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MG MGB Technical - Overheating?

The car is a 1974, HIF4, CB, mgb roadster with a Peter Burgess Stage-2 unleaded head.

She starts well enough and runs fine for the first 5 minutes or so before starting to splutter when the throttle is fully open at low revs. This symptom quickly becomes worse until touching the throttle at any speed instantly stalls the engine. Once stalled, the engine will not start until it has been left for a while.

The problem doesn't seem to be significantly worse in hotter weather.

Any guesses as to what's wrong and/or plans to diagnose the problem would be appreciated.
Carl Kambites

Carl,

Try testing your starter. Mine developed a short last week and was evidently pulling enough load to affect the ignition. The symptoms were similar to yours in that it started fine (until it burned-up the solenoid and wouldn't start at all) and the engine would run richer and richer as the solenoid heated.

Doug
D. Cook

Just taking a stab, if it were sitting in my garage I'd be going through the fuel system. First looking for the potential off a vapor lock in the lines caused by heating. Then it would be on to checking the lines, filter, vent and fuel tank for anything foreign materials that could be starving fuel flow or causing a vacuum.

my2bits
Brad Batchelor

Carl

I agree with Brad - could well be fuel evaporation causing a vapour lock. Does this problem arise when driving the car or only when stationary with the engine running?

Laurie
L Webb

Five minutes is not long enough to heat the carbs to the point of vapor lock. I'd take a look at the ignition system. Condensors have been known to fail as they heat up. The other cause could be a blocked filter or lines. Disconnect your fuel line from the carbs and check the flow.
R. L Carleen

There is no way you are going to develop vapor lock in 5 minutes. I would check the venting pipes to the carbs. I had a MGB in my shop that did the same thing. We found the steel vent line to the tank was plugged.
Jim Pelletterie

Your fuel line would have to be zip-tied to your exhaust system to vapor lock in five minutes. Even with that, it probably would still flow just fine, due to the "pushing" fuel pump as the MGB has.

I agree with RL, that it sounds like fuel starvation. As he says, fill up the tank, get a five gallon catch can and run the fuel pump long enough to fill it up. I'll bet you'll find the flow is slow, or slows with time. I can't recall the amount a new pump will flow. It's something like a pint a minute. I'm sure another will chime in with more exact info.

I'd add these few things to check: Clogged needle valve, plugged fuel filter, rubber fuel hose decomposing or crimped, dented steel fuel line, pump getting old, vented fuel cap clogged, (or if sealed system, return from charcoal can is clogged), clogged tank screen.

Matt Kulka

What about the coil chaps ?? It can frequently break down when it heats up and recover when left to cool. It's a quick change to eliminate it from the equation.

Also on the fuel side, try driving the car and leave the fuel filler cap off.
Iain MacKintosh

Vapour lock is highly unlikely, particularly in the UK and with HIFs. In practice the only place it can occur is in the exposed jet pipes of HSs, when the heat shield is defective, and in high ambients which we aren't getting in the UK at the moment. These cars run in desert states without this problem.

When it won't start always eliminate the ignition first by clipping a timing light on to the coil lead and each plug lead and make sure it flashes while cranking. Also check that 1 and 4 are flashing a few degrees before TDC. If all that is so move on to the fuel, and given the gradual nature of the problem it probably isn't ignition. Remove the fuel filler cap first and if you get a loud 'whoosh' and it then starts its vent is blocked - or missing altogether if the cap has been changed recently. (When a problem develops always ask yourself "What have I fiddled with recently?). Then remove a fuel feed pipe from a carb (it will spurt if the ignition has been on recently), direct it into a container and check that you get at least one pint per minute. If so, and it still won't start, blow into the carb overflows. If the float chambers are full of fuel it should come up from the jet, which can be seen with the air filters removed. If it doesn't then possibly the float valves are stuck closed, but both of them, especially having passed fuel recently, is unlikely.
Paul Hunt

(1) the fuel pump is relatively new and sounds at least like it's priming ok;
(2) the fuel lines are mostly braided and all look fine
(3) the filter is one of the glass cased ones and doesn't look dirty, however it does seem to "empty" as the car heats up (starts pretty much full of fuel and ends up looking empty) is this normal?;
(4) the tank definately isn't vacuum sealing;
(5) the coil is fairly new but is getting pretty hot (any idea how hot it should run?)
Carl Kambites

1. Nevertheless you should do a delivery test when the problem is occuring.

3. An empty filter can be quite normal on a normally runing engine, but it can also be empty when there is no fuel! So in this case it is no firm evidence either way.

