
Paul Hunt

Turn up the idle speed, advance the ignition 5 degrees from stock, and get to a place where you can get the exhaust sniffed. I still believe the carb needs tweaking and is rich at idle. The shaking is ohly making it worse.
I swear by the Gunson Colortune for the idle circuit.
If I remember correctly, you need to go leaner on the idle circuit jets if you cannot get the correct mixture between 1 1/2 and 2 1/2 turns of the adjuster.

Mike!
mike!

PJ. I agree with Mike! that you need to take the vehicle in to someone who has the proper test equipment. A four or five gas analyzer should be able to tell you some interesting things and point towards the direction you should be going in. Les
Les Bengtson

This thread was discussed between 03/08/2004 and 05/08/2004

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