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MG MGB Technical - '79 MGB starving for fuel

Hi,
I have a '79 MGB that doesn't want to run with its stock ZS carb. I've richened it totally and it still hesitates when punching the gas. I've played with the timing until it idles just right and it still hesitates, like its getting either not enough gas or too much air upon acceleration. Any ideas?
Jake

Jake,

Check the fuel pump to make sure it is putting out a good stream of gas (also check the fuel filter).

Is the air filter assembly intact? If not, the car will not run properly.

Set the timing to 10 degrees below TDC (factory specs) at 1500 RPM.

When you "richened it totally," what did you do?

What exactly are the symptons? Idles OK, but as soon as you give it some gas, it hesitates, right? If so, it's probably a fuel problem so you're on the right track.

Jim Campion
'79 B
Jim Campion

Jake. What Jim says is a starting point. I am not particularly found of the Z-S carb, so have replaced mine with Weber DGVs which are acceptable to our emissions testing people. However, a couple of thoughts come to mind. First, what condition is the diaphram in? They are, at best, only good for about 35K miles. In the desert areas they do not seem to last that long. Check it for cracks, loss of flexibility and too much flexibility. Best thing to do is to compare the one you have to a new one. If they get old, the rubber can harden. If some forms of solvents get on the rubber, it becomes very soft and flexible--too much. Either form of deterioration, and I have seen both, will cause problems with the piston rise.

Also, check to see that the oil in the piston rod is there and is the correct type. Regular engine oil is the base standard, ATF or 20 weight light machine oil are alternatives. The bottom of the hole in the piston rod is stopped up with an O ring. When this ring no longer seals, the oil drains out and the piston rises too quickly, leading to a too lean condition.

If you disconnect the fuel line, you can measure both fuel line pressure and volume. Opinions vary on what the fuel pressure should be. The only reference I have shows 1.75 psi. David DuBois, who has a great deal of experience, rebuilds the pumps as a sideline and, probably has more accurate test gauges, says about 3 psi.

You might also want to make sure your vacuum advance unit is working and that the centrifugal advance is advancing the timing as the rpms are increased. I have seen the weights stick in place on a few distributors due to rusting or old lube and dust. Les
Les Bengtson

Thanks guys, I'll look into the fuel situation and distributor. I recently rebuilt the carburetor with a new kit, so all of the o rings and the diaphram. The oil for the damper is ATF, and its at the right level. What I mean by richened it totally was that I raised the needle in the piston to its full height. The engine was rebuilt under 1K ago, but for dual SU carbs, but I'm not sure if that makes any difference. The distributor is the standard Lucas electrical ignition type, but I might throw in the rebuilt points style to see if that takes care of it. Thanks for the leads.
Jake

With the pipe removed from the carb and directed into a bottle the pump should deliver about one pint per minute or better.
Paul Hunt

Jake. You might try 20-50 oil in the carb. Since the function of the oil and damper is to cause the piston to rise more slowly, functioning in a similar manner to the accelerator pump on a Holley style carb, too thin an oil can allow the piston to rise too quickly, causing a lean condition.

The Lucas electronic ignitions came in two forms. The Lucas 45DE4, or Opus system, and the Lucas 45DM4, or CEI system. The Opus system failed on my 79 in the first four months from new. Symptoms were a cutting out of the engine, dropping of the tach, then picking up again. It was replaced, under warranty, with the CEI system which peformed well for the next five years and was going well when I sold the car. Most of the Opus systems have either had the internals replaced with Luminition or Petronix internals or have been replaced with CEI systems. If your distributor has the square box on the side, it is an Opus system. If it has a round bodied distributor and a black box under the coil, it is a CEI. I have never seen a running, intact Opus system this many years later. Les
Les Bengtson

Jake, also check for air leaks. The manifold
gasket is the most likely culprit -- are the bolts
tight?

Also are you getting mechanical advance? Check
with a timing light and see if advance increases
significantly with RPM (should go offscale at about
3000 RPM with the stock 79 distributor, when set
to 10 deg. BTDC at 1500 RPM as noted above.)
Ronald

Also, check for blockages in the charcoal
cannisters and breather hoses. This can
cause a vacuum above the fuel bowl leading
to a very lean mixture.

Try removing the hose on the rear of the
carb above the fuel line that goes to the
cannister. If the mixture becomes significantly
richer, you've found the problem.
Ronald

Hey, While yer checkinmg things, take a look at the carburator float and jet.

By the time ya do all these suggestions we jusy might be into spring like weather!!
glg gimbut

One part thast has not been mentioned... A number of cars no longer have it...

Check out the Inertia switch. It is between the fuel filter and the carburettor on the shelf next to where the throttle cable comes through. If this is malfuctioning, it can cause all kinds of fuel starvation issues. Simply place a new rubber line that illiminates this item. If your problem goes away, then you know what it is. Leaving it disconnected is not the safest thing to do, but plenty of MG Bs are operating without one now days.

-BMC.
BMC Brian McCullough

This thread was discussed between 08/04/2003 and 12/04/2003

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