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MG MGB Technical - please help - 76 B runs then dies.

I have a Canadian '76 MGB (no catalytic converter on Canadian cars, stromberg carb still on, emissions removed by previous owner), i just recently finished restoring the bodywork and now can't get the car to keep going since its initial couple of runs. What happens is the car started up fine, seems to run fine, then all of sudden unexpectedly dies once it has reached around normal temperature - after approximately 10-15 minutes of driving. First time this happened i replaced the points, condenser, distrubtor cap, rotor, spark plugs, set the timing, then cleaned and rebuilt the zenith stromberg carburetor using a kit. I didn't put a new needle in the carb yet however. Then I cleaned and rebuilt the autochoke. After doing all this the car seemed to work marginally better, the idle was a lot smoother, so i took it out again. Then same thing happened, car dies when it gets warmed up to normal temperature. The spark plugs do appear to be fouling some, but when replaced the car still doesn't want to start up. Now, I usually have to wait for the car to completely cool down to get it to start again. Any suggestions? Is this possibly related to the needle in the carb that i haven't replaced yet? At this point i'm strongly looking into converting to the dual HIF4s, however, if the carb isn't the problem i'd rather keep it original. Any help would be appreciated.
- Mike.
M. Carlile

Mike,

Do you know if you do or do not have fuel or spark when it dies and won't start? Once you know which is missing troubleshooting will be easier.

Cheers, Doug
Doug Keene

Hi

As Doug says, you need to determine whether the problem is fuel or sparks.

If it is sparks, I would suspect the coil.

Don
Don

Do later cars have a heat sheild?
J Moore

Thanks, for replying. Yes, later cars do have heat shields and mine is intact. Also, my car is getting both fuel and spark and also i replaced the coil a few days ago to test that possibility. I'm stumped for things to try short of replacing the carb completely.
M. Carlile

I would look at the fuel system specifically the gas tank. I've heard of this happening before and it was a fuel line inside the tank which got plugged up after running for a few minutes. After the car died it would start up as if nothing happened and then would stall out again just like Ground Hogs Day.
Mike MaGee

Mike's idea would be one to check out. Also, when the engine dies, if the tach winds down slowly, it is either high tension ignition or fuel. If it drops suddenly to zero, it is low tension ignition or coil. Since you have already checked the coil, it is probably good. But, watch the tach next time it dies. Also check the fuel pump to make sure it is working properly. You can generally "make it to about the time the engine warms up" on the fuel in the carb float bowl. Then, with a bad fuel pump, blocked fuel tank pick up tube/filter or a clogged fuel line, the engine dies (with the tach winding down slowly). Please let us know what you find. Les
Les Bengtson

I agree in part with Les here again.

It seems to lie in a faulty coil/condenser, that when they get hot, quit.

The coil may work well when cool, but up to temperatue fails, (which would cause a fouling of the plugs}.

First thing I'd try is to throughly clean the coil connections, holding bracket, and the socket where the high tension wire comes out.

If you have a it of "mad money" flip for a new coil and condensor.

Just curious: do you know how old the coil is? What does the exterior look like? -- is it banged up, paint flaking off, any cracks on the head or around the low tension terminals?

If you had the coil tested, was it cold or hot for the test? Get the car up to operating temp in the driveway until it quits. Then, remove a plug and test for spark -- should be a nice bright blue.

Of course, it could be something completly different, but electrical/coil pooped first in my world of demensia.

<;+}
glg

Thanks again for replying. I have replaced the coil a few days ago, it is in new condition and the points and condenser were replaced after the first time the car died. the connections on the new coil are all clean. The fuel pump appears to operate fine while starting the car, but i'm not sure if it works while its running. I have however had the fuel line disconnected from the carb and had the pump continuously pump out gas at a normal rate (steady clicking sound) into a gas can and it seems to be fine. I intend to check out the tank tomorrow and see if i can find anything that would cause a block in the fuel filler tube. I was also wondering if maybe the bi-metallic spring in the autochoke was worn out causing the choke to not work properly? would that cause the choke to shut off too early and stop the car? Also, Les, thecar suddenly dies, it doesn't slowly drop down in revs. When the car has died it has been when starting up again from stopping at a stop sign. Thanks.
- Mike
M. Carlile

One possibility could be gas tank venting. If the emissions stuff has been bodged about, the vent may be cut off and you could be developing a vacuum in the tank - quick check is to remove the gas cap after it dies; if you hear a whoosh sound, it wasn't vented right.

Vapor lock would be another possibility if your gas line routing is odd; it is a low pressure fuel pump, so it will have trouble overcoming any vapor bubbles in the fuel lines.

Another possibility would be a bad ballast wire or other primary circuit connector which is heat sensitive. Sometimes the ends of wires get some corrosion and have resistance which gets worse as they heat up.
Tom

Could be the carb vent line causing the float bowl to not receive fuel. Check that it is not plugged on its' way to the charcoal cannister. When you close the engine hood does it come into contact with the rubber vent hose that conects the carb line to the cannister? Try disconecting the vent line at the carb to test this. Be carefull that the carb vent is not leaking fuel on the exhaust manifold or you could have a fire.
Jim

