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MG MGB Technical - Poor performance. Is it the Weber Carb?

I have a new Weber 32/36 DGV (manual choke) carburetor on a 1966 MGB-GT. The car has a stock engine with uniform 150 lbs compression, and the original 3-synchro transmission is in excellent condition. The distributor was just rebuilt by Advanced Distributors. It has a Pertronix electronic ignition, and the spark plugs and coil are new.
I have all of the Weber books, and I have read all of the Weber material in the archives. The ignition timing is set at 20 degrees BTDC, and the full advance is 32 degrees at 4,000 rpm. The car starts easily and runs well around town, at speeds under 40 mph. The fuel mixture seems to be good, as the spark plugs have the right brown-gray color. The problem comes on the highway, where that car has to be at 4,800 rpm to reach 60 mph on flat ground. The plugs remain brown-gray after highway runs, and I am getting about 27 mpg
I donít have a fuel pressure regulator, although one is on order. Iíve checked the carís tachometer against an independent one, and they agree. The speedometer is accurate. All 4 wheels spin freely, and the brakes donít drag. Can anyone suggest what I should do next, (besides switching to SU carbs)?
Thanks in advance for your comments!
Mike Whalen
M.E. Whalen

Sounds like you are in 3rd pulling 4800 for 60mph, are you saying you can't accelerate or even worse maintain that speed in top?

Throttle pedal and cable opening the butterfly all the way? Have you checked the fuel flow rate? Should be at least one Imperial pint per minute, and in practice closer to 2, with minimal bubbling, with the carb feed pipe removed from the carb and directed into a container and the ignition turned on. If you have an SU pump watch out for a spurt of fuel when removing the fuel feed pipe from the carb if the ignition has been on recently.
PaulH Solihull

Mike. The Weber DGV does not require a fuel regulator when used with either the factory original SU fuel pump or the low pressure Faucet type pump. If using the high pressure Faucet pump, so something similar having a pressure higher than 3.5 psi, you will need a regulator.

What fuel pump are you running?

Like Paul, I am not sure what you are saying here regarding your engine rpm vs. the indicated speed on your speedometer. With the non-overdrive transmission you are traveling about 18 mph per 1,000 rpm of engine speed. Thus, in fourth gear, your 4,800 rpm engine speed should be causing you to be going over 85 mph. Are you in fourth gear when you observe the 4,800 rpms on the tachometer and 60 mph on the speedometer? Have you cross checked the engine rpms, using a diagnostic tachometer, and the speed using a GPS unit while actually driving on a highway?

The fact that your engine will turn 4,800 rpm is a good indication that the carb is working properly. You would need an air-fuel meter, or an exhaust gas analyzer to make exact determination of what your approximate mixture is. But, having had three cars with the Weber DGV on them, they are fairly well set up as they come from the factory. (Assuming you purchased one intended for use on an MGB--they are set up for a number of different engines.)

My best guess is that your clutch is slipping at highway speeds.

Les Bengtson


Here is a clarification of my situation. In 4th gear, I have to accelerate up to 4,800 rpm to reach 60 mph. I then have to hold the engine at 4,800 to maintain that speed. The car will do it, but I should be running about 3,500 rpm for 60 mph.

I have tested the tach with another unit and the speedometer with another car, both at highway speeds. They both seem to be accurate. The Weber I bought was supposed to be set up for an MGB.

Paul: I tested the fuel flow rate, and I was getting about 5 psi, which is too high for the Weber. Could this possibly be the problem?

Les: short of pulling the engine (which I'm willing to do) is there any way of checking for clutch slip at speed?

Thanks for your suggestions! Mike
M.E. Whalen

Mike. Put the handbrake on, engine running, put the car in gear and let the clutch out gradually. If the clutch is slipping, you should be able to determine it as the engine will not bog down as quickly as it would with a good clutch. Beyond that, the remedy is to take it to a competent mechanic who will do the same test, but should have done it many times before and may have a better understanding of what he is seeing and hearing.

By the way, my Weber equipped cars will run at over 90, the most I have had them up to, with ease.