5. It does get very warm, but should only get too hot to touch if you leave the ignition on without the engine running. However if you have a coil for a rubber bumper car (which uses a 6v coil on a ballasted system) on your car (assuming an original CB and not a converted RB) then it will get very hot and probably overheat the points. In my experience this can lead to ignition problems. Remove the wires from the coil and measure its primary resistance, a 12v coil should measure about 3 ohms, a 6v about 1.5 ohms. If this is the case it should be revealed by there being no flashing in the timing light test as recommended in my previous post.
Paul Hunt

The car has been converted to run on a 12v battery. does that mean it should have a 12v coil (cars' electrical systems are something of a mistery to me)?
Carl Kambites

All MGBs used 12v electrics, there is no difference electrically speaking for the rest of the car if that was achieved by using two 6v batteries in series as on the earlier cars or a single 12v battery on the later cars, or an earlier car converted to a single 12v battery. This is completely independant of whether a car should have a 12v coil or a 6v coil.

Originally a chrome bumper car had a 12v coil and a rubber bumper car had a 6v coil with loom ballast resistor. Either car could have been changed to the other way, but generally it is only rubber bumper cars that are converted to a 12v coil, if an after-market electronic ignition system is fitted for example. But having such a system doesn't always require the change to a 12v coil. To see what you need to have you must measure the voltage on the +ve terminal of the coil *with the points closed*. If you see 12v then you must have a 12v coil, where the primary measures about 3 ohms, or about 2.4 ohms for a Sport coil. Having a 6v coil in this case will cause premature points failure. If you see 6v, or possibly 9v, then you have a ballasted system and need the 6v coil, which measures about 1.4 ohms. If you see 6v then you probably already have a 6v coil, but still check. If you see 9v then you probably have a 12v coil and should change it for a 6v coil. Running with a 12v coil won't harm anything else, but you will be getting a weak spark.

If you measure the voltage with the points open, or disconnected, you will see 12v on all systems, which is why you must measure it with them closed.

Don't leave the ignition on with the engine stopped any longer than you have to to make the voltage measurements
Paul Hunt

Another line of investigation - do you have sufficient oil in both carburettor dashpots? Many years ago I had a problem with a Maxi 1850HL fitted with twin SU's. The car started fine on the choke, but if you tried to floor the throttle it spluttered like mad. It could only be driven using very gentle throttle opening and the reason was one dashpot was nearly empty and the other only half full causing the pistons to rise too quickly and the mixture getting too rich too quick! FWIW

Laurie
L Webb

Interesting comment about not leaving the ignition on without the car running.
I'm on my first MGB, which I have had for about 8 weeks. Things were running OK until last weekend. I have had electrical issues since I bought the car, no turn signals or brake lights, so I was working on that. I had the ignition on for about 2 hours while trying to resolve the brake lights. I eventually found the problem, the lights were wired up wrong at the back. However, since then I have been having the same problem as Carl. The car runs for about 5 minutes then dies! I will check the coil primary resistance.

Apologies for butting in there Carl ;-)

Glynne
Glynne Smith

"3. An empty filter can be quite normal on a normally runing engine, but it can also be empty when there is no fuel! So in this case it is no firm evidence either way."

Quite true. I would only add that the fuel filter should not take on more air while the car is idling in the garage.

In my estimation, the air in your fuel filter comes from brief moments when the fuel pickup is not completely submerged. For instance, you hit a bump and the fuel sloshes, or you corner hard and the fuel runs to the side of the tank. For a moment or so, the pump is sucking air. That air gets pumped up through the system, some of it getting trapped in the fuel filter.

These conditions should not occur if the car is sitting still. So if the fuel filter empties as the car sits, then the pump must be sucking air from somewhere. Either from a crack in the flexible hose (suction side of the pump) or rusted-through spot in the metal line outside or pickup inside the tank, or from a loose connection.

Matt Kulka

The fuel filter appearing "empty" is a red herring. Air is NOT entering the system. When gasoline (with a vapor point like at -100 degrees) finds a larger space it expands and fills it with... vapor. The fuel is sill flowing out the other side in a thin film along the surface of the filter.

From the sypmtoms it really sounds like a fuel delivery issue. Check the fuel pump delivery rate over a few minutes. REMEMBER there is a small screen "filter" at the fitting on your pump!!!
You may well have a clog at the tank.

Mike!
mike!

This thread was discussed between 23/08/2004 and 27/08/2004

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