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MG MGB Technical - what is the correct idle speed

hi can you tell me what is the correct idle speed for my 1979 mgbgt and how to set or adjust regards vince
vince aldy

The original figure was 850rpm but with modern fuels you may need to set it a little higher to get a smooth idle.

The idle speed is set with the idle screws on each carb, shown on the attached. Only ever turn both screws by the same amount in the same direction to get the desired speed. If you only alter one, or move them in different directions, you will upset the balance between the carbs, the engine will run less well, and they will have to be rebalanced from first principles. That is, if they are balanced to begin with!

PaulH Solihull

An idle setting of 1000rpm is reasonable for a 79B with electric fans.

When the fans come on in traffic on a hot day the rpm's drop some and then when you depress the clutch the rpm's drop some more.

That's a total drop of around 100rpm's that you should compensate for by having a slightly higher idle. Otherwise you may have an embarrassing stall when you attempt to move.
Steven Rechter

thank you some much. i have adjusted the carbs from your image, and the point made ref the elecrtic fan. so have set around 950rpm runs great. regards vince
vince aldy

I must nose in and echo Paul's observation. Over the years, I've seen lots of guys doing the exactly wrong idle adjust technigue. As noted, the adjust screws must be turned the exact same up or down, and the suck, measured by a carb-sync vac or a hose to ear, must be the same when you finish. I've seen many carbs, with the pistons apparently mixed up: one drops in 6 seconds, the other in 3!
Finally, u b just wasting your time if you don't have the proper 11-14 AFR, with the jet height equal in both carbs. This done,and with correct ignition advance with proper dizzy vac, the idle is steady between 600-800 RPM. With manifold vac and new HIF SU, even with a Piper 285, I can sustain idle down to 450 RPM fairly smoothly. VEM
vem myers

My 79 mgb idles in nice and smooth around 1000-1050. Dip the clutch it drops to 800-850.

cheers

Gary
79 mgb
gary hansen

This thread was discussed between 07/04/2012 and 28/04/2012

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