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MG Midget and Sprite Technical - Midget 1500 starting woes

We are working on a 1976 midget 1500 w/zenith stromberg carb. It has a manual choke and does not have a compensating valve. The engine has also been de-smogged.

Here is the problem - From cold it starts and runs good. After driving around for a while, we turn her off, go get a cup of coffee. After enjoying the coffee, head to the Midget to head home, only the Midget will not start, the carb is overloading, dumping gas out the front. Does not even make an attemp to start, but we let it sit over night, go out the next day and she starts. This is the second Midget I ran across that has this problem. The first one we sold! We have replaced the mechanical fuel pump with a new mechanical fuel pump. Still has the electronic igition and the coil with the resistor beside it. Any help would greatly be appreciated.

Would the weber downdraft carb and manifold solve this problem?

Thanks
Jim Watson
spridget@charter.net
JTW JIM

Jim, I had a similar problem and it turned out that the carburetor diaphragm was the problem. When cold it worked but when it was hot, the diaphragm was too soft and the piston wouldn't lift.

I suggest starting the car and letting it get hot. Shut it off and pull the top off the carburetor and see how soft the diaphragm is. That might be your problem too. I do know one thing, it stumped all the experts even those on here!
Clive Reddin

>>> ...the carb is overloading, dumping gas out the front. <<<

Been there. That's what happens when the float valve in the Zenith doesn't shut properly, and the chamber overfills. And yes, this will prevent the car from starting. Been there.

Are you running the standard (mechanical) fuel pump? If so, then you can rule out excessive fuel pressure overcoming the float valve. If you have an aftermarket electric pump, you might want to add a fuel pressure regulator upstream of the carb.

There may be crud in the float chamber that's gotten into the valve and is preventing it from sealing. If you remove the carb and drop the float chamber, you should be able to get an idea how clean it is... or isn't. I've added two inline filters to my car, one upstream of the fuel pump, and another right at the carb. I used the clear type so I can verify the presence of fuel when diagnosing non-starts.

At any rate, I recommend cleaning the float chamber and replacing the float valve. They're cheap... also check the float to make sure it's not cracked and leaking, and consider the filters mentioned above. Be aware also that running with the fuel mix extremely rich will cause raw fuel to blow past your rings into the sump, where it will contaminate and dilute your oil. Check your oil and make sure the level isn't higher than normal. If so, and if it looks thin and smells like fuel, change it. Again, been there.

Best of luck!

-:G:-
Gryf Ketcherside

Oops, just noticed that you mentioned that you were using a new mechanical pump. If it's correct to the car - i.e., if the pump spring is the proper rate - the pressure should be okay. Again, check the float valve.

NOTE: if you find that the float valve has been replaced with a Grose Jet (with a ball sticking out of the bottom, vs. a pin) remove it and throw it as far as you can, avoiding windows. The reason my Zenith overfilled was that I'd fallen for the blurb in the catalog and swapped the old float valve out for a Grose Jet. Big mistake. I don't know why they don't work well, but I've spoken with other British car owners who have had the same experience.

Sorry for the double post, but this is an important point.

-:G:-
Gryf Ketcherside

Yes, the Weber will fix it, but take pictures of your dis-assembly as you will need to put it back together someday for emissions compliance or some such.

FWIW I'm running the Weber conversion and it has a very irritating flat spot at low-rev acceleration that One must feather through gently to avoid a stumble. I've read this can be fixed by installing custom jets, but for the price of the thing should One have to?

R

Richard 1979 1500

Jim both Gryf and I have the Z-S carburetor on our cars. I'm happy with mine and I think Gryf is too and I see no reason to put on the Weber system. A rebuild of the Z-S is dead easy and dirty needle valves or the Z-S equivalaent can cause a lot of problems. I also had trouble with a stretched when hot diaphragm. That caused a hard start when the car wsa hot. A new diaphragm cleared up that problem.
Clive Reddin

This thread was discussed between 20/12/2008 and 26/12/2008

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