If it dies, and clipping a timing light onto the coil lead and each plug lead still shows at spark, and at the right time, then it cannot be ignition. Note that a flash from the light on the coil lead but not on the plug leads indicates the rotor and/or cap are breaking down. When the engine dies leave it in gear so the momentum of the car is spinning the engine and look at the tach and ignition warning light. If the tach is still registering the ignition LT (at least) is OK. If the warning light is on and the tach showing zero the 12v from the ignition switch has failed. If the emissions kit has been removed and the tank vent has been sealed then using the original non-vented filler cap will cause fuel starvation, but pumping into a can would appear to preclude that, removing the filler cap and listening for a big whoosh when it has died would indicate it. A blocked float chamber overflow, which should now be open to the atmoshphere, would cause the pump to pump fuel straight through the jet (on an SU at least) and hence very rich running (i.e. foulded plugs?), which could well be enough to kill it, but should be obvious from the smell. Is the pump running or not when it dies and the ignition is still on - assuming the original SU type? Speaking of the pump maybe the clacka-clacka of pumping into a can is enough to keep it working but the intermittent clicking of running isn't. If the pump *is* the problem the float chamber would be empty and the pump would chatter like mad for a few seconds immediately before the engine fired again.
Paul Hunt

Mike,

Paul may be on the right track here. Not long after I first got my "B" I experienced a problem with stalling out after driving for a while. I would let it set for a short time, and Voila, it would fire up. The problem got increasingly worse over time, and I finally figured out it was the fuel pump.

Cheers, Doug
Doug Keene

With me on my '73, it was not the fuel pump but 'grunge' in the fuel tank that would float around and then block the sending unit. This didn't start happening until after 3 tanks of fuel. At first I thought it was the pump so I swaped out with a cheap unit from NAPA and then it happend again - 15 minutes later. I could hear the pump clicking madly trying to suck fuel but nothing coming out at the carb. I replaced all of the fuel lines and the filter, 15 minutes later it happened again. This time shined a light in the tank and saw the chunks of grunge floating around. This time I replaced the tank and all has been well since.

good luck
bob g
1978 Pageant Blue MGB
1976 Inca Yellow TR6
1973 Mallard Green MGB
Robert Gloyd

Yeah, just sittin here wondering if it is more a time related problem than heat, (being warmed up).

vapor lock, fuel starvation, came to mind -- as did ignition switch, ballast resistor (if applicable)

Has electric choke? did you see black smoke from exhaust? (sign of flooding) -- Heavy smell of gas around carburators?

What about white wire from coil to tach that goes through ignition lamp then onto the ignition switch?

Would a bad ground or faulty ignition lamp cause the engine to stop -- ?
glg

Another remote possibility is the anti run on valve.
There is a vent tube on the valve that I believe allows air into the float chambers to allow fuel to enter the jets. Normal operation is: when you turn the ignition off the valve closes, blocking off the venting to the float chambers and the fuel flow to the jets stops thus preventing running on. When the oil pressure drops indicating that the engine has stopped running, the valve opens again. I wonder if the valve is malfunctioning. You could try bypassing the valve by disconnecting the tube that goes to the float chambers and do a test run. If the car stalls as before it is not the anti run on valve, and just hook it back up. It's easy enough to try. Oh I just realized this car has a zenith carb & I'm not sure if they have the anti run on valve. It is on the passenger side fender wall.
Ralph
Ralph

Yes, Zenith had anti-runon too, and it would be a good point, but original poster says all emissions kit removed. Could still have been jury-rigged to retain the valve though.
Paul Hunt

I had some incorrect information as I was going from memory. (apparently not that good of memory). The outlet hose from the anti run on goes to the charcoal canister and from the canister a line runs to the float chamber(s). When the key is turned off and there is oil pressure the valve connects inlet manifold vacuum through the valve, through the canister to the float chamber causing the engine to stop. Then after the engine stops and the oil pressure drops the valve resumes it's normal position closing off the manifold vacuum. On the bottom of the anti run on is a large dia. vent tube open to the atmosphere. It appears that during running, it allows venting through the canister to the float chamber to allow the vapours to flow from the canister to the carb, and also allows venting to the fuel tank. Since there is no venting at the fuel filler cap this seems to be how the fuel system is vented. If you block this tube off with your hand while the engine is running it dies after about 2 seconds. I tried this with my '73 last night, and may be a remote possibility if it is still fitted on your car. I guess an easy way to eliminate the fuel venting system and canister would be to disconnect the tube that goes from the float chamber to the charcoal canister, and the one that goes to the fuel tank, and do a test run.
Good Luck,
Ralph
Ralph

Not quite right, although the effect is the same. With the valve released all three ports are connected together i.e. both the port to the canister and that to the inlet manifold are open to atmosphere, so the inlet manifold port has no effect on the canister port. When the valve operates it closes off the big port at the bottom which is open to the atmosphere, so inlet manifold vacuum is now applied to the canister. This, together with carb vacuum applied via the crankcase and rocker cover, sucks the fuel out of the carb jet(s) stopping the engine. It might seem surprising that there is a permanent 'vacuum leak' into the inlet manifold but it is only a small tube and is taken account of when tuning the carbs. The large port on the valve *is* the way that the tank and float chamber are vented, via the canister to adsorb fumes. The continual suction (with the engine running) through the canister from the pipe on the top that goes to the rocker cover and via the crankcase to the carb(s) scavenges any fumes from the canister. In practice this also burns off any fumes from the tank and float chamber directly, the only time there is a net outward flow to the atmosphere via the canister and valve is when the fuel level in either the tank or the float chamber is rising. In the former case during refuelling or heat expansion, in the latter case when the ignition is first turned on or heat expansion after switch off.
Paul Hunt

Okay, I ran some tests on the MG today. The car was somewhat difficult to start up at the moment and would only start with the accelerator pedal floored, at which point it would only go up to 1300 rpm max once started, then die again very shortly afterwards. While running it I pulled the anti run-on valve connections and it didn't really change things, the car still died. So i tested the fuel pump again into a gas can and it still seems to be pumping alright. When i disconnected the fuel line from the ZS carburetor I found the fuel line full of gas, yet the car seems to be giving symptoms of it being starved of fuel. I'm thinking it may be a problem of fuel delivery within the carburetor itself.
- Mike.
M. Carlile

This thread was discussed between 23/10/2003 and 26/10/2003

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