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MG MGB Technical - can't turn down the idle, too high


when the engine is warming up all is fine,idles at 900. timing set at ~20 btdc, and a little lean (gunson color tune)
runs great, nice pickup, etc. it takes a bit of time to get really hot.
but, when engine is hot, it idles too high ~ 1500 and i can't bring it down. it also idles a little rough,
when on the road, at a decent speed, you can't tell that something is not right.



john sutter

What carburation do you have?

If you can't turn the idle down, is your accelerator cable too tight?
Dave O'Neill2

After Dave's point, I would focus on timing - I can't give you a definitive figure (don't know whether you have a high or low compression engine) but some versions of the 18V work best at 10 degrees - others at 12, 13 or 14.

If the cable is not too tight, try running at 14 degrees and see what happens - you won't do any harm.

On the question of fuel mixture, take care that it's not too lean - this is not a lean burn engine - too lean and you will burn out the valves.

HTH Roger
R Walker

Dave Braun should be chiming in on this. He rebuilds SU's. Just told me that my carb shafts were very tight and this would result in high idle. I am wondering if yours are also, so that when they warm up they get just a bit tighter and don't allow the idle to drop.
Bruce Cunha

A few points here 20deg BTDC is too advanced, unless of course you're not diconnecting the vacuum pipe and the engine speed is too high when you are setting it! You may have a dizzy problem over advancing the timinge. e.g., sticking bob weights or weak or broken springs.
A closed butterfly on any carb should stop the engine. If you are on SU's first of all check that they are able to close, also that the choke is returning fully Similarly with Strombergs, but with the latter check the diaphram for holes.
Air leaks can also cause raised idle speed Can be worn spindles, manifold/gasket leaks.
The centre spindles joining the two carbs should have some lateral movement as well.
Allan Reeling

Not being able to turn the idle down to stalling can be caused by the accelerator cable being too tight or worn throttle spindles and bushes as has been mentioned.

It can't be caused by manifold/gasket leaks though, as some fuel must be getting through the carbs for the engine to run. It can't be caused by incorrect timing for the same reason.

Vacuum leaks and over-advanced timing *do* cause a high idle for a given idle screw adjustment, but it should always be possible to stall the engine by unscrewing the idle screws.

Other causes are overrun valves in the butterflies not seating properly, or butterflies not seating in the carb throat fully, or sticky linkages. This last is easily checked by pressing on the butterfly cam and seeing if the idle slows.

With twin carbs it can also be caused by incorrect adjustment of the interconnecting spindles and clamps - one butterfly fully closed holding the other one open a little.
PaulH Solihull

It's a 79 US car:
ZS carb, one
Book timing is 10 BTDC @ 1500
These particular cars are remarkably insensitive to timing. I frequently find they run better at odd settings, but they don't seem to care very much. "Better" is definitely relative; they are never very good.
Mr Sutter has been fiddling with this in a pretty haphazard manner for a while. I suspect it has various carb and other issues that could only be found by a methodical go-over by a competent tuner.

FR Millmore

79mgb z-s carb

ok, i backed off to ~17degrees btdc, it still idles too high.
now, whats the matter with advencing the timing to 20 d btdc ? i thought that one advenced the timing until one gets preignition 'ping' and then backs off, this gets best performance, or is this just an old mechanics tale. anyway thats why it was at 20 d.
going to 17 d , no change. the 17 d was the recomendation of jeff schlemmer who rebuilt it a couple of years ago.

accelerator cable not too tight. mixture not leaned out very much, just a wee bit after the change from yellow to blue. gunson

vacuum leak ? likely sources are----?


mr millmore wants to take all the fun out of having
an mg, shame !
john sutter

Sounds like your auto choke. You can loosen the ring that holds it in the position that it is in and rotate either clockwise or counter clockwise to affect the idle rpm. There should be some good articles on the zenith auto choke and I believe john twist has a video as well.

Best of Luck,


P.S. Worst case scenario you can fit a manual choke.

"mr millmore wants to take all the fun out of having
an mg, shame !"
Far from it. I want to lessen the haphazard approach to car fixing, and with it the great useless mysteries that pervade the automotive universe. You could have a great deal of fun becoming a "competent tuner" yourself.

Changing idle speed by changing timing is not correct; you set timing then set idle speed by appropriate adjustment of the carb idle and throttle stops. I'm sure Jeff's timing figure is good, but be aware that the speed at which it is set is critical. For emissions reasons, this was specified at a very steep point on the advance curve, which makes life difficult.

And you do not adjust fast or "on-choke" idle by changing the temperature at which the choke operates, which is what JRB's idea does. There are separate setscrews for regular idle and fast idle, but if the fast idle mechanism is not working correctly, it can be impossible to set these.

This is the best piece on the ZS carb and choke:

The Moss MGB catalogue has a pretty good but incomplete discussion from John Twist, especially missing some choke info. I have not seen the Twist video, being unable to get such on my piss poor internet connection.

FR Millmore

Forget the timing and a vacuum leak. If the carb butterfly is fully closed the engine simply can't run - *unless* the spindle and bushes are so worn that mixture can creep round past the closed butterfly. Auto choke is a possibility - holding the butterfly open even though the accelerator cable is fully released and the idle screw backed off.
PaulH Solihull

I saw the 79B in John's first post and knew it had to be a ZS carb and I don't have as much experience with those. As an aside, Bruce's throttle shafts were riding in teflon bushings and one of them wasn't reamed / installed very straight at that. Teflon can cold flow and cause an egg shaped opening which can bind the shafts. Some HD carbs had a flat teflon bush held in by a ring, but those started as a flat ribbon.

