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MG MGB Technical - wants to die when warm?!

O.k. follow along and see if you can help..Please!
The cap and rotor are recent and checked out clean and not cracked. I checked the motor in the dark and saw no arcing. Fuel and air filters are recent. The pertronix pointless is about 2 years old and never showed signs of trouble. It has HIF4s and the springs inside are about a year old and oil is fine. The coil may be the culprit since it is not recent.
The air filter elements are not covered. The exhaust is Peco and not recent. The fuel pump is aftermarket and about 6 yrs old, reliable so far. There is an inline regulator just before the carbs and it is not recent. I adjusted it up and down to make sure it was not seized. It is about 5 years old. The tranny is not OD.

Now the problem: It all started a couple days ago. At about 40-45 mph the car wants to die. It slows down and can be kept running by slowing the car. It does not miss or pop etc. just slows and wants to shut off. Once it slows to 10-15mph, I can pop the clutch in 2nd and it starts (if it dies) and it will then run fine until it gets back up to speed. I cannot reproduce the problem when parked by revving it. Once it cools, it is fine until it warms up again, around 2-5 minutes of driving.
kids1

One more thing, I changed the plugs to see if it helped; it did not. The plugs are same as the old ones, NGK same as I've used for years. The car was great and reliable until this problem. I checked the timing and the distrib had not moved.
I can't get around in it until this is solved.
kids1

Hi.

I would try substituting the coil, distributor cap and rotor arm. New(ish) doesn't necessarily mean good, and cracks aren't always visible.

If it has the throttle butterfly has spring-loaded valves I would consider removing the valves (which, as a bonus, improves airflow at full throttle) and solder the holes up.

Will it run better with some choke ?.

Is there a significant hiss when removing the fuel filler cap ?.

HTH.. Don.
Don

It sounds like your problem is fuel related, not ignition. If your ignition was cutting in and out, you would get a "bang" when the abundance of fuel in your exhaust ignited.

I would look at the fuel filter first, then the fuel pump, then the carbs. Does it ever go faster than 45mph? If not, your fuel supply is being cut short somewhere. Is your fuel pump solid state or points? It may be ready for a tune-up.

The coil is blamed much more frequently than deserved, as they rarely go bad. If it works at low rpms, it should work at high rpms.
Jeff Schlemmer

I don't have the problem to work on when it is idling and hood open. If the problem begins, I can't pull over and check anything because it dies. Once restarted, it is fine.
I did check the fuel filter after it died and I drifted to the side of the road. It was full. I'll check if it runs with the choke after it begins to die; good thought.
kids1

Put a volt meter on the battery terminals and take a reading. Then, start the engine, run it up to about 1,000 rpm and check the voltage on the battery clamps to see what the alternator is doing. I had a similar problem last week that duplicated what you describe. The problem was the plug was loose on the alternator and the alternator was not charging properly. On the battery, at night, the car did not want to accelerate well and would top out between 45-50 mph.

If the charging system is good, David Dubois has a tech article on the SU fuel pump on my website, www.custompistols.com/ under the MG section. You can use the figures for the SU pump to see how your pump compares.

One thing Paul Hunt taught me years ago is that if it is a low tension electrical problem, the tach should drop to zero immediately when the engine cuts out. If it does not, look to the HT circuit or the fuel system.

One thing to consider is the ignition relay which provides power to the white wire circuit. If it is going bad, you can experience the problems you describe. That is the black can located forwards of the fuse box. Bad connections, or a bad relay, will cause the white wire circuits to cut out, affecting both your fuel pump and your ignition circuit. Les
Les Bengtson

I checked connections at the alt, coil, replay, and anything else I could find. The aftermarket fuel pump is sealed and cannot be rebuilt as far as I can tell.
Pulling the choke on as it tries to die does nothing. If I shut the car off (or let it die), it will restart and run fine until it reaches about 40-45mph again.
I bought a fuel filter this morning and did NOT install it. The whole way home from the shop, the car did fine! Ugh!
kids1

I had exactly the same problem, and it turned out to be a cracked rotar arm. The car would run fine untill it reached speed and then the problems would start - loss of power, miss-firing, and then the engine would die. You could get it going again by popping it into gear at lower speed and most of the time it would then run back up to speed before repeating the same problems. It turned out to be a hairline crack on the rotar arm, which was new, which was arcing out the spark. I replaced the arm and it now works fine.
Mike
MIke

Kids - The fuel pump could be the problem. Since you have HIF carbs, I'm not sure how you can determine the fuel level in the float chambers when the problem happens, so the only way to check this may be to install a different fuel pump. I had a very similiar situation with our old Mazda truck which used one of the little, square, run all the time, make a lot of noise, Facet pumps and started doing the same thing to me. Not being a MG with a SU fuel pump, it took a while for me to finally get the idea that it was the fuel pump. When it did, the next time the problem occured I shut the ignition off, got out and checked the fuel level in the float lever (the carb had a clear front on the float chamber) and found it empty. A new fuel pump cured the problem. One of the cheap Facet pumps ($40.00 at NAPA) might be the way to test if it is the pump. After the problem is solved, the Facet pump can be permently installed and wired as a backup pump. Good luck - Dave
David DuBois

Hi.

If the choke didn't make a difference it isn't a manifold air leak.
The problem is beginning to sound like vapour lock, do you have the fuel pipe / regulator / fuel filter close to the exhaust manifold ?.
Does removing the bonnet seal (if fitted) help the problem by allowing more underbonnet airflow ?.
I suspect that it is time to disconnect the fuel pipe, put it into a suitable container, and (with help and great care) switch on the ignition and check the delivery rate.

Don
Don

Don, will do. I'm at wits end and willing to try anything to solve it.
The fuel line has not moved in years and the car has been fine until this.

kids1

Here's a long shot, but simple to try. Remove the fuel cap when the car is acting up and see if the tank is pressuring. US model MGBs of later years have a vent line from the tank to a cannister in the engine compartment. I had serious issues with my '71 model...symptoms somewhat similar to yours. Turns out the tank would pressurize if the car was driven in the sunshine and the car would start running real badly. Not entirely sure why, but it did. I solved the problem...I'll tell you how if you're interested...but you can easily troubleshoot by removing the fuel cap as described above. Best of luck...these things can be frustrating

Dave
NW Florida
David Tye

Daily storms are keeping me from checking and tinking but thanks to all for the suggestions. I will get on it asap.
kids1

O.k. Today I put in a new fuel filter, air filter and a fuel sending unit while the weather let up. It seems to run fine so far this evening. Keep your fingers crossed. I'll check the rest when the weather breaks again.
Thanks everyone!!
kids1

Cars with emissions plumbing can suffer from *vacuum* in the tank, causing fuel starvation, if the vent is blocked and this would normally show up after several miles of driving. The same blocked vent could cause the tank to pressurise if parked in hot sun, which could cause problems of excess pressure causing a rich mixture and probably difficult starting. Removing the fuel filler cap will 'cure' both. Other blocked plumbing could cause manifold vacuum to tend to suck the fuel out of the carb jets and cause fuel starvation, which is exactly what the anti-runon valve does. Disconnecting the pipes from the carb float chamber vent pipes (just as a diagnostic) will cure this. Check the fuel delivery by removing a pipe from a carb and directing it into a container and turning on the ignition, it should deliver *at least* one (Imperial) pint per minute consistently and with few if any bubbles. When it starts happening switch off to stop the engine and turn on the ignition again. If you can hear the pump click more than once every 20 secs you could have a faulty float valve or blocked float chamber vent pipe causing a rich mixture.
Paul Hunt

This thread was discussed between 28/05/2005 and 05/06/2005

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