Mgb gt idle drops once it’s warm

[Accelerated]

The initial message was deleted from this topic on 01 September 2019 at 22:35

[phumy]

There is an idle "stop/adjust" screw on the throttle linkage on the carbs, any idle adjustment should be done on that screw in small increments until the idle is at where it should be. You dont want to be running on your choke at normal running temperature.

[harrycovert]

Idling stop screw

[Allan L]

phumy said:

You don't want to be running on your choke at normal running temperature.

As is shown in the drawing above, on post-war SUs there is a jet/idle interconnect cam which speeds up the idle when the mixture enrichment is operated. For the first small movement of the "choke" knob the jet doesn't move and the slight increase in idle speed is all there is, so no harm in it.
ETA you say it stalls when you "drop" the clutch, so I have to ask why you expect anything else. Normal driving involves the use of the throttle pedal when engaging the drive.

Edited by Allan L on Saturday 31st August 09:34

[9xxNick]

I think I'd be trying to establish why a warm engine idles more slowly than a cold one, all other things being equal - in this case that means with the choke "off" for both situations.

If the mixture's too rich that could cause the depressed hot idle, but you really need someone who knows their way around an SU carburettor to take a proper look. Haynes also do (did?) quite a decent book on the SU which showed you how to set them up from scratch.

[Peter3442]

To me an idle speed of 600 to 800 rpm when hot is about right. I'm not an expert on B series engines or MGBs (if you re-post under technical/engines, there's a world expert), but that's typical of a four-cylinder of that period. Modern engines idle faster because they have a load of gubbins to drive.