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MGB 73 MGB ignition troubleshooting

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MartyMGB73

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I own a 73 MGB with an XR700 electronic ignition. The car runs fine until I take it out on 90 degree plus days. Then after about 10 miles of driving the engine starts to sputter and quit. It seems to lose its spark. After an approximate 15 minute cool down, it will usually start up normally. I replaced the coil last summer thinking I solved the problem. But a hot summer day this year proved me wrong. I'm suspecting the electronic ignition?
 
Thanks for your insight. The heat shield is in place. I see gas in my glass filter. I hear the ticking of the fuel pump. But still the fuel delivery my be the problem. During the last stalling incident, instead of letting the engine cool down, I was able to start and run the engine by choking the carbs.
I guess my question now is, if the fuel pump is failing or there is a vapor lock, will the choked carbs pull gas through the fuel system/pump???
 
If the engine will run by increasing the mixture on the carbs, that pretty much rules out an electrical/ignition problem.

The vacuum on the carbs will not pull fuel through a dead pump, but the pump could be weak and supplying just not quite enough. Is adequate fuel getting thru your filter (if its full that means fuel is getting into it, but there's no assurance it is exiting at the same rate).

When you hear the fuel pump ticking, does it continue or does it tick for a minute or 2 then stop? The SU fuel pump stops once the float bowls are full and the fuel system is pressurized. If it never stops, that either means the floats are not closing off and flooding the cylinders, the pump is not moving enough fuel to fill the floats, or there is a leak somewhere.

Simple test: Disconnect the line from the fuel filter to the carbs, stick it in a glass bottle or jar. Turn on your key, the pump should tick fairly quickly and fuel should be pumped out of the line into the jar with a good solid flow. If you do get a good solid flow, block the end of the line and there should be a small amount of pressure build up (and the pump should stop ticking or at least drastically slow down if you don't get a perfect seal on the line end). If that happens your pump and fuel delivery are probably OK, and its time to start investigating the carbs.

If you only get a trickle, and/or the pump seems to be ticking slowly with the line fully disconnected and open, start working backwards. Disconnect the line going into the filter, see if you get good flow there. If not, check for a leak or kink in the hard line running down the right hand side of the car under the floor back toward the pump. If all seems OK there, then the pump or the lines from the tank to the pump become suspect.
 
Thank you for your input. The fuel pump is an after market DriveMotive REPV314515 unit. It tests fine when cold, plenty of delivery, no leaks. The next time I take the car out , I will be prepared to test it when the engine fails hot. What are your thoughts on whether it could be a compression problem or maybe the carburetors set to lean?
 
Pull a spark plug (fortunately on the "B" they are a lot easier to get to than an American V8) - the plugs cat tell you a lot about rich/lean condition based on the color and condition. Dark and sooty would indicate too rich, an almost bleached brownish-white and/or cracked ceramic would indicate excessive lean. There are lots of youtube vids and images that you can compare your plugs to and get a good idea of what is happening inside the cylinders.

Compression test is easy to do, especially if you are already looking at the plugs. The gauge is pretty cheap and just screws in like a spark plug - disconnect the coil so it doesn't try to fire, hold the throttle plates open (a brick on the pedal can do it). All cylinders should come up pretty even - mine show around 130 which is a little low but as long as all are withing 5-10 of each other it should be fine. If you have 3 at 130 and 1 at 60, theres a problem but that would show up hot or cold.
 
It's ironic you responded with these suggestions today. I preformed a compression test on the cylinders today, hot and cold engine. All the cylinders tested around 135 psi. Dropped only about 3 degrees when hot. My spark plugs always look the same since I've had the car. The plugs in cylinders #1 and #4 are firing normally, but plugs #2 and #3 always look shinny and black, maybe oil fouling. ???
I tuned the carburetors today using a tool called "Gunson Colortune". Unfortunately the carbs were not that far off. Time will tell if the Colortune has any effect on the plugs in cylinders #2 and #3.
 
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