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MG Midget and Sprite Technical - MG won't start hot, but cranks fine

I have a 1976 MG midget, when I first start it up, runs great. Afer I have driven it for a bit, turn it off and try to restart, cranks but won't start. Eventually, just before I have given up, it starts and again runs great again.Happens repeatedly. I have sprayed start up fluid in the carb when it happens, but no help, leading me to think it's electrical in nature. The engine is just rebuilt and tuned up. Any help is appreciated.
BB Bauer

Mine did this and it drove me nuts as none of the MG experts could solve it. I have the factory Zenith-Stromberg carburetor on mine.

In my case, it was an old diaphragm in the carburetor. When it was cold the rubber was hard and the vacuum lifted the piston allowing the car to start. When it was hot, it was so rubbery and pliable it just stretched and wouldn't lift the piston enough to start the car.

I changed it for new one and even after running in 90 degree heat, it will start up right away.

When it does it again, try lifting the piston in the carburetor a little and keeping it raised with a screwdriver or a friend's hand and see if it will start.

Perhaps this might be your problem too. Oh, spraying starting fluid into the carb will soon ruin your diaphragm. Always carry a spare in your toolbox just in case.
Clive Reddin

Start-up fluid will ruin the diaphragm.
darnoc31

If not the diaphram:

Was the heatshield replaced between carburettor and engine when the engine was replaced after it's rebuild?
Sounds a bit like fuel vapourisation problems to me.
(I presume US Midgets have heatshields?)
JB Anderson

If you have discounted fuel being the problem and are starting to look at the electrical side of things, I'd be inclined to investigate and eliminate the coil first off. When warm/hot they can become problematic with internal shorts; when cool working again.
Andy Hock

I agree w/ Andy.
If not the fuel, check the coil next. If the key is left on for too long, the coil will burn up. It may work when cold, but shorts out when hot.
Phil Burke

Thanks for the feedback from all. I will try lifting the piston in the carburetor a bit and keeping it raised to see if it will start then.
No, I don't have the heatshield you mention and as you can imagine down here in Texas we are in the upper 90s every day lately. Will let you know the outcome. Thanks..
BB Bauer

And I'll replace the coil right away.. I have noticed it gets very hot to the touch.
BB Bauer

Even in our perpetual winters here in Scotland, compared to Texas, a heat shield is absolutely vital for the engine to start easily and run properly when hot.
It is certainly worth having. I tried going without it years ago and I regretted it.
JB Anderson

Repeat all above.
+++

I have repeatedly posted the link to the definitive ZS carb site.
http://www.sterlingbritishmotoringsociety.org/files/choke.pdf

And I have many times said that first priority is to check that all the special screws holding the choke assembly to the carb must be present, tight, and in the correct holes. Loose choke cause the carb to act like the choke is ON; looser choke leads to fuel running out the carb and dripping on the exhaust. That plus a red hot catalytic convertor = no more car.
FYI. same applies to MGB or other cars with ZS.
Search this and MGB Tech archives.

FRM
FR Millmore

This thread was discussed between 12/06/2012 and 14/06/2012

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.