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MG MGA - High Temp & Low Idle

Hi All,
Looking for help with my latest problem. The weather has turned hot here (B.C.) and my problem is that the engine will start and run fine but after warming up to a gauge reading of 190 or so the idle speed drops so significantly that the car will actually stall completely if I don't "blip" the thottle. When started the idle is fine at about 8 to 900 RPM. This is a stock 1500 with the twin SU's and it behaves perfectly unless warmed up. Any ideas? Thanks in advance.
Tom Heath

Might be vapor lock. Are the carb spacers, heat shield with insulation pads in their proper place? Your Idle setting is a tad low. Set it to 900-1000 Rpm.
D Sjostrom

Hi Tom, my 1500 behaved the same way. if I adjusted for a nice tickover with the engine warmed up in the garage, I found the tickover to be too slow with the engine really hot on a run. The answer is to adjust the tickover when the engine is as hot as it can be. There are two things you can do to increase the tickover when hot, first, I would try advancing the ignition a little on the micrometer adjuster, try just a few clicks, no more than about five, and see if it gets you where you want to be. If that doesn't make any difference, back the ignition to where it was and turn the tickover srews up on the carbs; the same on each of course. I don't think its any kind of fault, just the nature of the beast. Adjust it after a good hard run, it worked for me.
Lindsay.
Lindsay Sampford

Tom - is it just this year that the problem's turned up? Or has it always been like that?
Nick

My favourite - rear filter box on upside down, blocking the vacuum balancing vent to the piston.

Steve
Steve Gyles

Tom,
You may have a sticky choke.
Once warmed up and you have pushed the choke knob fully in on the dash, reach under the carbs and push up on the choke mechanism where they both connect to the bottom of the carburetors. With too rich a mixture the engine starts loading up and slowly dies.
I would also check the idle and ignition as suggested.

Yes,... 103 degrees F, in Seattle was one hot day... glad it will only get to about 80 today..

Ray
Ray Ammeter

Problem is the mixture goes rich when hot. 20F is very noticeable. I find that most people set the mixture somwhat too rich at "normal" temps, and when it gets really hot, it's just plain drowning. I especially like the ones who brag about "starts first time cold, without the choke". It fouls the plugs, eats gas, washes the oil of the cylinder walls, and other evil things. Most cars should stumble on moderate acceleration without choke until the engine temp is at least halfway to normal.

When I was a Volvo/Triumph carb specialist, I would final set all cars outside the shop, compensating for unusual weather conditions. In winter, it was not uncommon to have to change the carbs by as much as 3 or 4 flats from 70F inside to 30F outside, especially the late 60s cars which were right on the lean edge for emissions reasons. Dat's why new cars have computerized FI.

On cars that I had tuned, I got so I could reset the carbs with the engine not running, just by the temperatures outside. Yep, I had an SU jet wrench on my keychain!

Fiddling with timing is out, and it should not be necessary to change the idle speed stops either. Normally I would set them once in spring, and again in the fall. Extreme cold - 0F, or heat - 100F, meant reset again. I once drove a Spridget in -30F and ran out of jet adjustment, had to run with choke out, nearly all the way, even with the rad blocked completely.

FRM
FR Millmore

Thanks for all your suggestions - I will start working my way through them. The problem is fairly new - likely coincides with carburetor work I had done a couple of years ago so I may start with the mixture first.
Tom Heath

This thread was discussed between 01/08/2009 and 04/08/2009

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