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MG MGB Technical - idles too high



hi

79mgb

did some work on my distributor, nothing serious.
i don't believe that my work is the cause of my problem, but it is the only thing i have done recently.

i can't get the car to idle less than about 1100 rpm, i want 900, at least.

i have checked my haynes manual for help but got no improvement in the ldle.
i also think it idles a little bit rough, but that may be my imagination.

any help/suggestions appreciated.

john
john sutter

Timing changes i.e. increasing the advance *will* increase the idle speed for a given throttle opening, however you should *always* be able to unscrew the idle screw(s) to stalling the engine. It's *not* a vacuum leak, which is often mentioned, an engine cannot run on air alone if the butterfly is fully closed.

It could be the throttle cable with no play i.e. the butterfly is hanging open on the cable when the idle (and fast idle) screws are clear of the stops. Assuming the single carb you can forget interconnecting shaft clamps, but could be the butterfly not fully seating in the throat, or the overrun valve in the butterfly stuck open, or the throttle spindle and bushes so worn that mixture is getting past the fully closed butterfly, or the spindle binding in the bushes. See if pressing the throttle cam closed stalls the engine or not.
PaulH Solihull

If you've got a Stromberg carby.check the auto choke. They are prone to failure. Otherwise I suspect "extra" air getting into the manifold.The brake booster or its plumbing could be a cause.I'm not familiar with all the anti smog fittings on a US car, but I think its worth looking there also. Barrie E
B Egerton

I'll say again, whilst a vacuum leak will cause a higher idle *for a given butterfly opening*, you should always be able reduce that with the idle screw(s), to stalling point, unless something is allowing *mixture* to get past the butterfly. An engine cannot run on air alone.
PaulH Solihull

Paul, I totally agree with what you're saying, but I don't think it helps much to find the fault. If mixture is getting into the manifold, it will need quite an amount of air to reach 1100rpm. What we should have emphasised from the start was to make sure the ignition timing was spot on. Barrie E
B Egerton

Go to the Spridget tech board and find "PRIMARY CHARCOAL ADSORPTION CANISTER" about halfway down. Follow that until you get to the updated links posted by Lee Fox on the ZS carb and choke. Copy these to your computer. The Rick Jaskowiak choke article and the comments with it are THE BEST info on this thing, and it is NOT available anywhere else.

FRM
Fletcher R Millmore

"79mgb"

Standard car or modified? (I assume NA version) If latter, how so?

"did some work on my distributor, nothing serious.
i don't believe that my work is the cause of my problem, but it is the only thing i have done recently."

What work - exactly.

What was the car idling at immediately prior to the work you did? Was the work an attempt to 'correct' an idling issue? If another issue, what?

Also, be aware that, with wear over the years and changes to fuel chemistry, the factory recommended idle speed becomes somewhat problematic. It's pretty much a case of what works for the individual car. There will be cases where dramatic differences from spec will indicate issues which are resolvable for an improved outcome.

Not sure that your 1100 rpm is that far from what some others here have offered as their norm effective idle.
Roger T

The point is that mixture *is* obviously getting into the manifold when it shouldn't, if the idle screw is fully backed off. So one examines the butterfly seating in the throat, the overrun valve, and the bushes, and possibly other routes via the emissions plumbing.

Whether air is getting in anywhere it shouldn't, ignition timing, and what the best idle speed for today's fuels are all initially irrelevant to John's problem. Find and cure the problem which prevents you stalling the engine with the idle screw, *then* you can start looking at the niceties.
PaulH Solihull

I'm not sure John can't stall the engine with the idle screw. He did not mention that in the initial post. What I understood John to say was that he had done some work on the distributer and then was not able to reduce his idle speed from 1100 to a desired 900.

