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MG Midget and Sprite Technical - Fuel Air Mix.

I've got a 1500 single carb 75. Over the weekend I started messing around with the fuel air mixture adjustment as Pop said "your running rich- you'll fowl your plugs". Well in the course of screwing around with it we discovered that the fuel filter was completely plugged- After adjustment (a multitude of adjustments) it ran considerably worse with each adjustment (due to the fuel filter which was a unknown problem at the time) My question is how do you get a good fuel/air mixture so you don't run rich or lean? I don't have a Co2 meter and I also have no idea where it's running now- we've adjusted it so much the last couple of days I don't have a clue where we stopped the adjustment.
Thanks!
M&M 75.
Mighty Midget 75

Hiya

If I have no access to metering equipment I tend to try to get an even idle. If the mix is too rich the idle slowly dies away. If the mix is too weak the engine will dip and stumble then rise slightly again when it comes back down to idle if you blip the throttle.

Peter
P Burgess

Here in the UK we have a device called a gunsons colourtune which gives a good indication of how lean or rich your mixture is. I don't know if you can get one in the states.

Graeme
graeme jackson

You could try the "lift the piston" method, which will get you quite close to the optimum mixture.

Remove the air filter so you have access to the carbureter.

Start the engine and let it idle.

Using a slender screwdriver, carefully lift the carburetor's air piston slightly, say 1/8".

If the engine tries to stall, the mix is too lean.

If the idle speeds up and stays there, the mix is rich.

If the idle increases slightly and then falls off again, that's right about where it should be.

You're adjusting the mixture using the correct tool - or at least an Allen wrench while holding the piston, right? Just curious. The screw down inside the damper is the coarse adjustment, and raises and lowers the metering needle. Once you get it close, there's a Fine Idle screw on the forward face of the carb where you can adjust the mixture more precisely.

Hope this helps,

-:G:-
Gryf Ketcherside

If you have the ZS carb, the two main causes of "too rich" are a bad diaphragm and problems with the auto choke. Any fiddling with adjustments will not fix either one. If in fact it was too rich, and the filter was blocked, which would make it too lean, then you are really off base. Replace the diaphragm as they do often go bad and it is cheap and easy, then search for "ZS choke" here and on the MGB board.
The fine idle adjustment will do absolutely nothing that you can check without a CO meter, so just screw it in all the way and leave it.

FRM
FR Millmore

Good point from Fletcher RE: the diaphragm. For that matter, you may be well off ordering a rebuild kit, which will contain the diaphragm plus all the other gaskets, seals, etc. that can perish and cause problems.

As for the choke, this is well worth a read:

http://www.sterlingbritishmotoringsociety.org/files/choke.pdf

It shows how to rebuild the choke, and is a big help.

Cheers,

-:G:-
Gryf Ketcherside

This thread was discussed between 14/09/2010 and 15/09/2010

MG Midget and Sprite Technical index

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