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72 MGB Carburetor problem

Posted by 72MGBnovice

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72 MGB Carburetor problem #1
72MGBnovice Karl Sparklin Dayton, USA	Posted: September 09, 2012 06:39PM Registered: 8 months ago Posts: 3

Hi,
I've had a '72 MGB for a number of years and have had nearly trouble-free operation with it. But I've had some brake problems that's had it not running for a while. Those are now fixed, but now I'm having some carb problems. Once I get the car started with strategically placed jets of starter fluid, and pumping the pedal slightly to keep it running, it will idle OK, but coughs and dies if I try to rev it at all. I get out of the car and kick the front carb's "lever" and the car rev's up fine. But doing the same to the back carb will make the car cough and die if done for more than an instant. Any ideas?

I've looked through other threads here, and I either don't see this symptom, or the terms are a bit beyond me. Is this a float problem? Needle valve?

Thanks for any help; I just discovered the place in a YouTube video!

Karl

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Re: 72 MGB Carburetor problem #2
rrmgb robert schau Reston, VA., USA 1973 MG MGB 1973 MG MGB	Posted: September 09, 2012 09:47PM Registered: 6 years ago Posts: 8,984

Welcome to the MGE Karl ! You've come to the right place.
Stock carbs for a '72 are SU HIFs. Do you know if this is what you have?
Not sure what you mean by "kick the lever" but both carbs work in unison unless there is a linkage problem.
Last question for you....do you have a manual for reference?
I'm sure many will chime in on this but will also like to know which carbs you have.
RS

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Re: 72 MGB Carburetor problem #3
Dave Braun Minnesota, USA 1952 MG TD "Tommy" 1970 MG MGB "Maggie" 1974 Triumph Spitfire 1500 "Sammy"	Posted: September 09, 2012 09:56PM Registered: 1 year ago Posts: 2,002

If the metering needle is stuck in the jet and loose from the piston, it can do this as the accelerator is kicked down.

warmly,
dave

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dbraun99 LLC – Hastings, Minnesota USA dbraun99 LLC provides advice, repairs and restorations. We also provide complete bench services on SU Carburetors.

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Re: 72 MGB Carburetor problem #4
72MGBnovice Karl Sparklin Dayton, USA	Posted: September 09, 2012 10:16PM Registered: 8 months ago Posts: 3

Thanks for your responses!
Yes, I have the stock SU HIF's.
I'm sorry I don't know the correct terms. The accelerator cable goes to a common linkage between the two carbs. If I hit the gas pedal, the car will sputter and nearly die. If I walk around and lift this common point, of course the same thing happens, but if I lift the individual screws where this linkage attaches, the front carb revs high just fine, but the back one is the one causing the stalling. But it idles OK!

It was running fine a few months ago when I only had brake problems. So it would run, but it wouldn't stop. Now it stops, but it won't run!

Viva British motoring!

Karl

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Re: 72 MGB Carburetor problem #5
oily-hands Owen Frankland Stockton on Tees, Cleveland, United Kingdom 1937 MG TA "Numbum" 1971 MG MGB GT "The Bruise" 2002 MG ZR	Posted: September 10, 2012 01:39AM Registered: 3 years ago Posts: 5,183

It's possible but not that common for the pistons to stick when a car is sat for a while.

Easy to check, just remove the air filters and lift up each piston in turn and ensure that they are moving freely. I suspect that the back one may be sticking.

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Re: 72 MGB Carburetor problem #6
Bankerdanny Daniel Palmer Chicagoland, USA 1972 MG MGB GT "Dudley" 1977 Honda MC Gold Wing	Posted: September 10, 2012 11:01AM Registered: 1 year ago Posts: 1,055

Keep in mind that the linkage opens a butterfly creating a vacuum that lifts a piston and allows more fuel to flow.

I would pull the pots off the carbs and lightly clean the slide pistons and the inside of the barrel with some carb cleaner and 0000 steel wool. You don't want to remove any metal, just clean any shellac that might have built up that would cause the pistons to hang up.

Then I would remove the black screw on cap from the top of the pot, dump out the damper oil, put the pot back on and refill with the appropriate amount of 20-50.

