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MG MGA - Varying idle

This has been an ongoing issue with my MGA.

When the car fully warms up the idle drops by up to 200 rpm and becomes much less even.

Before the rebuild I chalked it up to leaky carbs or an old distributor.

I rebuilt the engine last year including having Joe Curto rebuild the carbs and Jeff at Advanced Distributor rebuild the distributor. The idle still drops when fully warmed up.

When I say fully warmed up I mean 10 or 15 minutes.

Anybody have ANY ideas? One thing that occurred to me is that with heat the oil in the dashpots would get less viscous and could change the idle (I use 20 weight) Any other thoughts.

Thanks
T McCarthy

I don't have a solution for you, but for sure it wouldn't have anything to do with the viscosity of the oil in the dashpots. The dashpot oil comes into play only during acceleration. It has no effect while idling.
Andy Bounsall

My old favourite, always worth a check - rear filter box on upside down?

Steve
Steve Gyles

Maybe you have a sticking choke cable. Have you checked it has some slack when it is pushed fully in?
Also, the "slow running cam" could be sticking, or any of the choke levers that pull down the jets could be hanging up and then releasing.
The dashpot oil will not affect idle.
Also check manifold nuts are all torqued to about 25lbs (cold), as reduced idle could be due to air getting in thru a loose manifold.
Peter.
P. Tilbury

Could be a too rich mixture caused by reduced air flow, or sticking choke cable or levers, or just incorrect mixture adjustment. Also, could be electrical component like the coil or condenser breaking down when hot.
Jeff Schultz

Thanks for the thoughts.

Steve, ever since your original post on the filter boxes, moons ago, I've been very diligent about that. I'll check again.

I have replaced the distributor with electic components, but the ingition coil is a very good suggestion. Its the only thing I haven't replaced in the system. Is there a way to test the coil as it heats up?
T McCarthy

When the coil breaks down due to overheating, you will get a missfire. It will not affect idle.
Reduced idle points to a weaker mixture, ie, more air getting in to lean out the air/fuel mixture.
This assumes you have warmed up the car fully, released the choke completely, and disconnected the linkage between the carbs, when setting the initial idle speed. Also both carbs should be pulling equally at idle - check the sound of the air intake using a hose/tube to your ear, with the airfilters off, at the carb intakes.
Let us know if and when you find the source of your problem.
Peter.
P. Tilbury

When you mount the carbs on the inlet mainfold, I've found that it's essential to get both carb throttle spindles aligned so they are perfectly in line with each other. Otherwise when you connect them with the "funky" linkage it will tend to bind the complete return of one or other of the carbs. This leads to an irregular idle which can be either too fast or too slow depending on the return springs binding a little.

Andy
Andy Preston

I recall a similar situation with my A once, turned out to be, very simply, torqueing the bolts that hold the carbs to the manifold, it was barely noticable but after they were tightend the problem was solved...not even sure how they loosened but over time I guess its always possible. Hope its that simple.

Christine
C Burnham

This thread was discussed between 05/05/2009 and 09/05/2009

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This thread is from the archive. The Live MG MGA BBS is active now.