Les Bengtson

Mike, the Weber has nothing to do with it. Brown/grey plugs and 27 mpg seems to indicate it is working as it should.

You have what sounds to be a classic clutch slip problem.

Tony Oliver

Several of you have suggested that my problem is a slipping clutch. Could this be the case if I can feel no slipping at lower speeds? Or is the slip still there and I just can't tell at low speeds?

Thanks for all of your input!

M.E. Whalen


A slipping clutch will first be noticed in high gears. So, it is quite possible that you would get a slip in 4th gear and not get one in 1st, 2nd, or even 3rd. Counterintuitive I know, but that's the way it is.

C R Huff

Make certain that you have the proper high speed jet in the Weber. Just because the seller says it has the right one doesn't make it so. You very well may be running out of fuel when the engine is put under load. RAY
rjm RAY

When and how did this problem start? Certainly sounds like clutch slip to me.
Art Pearse

Ray. You misapprehend the problem. If it were a carb problem, the owner would not be able to rev the engine to 4,800 rpm. Thus, while there may still be some carb tuning required, it is currently working within acceptable limits and any such work should be deferred until the basic problem is analyzed and, if necessary, corrected.

We currently have some data, but are lacking information. By this I mean that we are told that the owner is registering 4,800 rpm at an indicated 60 mph. The owner claims that he has tested the tachometer against another tachometer (but, we do not the conditions of the test) and has cross checked the speedometer against another speedometer (but, was that one ever calibrated?)

It is possible that the tachometer, when at idle, is reading the same as a bench tachometer. But, for the test to be totally valid, the two tachometers must be tested under the problem conditions--highway speed. It might be possible that, under loaded conditions, the ignition system, which the tach is tied into, may be causing the tach to read in an inaccurate manner. Also, these old tachometers are known to be inaccurate at certain readings. But, the range of the inaccuracy cannot be determined except by experiment.

This is why I suggested that a hand held GPS unit and a diagnostic tachometer be used to verify the readings of the speedometer and dash tachometer. If such a test is performed, the data becomes information--i.e. data that can be used to reduce the level of uncertainty in decision making.

Assuming that the test would show the reading of the tachometer to be accurate(counting 9,600 firing impulses per minute at 4,800 indicated rpm) and the speedometer to be accurate, there are only two possible conditions under which such a situation can occur.

The first, and most probable, is a slipping clutch.

The second, possible if the rear end has been set up for auto-crossing events, is that the ring and pinion gear have been changed out from the factory 3.9 gear to one offering faster acceleration at the expense of top end speed. Since I have only known one individual who made such a modification, using the ring and pinion from some British van, I do not consider this to be likely. But, if we have verified information, checking the number of turns the prop shaft makes for one revolution of a tire is a relatively simple exercise and would, quickly, demonstrate whether the problem is with the rear end gear ratio or a slipping clutch. Those are the only two items which could cause this situation.

Les Bengtson

I hear you heaving a great sigh and saying "Bloody hell!".
How can carburettors have anything to do with a speed:rev situation??
David Overington

The easiest way to diagnose clutch slip is simply drive in fourth (or OD 4th if you have OD) and move the accelerator back and fore! If the clutch is slipping the tach will go up faster than the speedo when you try to accelerate. What speed *are* you doing in 3rd at 4800 just maintaining speed i.e. not accelerating? If significantly less than 60mph, and there is no indication of clutch slip, then the axle ratio is non-standard.
PaulH Solihull

Les, My statement was based on the assumption that the car could pull its load in 3rd gear but not in 4th. Too small a high speed jet could be restricting the fuel delivery system and limiting the engine's ability to function properly in 4th. I ran a DCOE for 10 years and found it to be very sensitive to jetting. The downdraft Webers are much simpler items, but the same laws of physics apply. RAY
rjm RAY

Les and Paul: The MGB has its stock 3.9 rear end. Clutch slip is looking more and more likely.

M.E. Whalen

This thread was discussed between 20/09/2010 and 22/09/2010

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