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MG MGB Technical - Idling issues

I think this applies to just about every MGB I've ever owned (and that's a bunch!) from bone-stock to slightly modified, they've all run great with one caveat - idle (tick-over) is terribly erratic. I don't think any one of them has ever idled twice at the same speed, and sometimes it drops to a <500 rpm "lope" - even when set quite high. I've been putting up with it for 28 years, accepting it as just the nature of the beast, but it's occurred to me there may be a solution out there.

Case in point is my '73 B/GT. I have A/C on this car, so I increase the idle up to about 1500 in hot weather, so that it will still idle around 1000 rpm with compressor on. I'll get it just fine in tuning, go out and test-drive it for ten minutes and at least at one stop light it will die unless I turn the A/C off. While there is no evidence that this is due to any mods (same problem before any of them, including the A/C), I will list them just so readers will know exactly what I'm dealing with.

Bored +.030, stock cam, 8.6:1 pistons (as opposed to original 8:1), pertronics and MSD on a Schlemmerized Dizzy, freshly rebuilt carbs from none other than Joe Curto. Car runs great... well, except it starts to break up, backfire, etc just under 5000 rpm. It didn't do until two-three years ago and I can't correlate that with any other change.

I've always been a true-blue SU enthusiast, having little regard for the Weber downdraft, and since tuning is not my strong suit, I consider myself incompatible with the DCOE. But does the Weber DGV improve this situation?

I know Moss used to offer an EFI set-up, but no longer. Any other suggestions?
TIA
Allen Bachelder

Allen-
I've never had much trouble or customer complaints about varying idle speed, so long as the car was in basically decent shape. You may be setting carbs too close to the limits in either direction - rich or lean. I used to set carbs twice a year, for average temp conditions expected for the season. Other guys in the shop couldn't figure out why I always took the cars outside for final setting, but then I was hired to solve the dealership's carb problems - mission accomplished.

Having the idle speed in a steep section of the advance curve can aggravate this. But, if Jeff did your distributor, do you know where the mech advance starts and the rate?

The breakup at high speed is wrong, never had that on any MG. Normally I'd say ignition fault, but it could be fuel starvation, or once more, carbs set at the extremes of passable driveway tuning. You could plumb in a pressure gauge and check fuel delivery pressure under various conditions; this could affect both top end and hot idle issues. When the NGK plugs that everybody loves today were first introduced to the US, they gave them to us for free to get us to use them. I sent them right back, because my Jaguars would not pull past 4500 in 3rd, but were always 6000 redline with the Champions we normally used. Redline 3rd uphill was my standard XKE roadtest - not enough road for fourth!


As far as the AC/idle problem, I've been thinking about this for years, actually ever since you first brought it up a long while back.
Simplest way is to set up for carb vac advance, with a solenoid to switch to manifold vac when AC is on. This would increase idle speed when the compressor kicks in, and could have a parallel thermoswitch to increase idle when it gets Hot. Triumph used such an arrangement on TR6, but with the retard/advance distributor.

You could also arrange a solenoid wedge to open the throttle with AC or temp.
Or a solenoid operated fast idle bypass passage between carbs and manifold, with needle valves to adjust amount of increase (now called IAC valve).

All of these methods, singly or severally, were used by the factories in the last gasp of carbs days.

All sorts of useful solenoid gizmos on cars today, in junkyards.

FRM
FR Millmore

Worn Dizzy. WHen they wear the bob weights get sloppy, and the light, "first" springs doesn't hold. Hence the timing wanders a bit at idle. Put a strobe on the timing marks and you will see the timing marks "wandering" at idle. If you increase revs they steady up.
Allan Reeling

Fletcher,

Thanks, and good to hear from you!

Allan,

A big thanks to you also. Logical explanation but this is a freshly rebuilt, recurved dizzy from the most reputable dizzy guy on this side of the pond. Timing marks don't wander. But, I wonder if the MSD ignition has anything to do with it.

Thanks again to both of you...

Allen
Allen Bachelder

Fletcher,

It just occurred to me that the tachometer doesn't fluctuate when the engine starts breaking up at high rpm, so it must be a fuel problem. Right?

Thanks,
Allen
Allen Bachelder

That last comment of yours Allen fuel! Could be it. If the tach's ok, fuel starvation?Mike
J.M. Doust

Allen-
Not really.
Erratic tach is a sign of primary/LT fault. Your problem sounds like secondary/HT side.

I've not had any experience with the MSD thing, regarding it as one more inscrutable object that nobody has ever demonstrated a need for, but which costs money and will leave you stranded. Put it in the same box as the DGV, makes money for the seller and replaces fixable problems you have with unfixable ones you didn't.

My understanding is that it produces multiple sparks, yes? So, the tach must only read the first one, but the HT side must see all of them. Any decent coil can provide sparks up to beyond 6 or 7,000rpm on a 4 cylinder engine, but sees twice as many on an eight, so they get iffy at high speeds. Almost no coil will feed a 12 at speed, which is why most 12 cylinder cars used twin 6 ignition systems. Jaguar used a bizarre two coils hooked together somehow to fire the V12, but that was a pretty low speed thing.
If the MSD is firing three or more times when you only need one spark, then the coil is seeing a twelve or more cylinder engine. So I suspect a problem there.

I'm pretty sure you have enough bits around to put a plain coil on there, with or without the Pertronix (another black box surprise source!), and see what happens. MGs and other 4 cyl cars have been running fine for years up to 7000rpm on points and Lucas coils.
The stock MGB coil is a "Semi Sports", and will do the job. The "Sports" coil was developed to run racing fours and high rpm/performance sixes, and to extract cash from wannabes. I'm talking real Lucas stuff here, God only knows (maybe!) what you get today.

Or, find somebody with a scope; better yet a RR dyno, which could check fueling at the same time.

FRM
FR Millmore

Add: Model of coils was stamped on them, in addition to actual part number & mfg date. Pedestrian cars like Morris, Austin etc had LA12 coils, MGA/B etc had HA12, Sports coils are SA12.

FRM
FR Millmore

Don't know about the MSD specifically, but some multi-spark systems (I'm thinking maybe Jacobs in particular?) switch to single-spark mode above a certain RPM (ca. 3000 as I recall) for exactly the reason Fletcher mentions -- the coil has trouble discharging & recharging that rapidly. No idea if that's at all pertinent to your situation, but something to consider anyway....
Rob Edwards

This thread was discussed between 25/05/2012 and 28/05/2012

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