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MG MGB Technical - Car keeps on cutting out !

A few weeks ago I asked more or less the same question about my '69B roadster. The car will start perfectly drive for about 15 minutes, then the performace will get gradually worse until eventually cutting out. After waiting for about another 10-15 minutes the car will start again and then do the same thing after a while.
Upon starting it for the second time there is sometimes a loud hiss and I could not work where it was coming from.
According to my dad who was watching the engine at the time of this hiss the engine appeared to be running backwards (the direction of the fan).
A few of you people kindly responded by saying it was a possible petrol cap problem. But I have eliminated this by trying to start the car without the cap in place, all to no avail. Can someone please help as summer is nearly over and I have only done abouy 30 miles so far !!!!!!!!!!!
Chris Kendal

Chris

Could be fuel vapourisation. What's the state of your heatshield?

Regards

Dave
Dave Wellings

I have already crossed that bridge and replaced it, and that did not cure the problem. I also know that the fuel pump is good and sucking well as when it does not work I can quite happily disconnect the fuel line to the carbs and fill a pint bottle.

Cheers

Chris
Chris Kendal



My heartfelt sympathy!!1
I struggled with a problem like this for about 3 months earlier this year.
In my case it eventually turned out to be worn carb spindles(72 BGT),
I fitted recon carbs and all was miraculously cured!

DEREK THOMPSON,
FIFE,
SCOTLAND.
da thompson

OK Here are a few suggestions:

1. Possibly intermittant fuel pump operation. Check all connections.

2. Fuel pickup in tank partially obstructed (by rust?)

3. Possible condensor or coil giving out.

4. Fuel filter clogged.

5. Points gap closing up on the dizzy. (its happened)

6. If all the guages go off or if red dash light goes on, could be an electrical malfunction or loose connection at starter (check the small plug terminals they sometimes loose their contact through oil & crud accumulation) or at the alternator. Also check ignition wires or starter solinoid.

Good Luck,

Luigi
Luis

I have had the car for 5 years and I think I changed the carbs when I bought it, (but I'm not to sure)
The fuel pump is new and I'm quite confident in that.
The fuel tank is brand new, as the other one looked like a seive :-)
The coil is brand new, but we notice that it gets very hot, nearly to the point of untouchable...any views on this !!!!

The fuel filter is transparent and so it can be checked
I have fitted magnetronic ignition and have no points any more
I will have a look at the electrical connections.

Chris Kendal

Chris, Is your coil a standard 12 volt coil for a 69 with no ballast resistor? What do your magnetronic instructions say about the coil and ballast resistors? I know the Crane Fireball system suggests using a ballast resistor if the coil gets too hot to touch. Was the car running ok before installing the magnetronic system? I would try using the old coil and put the points back in and see if the problem is still there. I have seen coils act as you describe, cut out ignition, cool off an run ok until it heats up. Unusual but possible. Good luck Clifton
Clifton Gordon

You may have something there. I seem to recall it did run better before the electronic ignition was added. Unfortunately I changed it in the autumn, threw the old stuff away as it seemed to run OK in the garage, and so now I'm stuck.
May I pick your brian regarding the ballast resistor??

I am a telephone engineer and don't know too much about cars. Can you please describe the role of the resistor and what values it should be.

Thanks
Chris Kendal

Firstly put a timing light on the coil lead and each plug lead and watch the flashes. That will tell you whether the non-starting is ignition (except plugs, but they are highly unlikely to all fail then all come back again) or fuel.

A 69 didn't have a ballast resistor and would have had a 12v coil originally. A 12v coil should measure about 3 ohms across the primary and a 6v coil about half that. A high performance coil will probably measure something between the two.

An after-market ignition system may or may not have its own ballast resistor and/or coil, and ignition system, coil and any ballast must be used as per the manufacturers instructions. Electronic ignition, or electronic 'points', is largely a waste of money on a half-decently maintained road car, has much more to go wrong, and cannot be fixed at the roadside when it does, witness those people who recommend carrying a set of points as a 'spare' for such electronic systems - *if* you are able to retrofit them. AFAIK the Magnetronic system is simply a solid-state replacement for the points, what else did you 'throw away' (foolish boy, don't you know that if you haven't found a use for something yet you haven't kept it long enough)?
Paul Hunt

Chris. As a follow on to Paul Hunt's post, I agree completely on the "electronic points trigger". I prefer point myself. If the problem is in the low tension circuit--wire to the coil, coil (part), wire from the coil or the "electronic ignition", you will see the tach suddenly drop to zero when the problem occurs. It it is not the LT circuit, the tach will wind down as engine rpms wind down. A sudden drop to zero, on the tach, is almost always a LT circuit problem. It it is a high tension circuit problem--coil, king lead, distributor cap, rotor, spark plug leads or spark plug, it should show up when you perform Paul's test using a timing light. In my experience (which is a lot less than Paul's) the stopping of the engine can only be caused by the coil, the king lead (coil lead here in the US), rotor or distributor cap. I would remove your coil and bracket and see what they look like. I have seen the bracket tightened to the point it dented the case of the coil and shorted out some of the windings, causing problems similar to yours. The resistance of a 12V coil is about 3.7 ohms and the "Sports Coil" is somewhat higher. (I measured mine when I got it, but do not remember the exact reading. Memory says about 7 ohms resistance.) Have you checked the vents on the carbs and the lines. If they are blocked, it might cause a problem. But, this is only theoretical, because I have never had a problem with them. Keep driving the car and not what happens when it dies. This will help build up a data base so we can make better suggestions. Les
Les Bengtson

Chris, Ballast resistors are usually around 1.5 ohms and are installed in the the primary side of the coil (+ for cars with negative earth). The resistor is wired so that it is bypassed with the ignition switch in the start position and in the circuit in the run position. They are usually a ceramac device about 3 inches long.

If you are using the specified 12 volt coil it's not a good idea to add a ballast resistor unless your magnetronic instructions tell you to add one. If you are using a 6 volt coil it should have a ballast and it should run a little cooler, however it probably will not correct the intermittent problem you have. Follow the suggestions made by Paul and Les for checking the system. Clifton
Clifton Gordon

Les - a 12v Lucas Sport coil I have measures about 2.4 ohms on the primary i.e. somewhere between the 12v and 6v coils. The secondary resistance *is* higher than for the 12v or 6v coils, which seems logical as both differences would result in a higher maximum HT voltage.

Clifton - the coil ballast is a length of resistance wire contained within the loom. The seperate resistor which is a physically identifiable component was an *additional* resistor used on the Opus 45DE4 Distributors which were only supplied to North America.
Paul Hunt

Thanks for all this advice.
I feel my weekend(s) will be tied up for a while now.

I will keep everyone who has kindly posted a reply updated on the outcome, or I'll be back asking for more advice.

Cheers

Chris
Chris Kendal

Paul, I'm aware of the fact that many cars have a ballast resistor in the loom. I was referring to add on resistors because Chris has a 69. The external ballast resistors are very common here in the US and in the 70's they were standard on many cars. In fact our 74 Jensen Interceptor has one. Regards, Clifton
Clifton Gordon

This thread was discussed between 27/08/2002 and 31/08/2002

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