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MG Midget and Sprite Technical - Carb Runs Rich

Greetings from Canada to everyone. I am still alive although not near as active as I used to be.

Finally managed to get my midget up and running. I had a fuel delivery problem. I bypassed the mechanical pump which worked with an electric one but still no luck. Took the float bowl off and the floats were stuck from sitting so long.

Tore the carb down, cleaned it and put a rebuild kit in and it runs. Tried to tune it but it still runs just a tad rich and the screw for adjusting the needle has probably come to the limit.

It is a 1976, 1500 engine, manual choke, single Zenith-Stromberg carburetor, not a California car, no catalytic converter. Engine rebuilt to no lead specs, emission controls stripped when the engine was done. Shop tossed them away which they should not have done and also installed a points distributor in place of the correct electronic distributor.

Any suggestions welcomed.

Thanks,
Clive Reddin
Vineland, Ontario
Clive from Canada

What components were in the rebuild kit?
Some things that could cause the out of range mixture adjustment:
Metering needle worn at idle
Jet worn
Float height too high
Float valve not sealing
Fuel pressure too high (what sort of pump did you fit, aftermarket electric pumps frequently have too much pressure)
Paul Walbran

I am not fully familiar with a Stromberg carb but I did have a similar problem with the SU on my 1931 MG M Type. I tried lowering the needle in the piston but this did not help. I was then advised to check the fuel level, not in the float chamber but at the jet itself. If you remove the piston and look at the top of the jet it should not be wet. The meniscus in the jet should just be below the top surface. The choke tube in my carb was wet. After fiddling with the float fork I was able to get the meniscus in the correct place and the engine ran beautifully.

Jan T
J Targosz

My experience with the Zenith has shown that the tiny O-ring on the choke needle can wear out, allowing a richer mixture all the time. Have you had the choke apart? The O-ring was included in my rebuild kit… I don’t know about yours.

I also had a severely overrich condition when I tried a Grose Jet float valve. Lesson learned, stick with the original. ;-)

Also,
Gryf Ketcherside

Hello Paul, Gryf and JT;

I am running with the original mechanical pump. I only used the electric one to suck fuel up from the tank to save cranking the engine. Once fuel was in the line, it was removed.

The carburetor has a manual factory choke so that eliminates the automatic choke problem and I hear they can be a problem.

I will pull the piston and see if it's wet and maybe tinker with the float level setting. Right now it is at 16 mm as the manual recommended. The carb did run fine and it is almost set correct, so maybe the fuel level in the bowl is just the ticket. I think I might also switch the jet over but will only do one thing at a time and see how that works.

As they say, watch this space for future developments.

Thank you all for your input. Let's see how we do.

Cheers,
Clive from Canada.
Clive from Canada

Firstly what was the standard carb for the car? If someone has swopped it out for a non-standard carb, then I would start from there. As Paul hints, any non-standard carb is likely not set up correctly for the engine. Principle things include main jet, needle, float height etc etc. Personally if it is the wrong carb and so far away from modifying satisfactorily, I would chuck it and obtain a decent second-hand correct one. Usually it simply is not worth the effort of making a silk purse out of a sow's ear and you end up chasing your tail.
Oggers

I mentioned my choke above… mine had a manual conversion, but internally it was still the original setup, with a metering needle and O-ring. So even if yours is manually actuated, it could still have a failed O-ring.

-:G:-
Gryf Ketcherside

Hi Oggers, it is the factory carburetor for the car, the tag matches the one on the carburetor that was originally on there and I recently picked up a spare. Couldn't pass it up. The original had some problems and was replaced by the one on there now.
I think I will use that fuel inlet jet and recheck the float height. If there's too much fuel in the bowl, that can't be good.

Gryf, the carburetor on the car didn't have an automatic choke. The 77s and later did though.

Cheers,
Clive from Canada.

Clive from Canada

Clive

Fair enough. I'd still clean the innards out, especially jets/orifices etc. For sure an incorrect float height can give problems - and obviouly check to see if it actually floats! The old brass ones sometimes split and fill with fuel - no good at all. Check also the float needle and seat. If this passes fuel when the float rises past the cut off level, then again problems can ensue with too much fuel in the bowl.
Oggers

This thread was discussed between 14/08/2021 and 18/08/2021

MG Midget and Sprite Technical index

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