Dave Braun

I saw the 79 too, but that is no guarantee it still has the original carb.
PaulH Solihull

original z-s carb 175cd , but with the automatic choke stuff removed.

i think i have solved the problem of the fast idle that i could not lower.

it was the vacuum servo assembly, it leaks.
i put a clamp on the vacuum hose from engine to unit.
as i was tightening it down, the revs dropped to normal, 900. i let the engine run for a while and the revs stayed at 900. before, the revs would have risen to at least 1500-1600 by now.

comments ??

question: can one remove the vacuunm unit without having to also remove all the brake master cylinder stuff ?? i do not like that chore.

i havent tried, but are the brakes still o.k. , but that you have to apply more foot pressure to stop ??

i hope i haven't forgotten another guestion that i wanted to ask

thanks !!!


john sutter

Glad that you found the issue.

When you get this sorted out, check your mixture. Since the vacuum booster port is downstream of the carburettor the air is not metered. Likely you are too rich now at idle whereas before you were slightly lean per the "colortune".

Sorry, I can't answer the servo removal and replacement question. My 65 has only the "leg powered" servo.

Bob McCoy
Robert McCoy

Did they change the master cylinder diameters when they put the servo on? If I remember rightly, the size of the master cylinder remained the same. If the servo isn't actually doing any work, you then have standard MGB brakes.

My 1964 B also has the "leg powered" servo which seems enough!

Mike Standring

I'll say again that you should always be able to stall the engine by unscrewing the idle screws. Whilst a vacuum leak will cause a high idle *for a given idle screw setting*, an engine can only run if mixture is getting through or past the butterfly, and the idle screw should be able to close that completely. If you can't get the idle below 900, and in fact stall the engine, then your carb still has a fault. However 900rpm may be good enough for you.

Master cylinders with the integral servo are different to the earlier masters which used either no servo or the remote servo. I'm told that the servo gives *significant* assistance when it is part of the master, and loss of it leads to very heavy brakes.

The remote servo was optional therefore the brakes without must be perfectly usable, and having driven them with the servo disabled I can confirm the difference in those is negligible. I've recently driven one where I didn't realise it didn't even have a servo, and never noticed.
PaulH Solihull

you say, ' unscrew the idle screws ' i only know of one ? the one near my manual choke cable!!

also, a lot of stuff has been removed from the mg.
eg: no air pump or cleaner ,etc, which probably has no bearing om the problem

but, could not the leaking vacuum servo assembly be the sole cause of high revs ?

i will check more carefully the idle screws to see if it affects the revs

i don't really know much about the brake servo, its such a pain to remove i'd just like to get rid of it.

john sutter


yuz iz right, i cant idle below about 900-1000. in fact, i guess it has been that way for as long as i have had the car.

any hints on how i might correct the carb problem?
yeh. i know, take it to a real mechanic.

john sutter

You could give some appropriate respect for the people who have evidently wasted their time explaining things and giving you the links to what you need to know. Both the ZS link I gave above and the Twist piece in the Moss catalog tell you how the choke/idle systems work.

FR Millmore

The aforegoing says it all, as far as I'm aware. To know how to correct the carb problem you first have to know what is wrong, and previous posts should give all the places to look and things to check.
PaulH Solihull

sorry to be a pest but with my age and memory i need all the suggestions with variations i can get.
john sutter

John, actually you've made a lot of progress on this thread. Between you and the contributors you've narrowed the problem of high idle to your carb.

If you're uncomfortable getting into the ZS I suggest taking it an expert. You don't want to ruin your catalytic converter with too rich a mixture or burn a valve with too lean a mixture. I personally suspect a defective fast idle circuit in the choke mechanism. As stated by others, if you back out the idle speed screw you should stall the engine unless the throttle isn't closing or the fast idle circuit is malfunctioning.

One option is Joe Curto ( who specializes in British carburetors. You'd have to remove it and ship it to him. Another is someone who does British car work. There must be someone in the DC area. The carb should be easy to remove. Just get a good manual and follow the steps.

Unfortunately most mechanics don't understand carburetors anymore let alone British carburetors which operate on a different theory. So taking it to a general mechanic is probably not going to give you satisfaction.

You then will need to make sure you have no vacuum leaks like the one you found on the servo, that the timing is correct.

All the best.

Robert McCoy

Well John, we've been trying, but we can't fix the car by teleportation.

Go to the link I gave you above and find the picture of the two idle screws. Even if "...the automatic choke stuff removed", most conversions still have the choke fast idle screw as well as the basic one. It is very common that people set the base idle with the fast idle screw, which gives too much fast idle. It is even possible that one or the other is missing, but the (possibly stripped) hole for it will be there. And if the choke mechanism is sticky, idle speed and possibly mixture at all times will be erratic. You need to identify these things for us. Maybe a picture of the choke would help.

And congrats on finding the vacuum leak, lots of "Pros" miss those.

FR Millmore

This thread was discussed between 10/01/2012 and 20/01/2012

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