I was after the contect of the work to better understand what might be happening.
Roger T

dash it! 'context'
Roger T

If he can't reduce it from 1100 to 900 with the idle screw then he won't be able to stall it either, surely? Ordinarily you don't want to stall it with the screw, but if you can't then there is something wrong.
PaulH Solihull

Until Mr Sutter reveals what he did to the dist. & what he's done since if anything, this has become a lost course. Barrie E
B Egerton

Hi,

Maybe the accelerator linkage is to blame.
To set up the carbs, you disconnect (loosen) the linkage screws, both of them. Then you synchronize them by whatever means, then you set the idle revs on each carby, turning the adjust screws an equal amount, to retain the synchronization. Lastly you tighten the linkage screws, leaving 12 thou between the lever thingy and the choke linkage. If this lever doesn't have any clearance it may be acting as an end stop and all the fiddling with the idle screws, or manually trying to close the throttles won't help.

Herb
Herb Adler

As an American 79 with no indication to the contrary I assumed it had a single Stromberg, hence insufficient free play in the cable (mentioned way back) is the only likely possibility in the area of 'linkage'.

It doesn't matter what he did with the distributor, if he can't get the idle speed down with the idle screw the problem is with the carb.
PaulH Solihull

if you have a manual choke, check that the cable isn't too tight and keeping the choke on.
don g

hi, again

all i did with the distributor was to open the top to check that the rubber diaphram was in good shape, it was. that is all i did, that could not have affected the idle.

where are the first places to look for a leak ?

john


john sutter

Eh? Rubber diaphragm? There is no rubber diaphragm under the distributor cap, unless you mean the anti-flash shield between the rotor and the electronic trigger on 45DE4 or 45DM4 distributors.

Has the vacuum pipe been pulled out of the distributor vacuum capsule or inlet manifold ports? Do you have the TCSA valve (provided from late 76) in the vacuum pipe between those two points?

If that pipe has been dislodged or is split then it *would* cause a vacuum leak, and that *would* cause the idle to be higher than it was before, but as I have said many times you should *always* be able to turn the idle speed down to stalling point with the idle screw, regardless of any vacuum leak, if you can't then there is a problem with the carb. However if there *is* a vacuum leak, then when you fix that the idle speed may well drop again, but it will be just a matter of luck that it drops to a usable level, there is *still* something wrong with the carb that I at least wouldn't live with.

If you have the TCSA and it is working correctly any vacuum leak can only be between it and the inlet manifold, the valve isolates the distributor unless 4th gear is selected. Even if the pipe between the TCSA and the manifold is sound the valve itself could be leaking. If no TCSA then it could be the vacuum capsule leaking as well as the pipe or its rubber connectors. The first thing to do is remove the rubber connector for that pipe from the inlet manifold port, temporarily seal that port, and see what difference that makes. If none do the same thing with each of the other ports on the inlet manifold, one at a time. If you have the servo assisted brakes then do that port as well. If none of them make a difference then if it *is* a vacuum leak then it is elsewhere, like a gasket between carb and manifold or manifold and head, or a loose plug or bolt on the manifold, which you might be able to find by spraying carb-cleaner or preferably propane round them.

If none of that makes a difference check the timing, if that is advanced it will cause a higher idle, *which should still be able to be turned back down again with the idle speed screw*. If not that then it can really only be something hanging up on the carb. Or did I already mention that ...
PaulH Solihull

Don't want to insult the OP, but maybe he confused dizzy for carb and really means he opened the top of his Stromberg up, dislodged the diaphram maybe and didn't get it screwed back down correctly/air tight.

From what he's said that's about all that makes sense.

It's my experience that as soon as you open something made of rubber/plastic/(anything not metal) to check it, said item will look fine but secretly disintegrate on re-assembly in protest of being touched after 30 years.
RoadWarrior

That occurred to me, or that he opened up the vacuum capsule, or the EGR valve etc. But I opted for no one making that much of a mistake ... however when you consider one chap asked why his suspension was at full stretch when he didn't have the engine or gearbox installed, or the same question from another when his car was upside down on a rotator, maybe he did.
PaulH Solihull

Once had a woman with constant complaints of poor fuel mileage and rough running.

A "joint" driving session (with her doing the driving) indicated that her use of the manual choke,pulled out to max, and used as a "hanger" for her purse, was the source of the problem.
Never underestimate the power of the human mind to re-invent the wheel.
JR Ross

This thread was discussed between 28/07/2010 and 11/08/2010

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