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MG MGB Technical - High idle speed

Hi, my MGB GT 1800 has a minimum idle speed of 1300 rpm. The adjustment screw just touches the carb body, any more undone means a gap so I assume the butterfly is fully closed. The carbs are both balanced using a gunson balancer, but cannot reduce idle speed any more. Apart from that, the engine starts without choke and runs well. Appreciate any advice, thanks.
P J Dyble

I'm sure you'll get answers here and there'll be threads on here and in the Archives here to help you but this subject came up recently on the 'midget and Sprite Technical' forum so you could look at some of the suggestions there on the thread called ‘Cannot slow down idle speed!’

there's also Paul Hunt's excellent website mgb-stuff

I can also post links to John Twist videos on the carbs and setting them up if you want them
Nigel Atkins

Hi PJ,

A couple of suspects.
The fast idle screws on the choke cams need to be backed off.
The accelerator cable is too tight.
The throttle linkage is hitting the choke linkage.

Herb
Herb Adler

You could have a vacuum leak at the intake manifold - your other thread gives a clue that the brake servo might be a leak source. Try disconnecting and capping the intake manifold ports one by one.

Kevin
K J P Kelleher

Once again, a vacuum leak cannot cause a high idle that cannot be compensated for by the idle control screws, a common misconception. Yes, a vacuum leak will cause a high idle *for a given idle screw setting*, but you should always be able to back off the idle screws to stalling point. An engine cannot run on air alone, it must have fuel as well, and so if you can't adjust the idle low enough this must be getting past the butterflies (or through a faulty poppet valve in the butterflies on HIFs) somehow. If you can't then like Herb says either the throttle cable doesn't have the required free play i.e. it is holding both butterflies open, or the fast idle screw(s) is holding a butterfly open, or the linkages are maladjusted such that one carb fully closed is holding the other partially open. Another possibility is grossly worn throttle shafts and bushes such that mixture can get past the fully closed butterfly.

To start from cold without choke indicates other problems, but more like a rich mixture. Maybe the carbs need setting up for air flow and mixture balance from scratch.
Paul Hunt

Also HIF carbs can have seal failure on the choke valve which allows the excess fuel into the engine.

High fuel level in the float chamber will also supply additional fuel.

No thottle plate I have ever seen has been airtight - most have gaps around the throttle shaft and wear on the shaft bushes that are more than enough to supply the air with a raised fuel level or choke leak to give 1300 rpm idle even with the idle screws right off.
Chris at Octarine Services

I note you said "Idle screw" but there is one for each carb, you must make sure the linkage between the two carbs are undone and loose to balance them, don't bother with the gunson, use a length of hose and listen to venturi hiss.
c cummins

Hi,

About balancing the carbs, there is a very cheap and simple way to do it. No rubber hoses, no Gunsons thing, no Unisync, bit very accurate. Simply two bits of wire. I use "soft" wire to avoid damage to the tube walls.
Once the wires are set then they should stay level across the whole rev range.

Herb


Herb Adler

This thread was discussed between 20/06/2013 and 21/06/2013

MG MGB Technical index

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