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MG MGB Technical - Smog/Carb Problems

Hi,
I love California's smog laws....!Anyway, my 1977 MGB with Zenith carb failed the smog due to high HC levels. I tried riching it up but turning the adjusting screws on the carb did nothing to the mixture.
My guess is the float level is wrong. Any suggestions.
Plus, due to time constaints and my work schedule I can't do it myself. Does anyone know of a good MG mech. around the Yuba City, Ca area that can fix my carb? I know of one in Cameron Park but that is a bit far right now.
Any suggestions would be a help.
Thanks,
Joe
joe fabian

High HCs can also be due to ignition problems or
a faulty catalytic converter. Do you have a misfire?

If the problem is with the carburetor, rebuilding
it does not take very long, you can do it in a
few evenings after reading the Haynes manual.
But first check basics like compression, ignition
timing and valve settings. All these things can
be done in a few hours.

Unfortunately, its hard to adjust the mixture
on the Zenith without a CO analyzer. You definitely
need the Zenith adjusting tool from Moss or VB so
as not to tear the diaphragm. It sounds like
you're adjusting the idle air screw on the side,
which I've found has only minor effects on mixture.
You really need to adjust the metering needle
with the special tool. If you don't have the
Haynes Zenith carb manual, it would be good to
get it from Moss or Amazon. I found I had to
spend many nights reading it before I understood
the Zenith operation (especially the autochoke.)
Ronald

Sorry, can't help much with mechanics in your area. You might actually be better off with an experienced import mechanic than an mg specialist; for the MG guys, only a very small part of their business has to deal with the later smog carbs, while the general import specialist may have dealt with them on a range of older cars such as volvos.

Your cat converter could be non-functional; though it is possible (painfully) to pass smog with a hollow shell.

Make sure the air pump lines aren't clogged; running with the belt off the air pump can result in carbon clogging the fittings.

Adjusting the valves loose can help reduce HC; you don't want to go too far, but it effectively reduces overlap on the cam.
Tom G

Thanks for the input. I had a misfire but that was fixed by replacing the old plug wires. As for the cat, how can I tell if its's no good?
When I took the car for the pre smog the mech. said the carb wasn't getting enough gas.
On another note, I was able to adjust the mixture a bit using the screw and nut on the rear of the carb a few months ago but now turning them does absolutly nothing as to increasing richness or leaning it out. Does that help?

Joe
joe

Joe,

I am fairly sure that the screw and nut at the rear of the carb you are describing is an air bleeder for fine tuning the idle mixture only.

You should use the Z/S tool to turn the metering rod to adjust the mixture. The air bleeder screw and nut should be seated clockwise during adjustments to the metering rod.

As suggested above, also make sure that your smog pump is putting out plenty of air and that the air lines are clear.

I can highly recommend Lawrie and his crew (one of whom is a Zenith "expert") at British Automotive in Cameron Park to help you if you get stumped.
Steven Rechter

Joe,

Many times high HC turns out to be a tuneup problem, the high HC is unburned gasoline. This can be traced (usually) to faulty ignition or a too-lean mixture. When you replaced the plug wires, did you also replace the plugs, cap, and rotor and check the timing? 9 out of 10 times, it's the ignition before the carb in my experience.

Cheers!
Paul K

The plugs were just replaced a few months ago and I looked at them when I replace the wires, they were looking fine and the cap and rotor are only a few months old as well. The timing is set at 10BTC (had it at 15 but the smog place said had to be at 10 as per ...)
So that's why I think it may be the carb. I plan on talking it off this weekend and checking it out. I'll let you all know what I find.
Thanks.
Joe
Joe

Lowering the needle should significantly reduce CO and HC readings. As mentioned above, you need to use the proper tool to avoid diaphragm damage.

If you remove the piston/diaphragm assy, the washer at the base of the needle should be flush with the bottom of the piston as a baseline adjustment. Turn counter-clockwise to lower (lean), clockwise to raise (rich). It is very simple to make this adjustment.
Glenn Mallory

This thread was discussed between 12/02/2003 and 14/02/2003

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