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MG Midget and Sprite Technical - Tickover speed query?

Hi all,
1972 Mk 3 midget standard SU carbs
Could I have your opinions please on how to sort this one out?
My carbs are fuelling really well ( lift pin and gas analyser and plug condition etc ) _ and the car runs very nicely.
I set the tick over speed to about 1000 rpm, using the cars tacho and an external rev counter attached to the coil. They both agreed.
My query is that if I'm driving, when I come to a halt ( lights , roundabouts etc ) the revs drop to about 1500 rpm and then only gradually drops back to the 1000 rpm set up ....any ideas why the car doesn't go to tickover revs directly once my foot is off the accelerator so only the " tickover" fuel flow is going through ?
Grateful for any thoughts
Cheers colin
colin frowen

Might be a bit left field - what throttle cable do you use? Cars like ours really benefit from PTFE cables. I have made up several for various old cars from the Venhill kit and all have improved the throttle control.
Also, where the cable passes through the firewall the panel is "dimpled". Check that that area of the panel is straight. If it's bent the cable doesn't have a straight run to the pedal when the pedal is at the top of its travel, which is when the return spring tension is least.
I've seen this several times, it causes just enough cable drag to give the symptoms you describe.
I think the panel bending comes over time from heavy right boots but it's an easy fix.
Greybeard

Just a thought as I have the same year/model midget - are the throttle returns springs OK?
D Price

This can also be caused by a slight manifold leak. As you decelerate the carb butterfly closes, cutting off the air supply and causing the manifold pressure to drop dramatically. You stop but the low manifold pressure sucks air in (from a leaking gasket?). This weakens the mixture so the engine still runs a bit fast for a few moments, until the manifold pressure equalises, mixture richens again and tickover settles down.
GuyW

Colin

The inlet gasket would be my initial thought too.
if not is there any excessive play in the throttle spindles creating an air leak?

Bob Beaumont

Hi Guys,
Many thanks for the suggestions.
My throttle return springs are new this year , but I will certainly check out my inlet gaskets and also my throttle cable. ( Never even thought of that!!)
Cheers all , and many thanks again
colin
colin frowen

I was getting this, once of the potential causes is an air leak round the spindle(s). My problem has gone away since I had the carb rebushed.
Jeremy Cogman

This thread was discussed between 29/09/2022 and 04/10/2022

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