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MG MGB Technical - Running on 3 cylinders?

I've had a '72 B for 10 weeks now and it was running GREAT; in fact just pushed it a little on a 75 mile loop through the VT mountains. Note: this car was rallied and has a Webber side-draft, vacuum oil pump system, stiff suspension. The next day after a 15 mile easy ride when I re-started the carit was only running on 3 cylinders...rough and no power. I limped home at 55 MPH and now wonder where to start. Could it be a carburetor/gas problem? or burnt or leaky valve? or worse, bad lobe on CAM shaft? Any ideas where to start and how to diagnose? I am a novice at this MGB stuff. TIA

Jay T
JT Tracey

Are the plugs OK?

First, remove & clean the spark plugs, gap them and replace.
With the engine idling, pull off a plug wire, wait a moment,
then replace it and go on to the next plug wire. Listen for
any drop in idle speed or additional roughness while doing this.
If nothing happens - then that spark plug or plug wire is bad.

It could also be that all that rough driving stirred up
some debris and rust either in the fuel tank or in
the bowl of the side-draft Weber carb.

Check the fuel filter.

After that, try pulling out the idle jets and main jet
assemblies from the carb (fortunately, this is very easy
to do on a sidedraft Weber) and blow them clean with a
jet of compressed air or carb cleaner. Then, while the
jets are out, flush out the jet passages in the carb body
with an aerosol can of carb cleaner (the one with a long,
thin plastic nozzle). Unscrew the screen filter that is
housed on top of the carb and clean it.

Unscrew the idle mixture needles (count the number
of turns needed to remove them - and write it down).
wipe the needles clean and flush out the passage with
carb cleaner. Replace the needles, using the same
number of turns it took to remove them.

Wear goggles over your eyes while doing all this.

After that, then it's time to check the compression
on all the cylinders with a compression tester.

Pray that all's well and OK here.
Daniel Wong

In addition to all the above I had a similar problem when I was new to MGs too. Two cylinders weren't running very well, it turned out the valves were badly out of adjustment. I adjusted the clearances to spec and it made a world of difference.

If your valve train is making louder clicking noises than before then that's usually a good indication of large clearance. Although it's possible the clearance has tightened up and in that case they won't make much noise so go ahead and check them regardless of noise.
Jared Snider

Everything I've ever heard about a Weber sidedraught is that it is a difficult carb to tune, even for professional mechanics. I have stock SUs on my 71, and a stock Zenith on my 79. Good Luck!
Sam C

If your sure it's only one cylinder, I'd do as Daniel has stated and try the plug wires first, if you find one that is bad, this has happened to me a few times, go buy a new set. You can also take a voltmeter and check continiuty.
J Arthurs

Could also be the dizzy cap or rotor.
Mark Neisser

This thread was discussed between 20/06/2002 and 22/06/2002

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