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MG Midget and Sprite Technical - Some don't like it hot

My 79 1500 midget won't idle at temps over 85 deg.(F).After a 20 mile or so ride in hot weather I pull up to a stop sign or red light the engine dies immediately but starts right up at the turn of the key. I have to keep my foot on the gas pedal to keep the rpm's at 1000. If I take my foot off the gas it will immediately stall. At cooler temps and around town no problem. What could this possibly be ?
Don M
D.J. McQueen

I may add that the temp gauge shows the needle at 3/4 deflection. (Close to red but not boiling)
D.J. McQueen

Don. It sounds like the idle speed mechanism is not adjusted correctly. There should be a fast idle adjustment (cold operation) and a hot idle adjustment. It sounds like you need to adjust so that the hot idle is operating correctly. But, this is all from memory. I have not used the Z-S carb on my cars for a number of years now. Others will have more accurate information, but check out your workshop manual so you will be able to understand what they are referring to.

Les
Les Bengtson

Sounds to me like the automatic choke is faulty.
S.A. Jones

I agree with Les. Have a look at the idle speed adjustment, and see if you can dial it up to 900 or so RPM.

And yeah, the choke is always worth a look on the Zenith. Have a look at this website:

http://www.paulbunyan.net/~jasko/choke/

It's a great tutorial on how the choke works, whether automatic or manual conversion. Central to the picture is the tiny little O-ring on the choke metering needle, which can fail and give you an enriched mix at idle even with the choke off.

BTW, the tutorial mentions removing the brass sleeve around the choke needle. There's no need for that - just pull the needle out through the hole provided. You'll get it when you see the pictures.

You can also get similar symptoms if your float valve is leaking slightly. At idle, fuel is being used slowly enough that a gunky valve will allow fuel level to increase slowly in the bowl until the mix is too rich to sustain running. Holding the throttle a bit, or actually driving, uses enough fuel to drop the level. But the next time you idle for a bit, the fuel level will rise again, flood the engine, and stall it. Been there.

Best of luck, and report back,

-:G:-
Gryf Ketcherside

I disconnected the auto choke a long time ago and I now use manual choke
Don McQueen

Also if I adjust the idle up any further it will be way to high at cooler temps. (outside air temps)
Don

how recently were your carbs rebuilt don?
S.A. Jones

One ZS carb. Rebuilt a couple of yrs ago.I was just thinking could a dash pot low on oil cause that condition ? One way to find out I'll go check
Thanks
Don

>>> I disconnected the auto choke a long time ago and I now use manual choke. <<<

Mine's also got a manual conversion, but the innards still work the same as the automatic model, and need attention every now and then as described on the website I sent you.

Low oil in the dashpot will only affect acceleration. As you probably know, the dashpot's primary purpose is to resist upward movement during acceleration, thus enriching the mixture during times of peak demand.

This may be a long shot, but it's stalled me at least once; when you unscrew the damper cap and remove the little valve from the dashpot, look just under the upper edge of the threaded bit at the top of the carb body. There should be a little vent hole; make sure it's clear. I had it plug up once, and the engine died at a stop sign when the air piston failed to fall all the way to the bottom when I let off the throttle. I removed the air filter and looked in the carb, to see that the piston was hung about halfway up; so I pushed it up with my finger, and it then fell freely all the way to the bottom, and the car started up fine. All I can figure is that there was gunk in that vent hole, and it popped out when I lifted the piston. I haven't had that problem since.

Good luck,

-:G:-
Gryf Ketcherside

Hey, do you have a heatshield in place?

-:G:-
Gryf Ketcherside

Gryf, I took the heat shield off when I removed the cat. Think I should hook it back up ?
Don M
Don

>>> Gryf, I took the heat shield off when I removed the cat. Think I should hook it back up? <<<

Couldn't hurt. Even with the cat gone, there's still a lot of heat radiating from down there. I'd give it a shot and see if the problem disappears... if not, at least you've eliminated the missing heatshield as a cause.

Cheers,

-:G:-
Gryf Ketcherside

Gryf, OK put the heat shield back on. took a 40 mi. trip and it idles just fine when hot. I guess it was the shield. Thanks for the tip
Don M
Don

Yeah!

Great to hear that worked! Now we need to see if it KEEPS working.

-:G:-
Gryf Ketcherside

This thread was discussed between 23/05/2008 and 08/06/2008

MG Midget and Sprite Technical index

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