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MG MGB Technical - Front carb mixture way off

Hello-
When setting up the mixture on my
HS4's I have to turn the front carb down (richen) a *lot* more than the rear.
When I lift the piston with a small screwdriver the engine will stumble
until the jet is about twice lower than the rear carb then it will start to pick up.
The rear carb sets up fine at reasonable height.

I have good floats & float needles, both accurately set at 3/16". Fuel tubes clear.
Carb needles are even. I had FX then tried #6 and same thing.
No vacuum leaks, my vac guage averages 17hg at idle and 20hg @ 2500 rpm. Valves recently adjusted.
The PCV system is the "Y" tube from tappet cover to carbs.
Recent rebuilt engine bored .060. The head has the larger intake valves I
think they used in 1974.
pertronix ignition, Magnacor wires, NGK nonresistor plugs gapped at 32.

Any suggestions or ideas appreciated!
Samuel Sullivan

Samuel - Did you balance the carbs, front and rear so there is equal air being drawn through each? Cheers - Dave
David DuBois

Yes, with a Unisyn as I usually do.
Samuel Sullivan

Maybe the metering needle is different or bent on the front carb or has moved in its mounting hole?

Ralph
Ralph

What condition is the throttle shaft on the front carb in? If worn, air could be bypassing the throttle disc. RAY
RAY

The throttle shaft is ok, it was rebushed a while ago.
Needles are new and same.
Thanks
Samuel Sullivan

Too obvious, but are your jet heights even? I had a similar problem recently when the choke linkage was holding one jet slightly lower.

David
David Overington

Samuel,

Are the carb pistons matched? John Twist shows how to check this in one of his brilliant videos. If the two got interchanged at some point this could throw the whole thing out of balance.

http://www.youtube.com/watch?v=IfU47Oqq9wA

David
D Balkwill

Double check that your needles are centered properly in the jets.
Robert McCoy

Jets are centered. I did a drop test some time ago and
the pistons were fairly close.
Samuel Sullivan

Hi Samuel,

Have you gotten this straightened out yet?

Just a thought. It is possible to press the brass portion of the jet farther into the plastic base, inadvertently or on purpose. I've done both. I recently did it inadvertently while trying to center my jets with my jet centering tool - with the carbs on the car and the jets installed but lowered - because I was too lazy to take the carbs off.

Anyway, when that happens, in effect, you end up with one jet that is longer than the other, and more of the jet orifice is exposed relative to the needle in the longer jet. I'm sure that would cause a difference of mixtures between the two jets.

So, anyway, see if both of your jets are the same length, from the bottom of the plastic to the top of the brass.

If you need to lengthen the jet, here's the method I learned from Joe Curto: Chuck the brass portion in a drill press, with just a little brass exposed above the plastic, and lever the plastic part down with a screwdriver.

A long shot maybe, but hope it helps.

Cheers,
Greg

Greg Van Hook

This thread was discussed between 01/06/2009 and 05/06/2009

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