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MG Midget and Sprite Technical - 79 Midget 1500 idle speed

Looking for some advice from the guys in the know!

Idle speed setting for a 1500 is specified as 680-700 RPM in the Haynes manual. I can only get my girl to run smoothly at about 1000 RPM even after I have checked the valve clearances (0.25mm), set the dynamic timing to 10 degrees BTDC (gunson timing light), balanced the carbs (Gunson carbalancer) and checked the mixture to be correct (gunson Colourtune).Fast idle screws are also not fouling the cold start mechanism (correct clearance) and I have pushed down on the throttle linkage to take care of an old and knackered throttle return spring which I will replace anyway later this week.

I've been quite thorough in the process and was wondering whether or not 1000 RPM is acceptable for a well run in engine and carbs? or have I missed something maybe?


D Williamson

Dave can't you drop the rpm using the throttle stop screws on the carb body? if not then must be throttles worn and leaking too much air or stuck open a bit...
John Barber

Hi John

I marked the throttle stop screw head flat positions against the top of the screw recess on the carb and moved each screw anti-clockwise the same amount a little bit at a time to drop the RPM. The only problem with doing this was that the running became a little more rough, a little more put-put through the exhaust and a steering wheel judder becoming more apparent. I found that if I moved the throttle stop screws back into their original positions that the exhaust note was much smoother and the steering wheel stopped juddering completely.

Maybe as I have changed the air filter to K&N Pancakes on each carb, then I need to repeatedly alter the ignition timing and throttle stop screws until I get the best balance of RPM versus smooth running without any pinking. What do you think?

I may sound like I know what I'm talking about, but really I don't!


D Williamson

K&N will not affect idle, unless your old filters were appallingly clogged.
Rough running at slower idle speeds indicates some other fault or discrepancy between cylinders - recheck valve clearance first, then compression test, plugs cap wires, vac leaks. Any stock engine should be capable of smooth idle down to 500rpm, although that will be a nuisance to drive because it is too easy for YOU to stall.

1000 is acceptable to me if it is to you, but run-on may become an issue, especially at high ambient.

FR Millmore

If you set your ignition timing at 1000rpm - it may be a little retarded at 600-700. The centrifugal advance at 1100 is anywhere between 2-6 deg. It may be as you then lower the engine speed you could be at 4-8 degrees advanced instead of 10?

I think to really fgive it a chance at 700 you need to check this and then maybe the mixture is worth checking with the piston lifting pin routine ( its in the owners manual).

If these don't improve matters then as FRM says it could be something is amiss somewhere...

I know the spark scatter can be big on old worn distributors and this won't help - but your strobe should show this when you shine it on the timing marks...

good luck !
John Barber

Thanks for your help guys, much appreciated!

I think what I have done from reading your replies is set the dynamic timing at 1000 RPM. I will try and reduce the idle speed (OK it will be a bit lumpy) and then mess with the ignition timing to see if I can make it run smoother. Failing that, I'll have a go at the mixture setting (revs should rise and then fall back to steady idle speed) if I remember this correctly that is :-)

D Williamson

I wouldn't always trust figures and information in a publication like Haynes - I seen a couple of errors for the 1500 in Haynes and I'm only ever looking for info on 1275s

swapping to K&Ns would probably mean adjusting mixture and possibly needles (adding stubs stacks is also recommended by some)

adjustment sequence would be tappets, point/plugs, timing then mixture

using factory settings as an initial guide is a good idea but your car isn't new and you made not know what adjustments, alterations, modifications, uprovements, upgrades it has had before your ownership so your car may run better slightly (or more) away from the book figures
Nigel Atkins

To add to all that, what are you using to measure the idle speed? The tacho gauge is likley to be nonsense.

Does it sound like 1000?

I'd get a magnetic induction pick up on a mulitmeter to check the rpm.

700 is quite low... my steeringwheel judders a lot at that speed and I can barely hear the engine tick over.

Personally I like to have idle at 800
C L Carter

I'd just go by how it sounds and feels, allow for incurracies of speedo and wear of dissy, engine, and ignition a 1,000 might be ok until, or even after, the car is fully serviced, set up and run regularly

it's all a compromise with carbs too, if set to run perfectly on one day then the variables of weather, car wear and tear, traffic conditions, ect., ect. will mean it could be out for other days

if you've not already done so, once you've fully serviced, set it up and run the car regularly look at it again if required
Nigel Atkins

This thread was discussed between 10/07/2012 and 17/07/2012

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