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Re: 72 MGB Carburetor problem #7
tomkatb Larry Baygents Dayton, Ohio, USA 1963 MG MGB	Posted: September 10, 2012 11:16AM Registered: 8 years ago Posts: 3,374

Karl

You are in a good place(Dayton Ohio) to work on a B.

There is a great shop here if all else fails.

The problem you describe could be one of many.

Is the fuel being delivered correctly? That is what I would check first.

One can learn a lot about these carbs by removing the air cleaners and mess with the linkage a while. Lots of light.

When you move the cable the carbs need to move together.

L.W.(Larry)Baygents
63B
77 Spit

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Re: 72 MGB Carburetor problem #8
geezer charles durning Magee, MS, USA 1958 MG Magnette ZB "Chick Magnette"	Posted: September 10, 2012 11:20AM Registered: 2 years ago Posts: 400

Could have a stuck needle and seat that would keep the back carb from getting any fuel. Are the carbs still hooked together? If you can operate the front carb independantly from the rear carb you may also have a linkage issue that need to be resolved.

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Re: 72 MGB Carburetor problem #9
GILMGA Gil Dupre Chattanooga, TN, USA 1962 MG MGA 1974 MG MGB GT	Posted: September 10, 2012 11:41AM Registered: 6 years ago Posts: 7,308

Remove the cap on top of the carb and make sure there is oil is in the hole and the piston moves up and down easily

Gil

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Re: 72 MGB Carburetor problem #10
72MGBnovice Karl Sparklin Dayton, USA	Posted: September 10, 2012 07:28PM Registered: 8 months ago Posts: 3

Thanks to everyone that's helped so far!

Yes, "MG Automotive" is a great resource if things degrade that far, Larry. winking smiley

I assume that fuel flow to the carbs is OK, since it idles all right. Foolish?

Owen, I should have said that I do indeed have the air filters off; they've been that way for months. I can indeed manually raise both pistons manually, although the one in back seems to take a bit more force.

Charles, would the car idle OK if the needle and/or seat were stuck? The carbs are indeed still stuck together, and the linkage seems to work OK. If I bypass the linkage and push on the relevant pieces on each carb individually, the front car revs the engine OK, but the back one makes it sputter and die.

GIl, there is indeed fluid in the hole. And the plunger seems to move in the back carb the way the one in front does.

Thanks for all the help!

Karl

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Re: 72 MGB Carburetor problem #11
geezer charles durning Magee, MS, USA 1958 MG Magnette ZB "Chick Magnette"	Posted: September 10, 2012 07:59PM Registered: 2 years ago Posts: 400

In reply to # 2166762 by 72MGBnovice
Charles, would the car idle OK if the needle and/or seat were stuck? The carbs are indeed still stuck together, and the linkage seems to work OK. If I bypass the linkage and push on the relevant pieces on each carb individually, the front car revs the engine OK, but the back one makes it sputter and die.

Karl,

If the needle and seat are stuck in the closed position (no gas getting into the float bowl), YES, the engine will idle. However the power will be low, but, the engine will run on one carb. That just happened to me a couple of weeks ago.

Charlie D

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Re: 72 MGB Carburetor problem #12
NOHOME Peter Plouf London, Canada 1961 Austin-Healey Sprite Bugeye "Lil" 1967 MG MGB GT "Maggie (GT From Hell)"	Posted: September 10, 2012 09:00PM Registered: 6 years ago Posts: 11,898

I suspect the rear carb is gummed up due to the fuel sitting in it during the lay-up. Best bet would be to remove it and dismantle to clean all the orifices. The carbs really are simple.

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Re: 72 MGB Carburetor problem #13
tomkatb Larry Baygents Dayton, Ohio, USA 1963 MG MGB	Posted: September 10, 2012 09:56PM Registered: 8 years ago Posts: 3,374

i would disconnect the fuel line at the carbs. Put the line in a quart container. Your pump should give you 16 ounces Iin about 60 seconds as I remember.

If that is ok then likely the carbs are all gummed up. My son buys four carb Hondas for cheap and cleans the carbs for a few dollars.

They are fairly easy to clean. Modern gas goes bad in six months.

Usually step one is valves, then ignition then carbs. However, it seems obvious the carbs are in trouble.

L.W.(Larry)Baygents
63B
77 Spit

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