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MG Midget and Sprite Technical - Leaky Carb

I've been having fun driving around in my 77 midget since rebuilding the engine, but have noticed trouble idling and some faint black smoke out of the tail. Also a lot of hissing around the carb, which I traced to the mixture screw area (ZS carb). I tightened the big plastic hex, which helped, and then turned the screw anti-clockwise which I believe leaned the mixture. Less hiss and smoother idle, which seems good. Am I ok with a carb that seems to idle nicely, or should I track down a pro to set up the carb?

But then I noticed raw fuel dripping from the opposite side of the carb (meaning nearer the firewall). There is a piece (which Haynes didn't name so I can't name) that my manual choke attaches to. It wobbles to the touch, and I think its connection to the body of the carb is the source of the fuel. But I can't figure out how to tighten it. Is the mounting bolt hidden under the choke attachment?

I realize this would be a lot easier if I posted a picture, but I don't have one handy right now.

Thanks.

--john
jf Falconer

Is a ZS a single carb?

My thoughts would be if it runs and idles nice then let it be. I had my twin SUs pro setup, I have no idea when it comes to twins!

cant help with the second bit, never come across anything single carb'd with anything other than a webber.

BC
B Cavagan

Yes, sorry. Single carb. Thanks.

--jf
jf Falconer

John -

If the choke assembly is wobbly, that may well be the source of the leak. The ZS choke should mount firmly against the carb body using three screws, with a gasket in between.

The function of the Zenith choke involves fuel ports - one inlet, one outlet - that allow fuel to flow from the carb float chamber through a needle valve in the choke, and from there into the carb intake air stream. So, since fuel is passing from the carb into the choke unit and back to the carb, if the mounting is loose you'll see a leak.

As for mixture adjustment, the main needle adjuster at the bottom of the dashpot should get you in the ballpark. This requires a special tool, although if you use a finger to keep the piston from rotating, you can use an Allen wrench. Once you're close, use the Fine Idle CO Screw (the one you mentioned) to fine-tune the mix to the specified setting, found in the manual. I borrowed a gunson's GasTester exhaust analyzer to do mine, and while many will debate the accuracy of that device, it seemed to work fine for me.

Some hissing at the carb is normal. That's the sound of air being drawn in - unless you were talking about something else.

Keep us posted,

-:G:-
Gryf Ketcherside

Gryf--

I know the three screws you mentioned... I knew they held the choke together, but didn't get that they connected the unit to the rest of the carb. Since I installed the manual choke, I suspect I know the culprit. Thanks.

I'll talk to my shop about the GasTester... perhaps I can elevate from art to science.

According to my stethescope, the hissing is confined to the carb. I was afraid of a vacuum leak, but now I'm not.

Many thanks.

--john
jf Falconer

This thread was discussed between 21/07/2008 and 22/07/2008

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