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MG MGB Technical - 79 MGB missing when hot

Posted 04 September 2016 at 02:03:36 UK time
MVB Brothers, Pennsylvania, USA
1979 MGB begins missing badly when engine is hot
I have had problems with the 1800cc engine stopping while I am driving. When stopped, I then could not start the car and have had to have it towed to a garage on four different occasions. The mechanic thought it was the Stromberg carb so I had him replace the Stromberg with a Weber. This seemed to take care of the problem. I then had problems starting the car. After another few times at the garage, they installed a new electronic ignition system. This seemed to again take care of the problem until now, the car starts fine and drives like a dream but It begins missing terribly when the engine is hot. It misses in all four gears. I, we are out of solutions. Can anyone help with what other problems could be causing the missing. The car has 43,000 miles and everything else with the car is beautiful. I love this car but my wife won’t ride with me because it has stopped twice while she was in the car and insists I have my phone with me due to a time when it stopped and I had to walk home because I left my phone at home. Thank you for anyone who may help.



MVB Brothers

A few items to check:

Does the tach "bounce" when the miss fire occurs? Check all ignition electrical connections. Distributor cables, coil wires - lead and secondary leads. Pull the plugs and check the gap as well as color.

Check all vacuum lines for deterioration and splitting.

Check all intake/exhaust manifold nuts to be sure there is no vacuum leakage.

To check for vacuum leaks, spray carburetor cleaner at mating surfaces around the manifolds and carburetor base, as well as vacuum lines. If there is a momentary change in the engine idle, you have a leak.

Cheers

Gary

79 MGB
gary hansen

The first thing to check is whether your fuel tank is "breathing". Take the fuel cap off and take the car for a ride. If it runs OK the emission breathing system is clogged up. When the car acts up, try loosening the fuel cap and listening for air entering the tank. RAY
rjm RAY

Next time it stalls, have someone key the starter while you hold:

1- one of the spark plug wires approx. 1/4 inch from the spark plug to see if you are getting any spark. If not,

2- then hold the coil wire to the distributor and again see if you get any spark.

If you are getting spark, then follow the fuel system from tank to carburetor. IE. clogged line, fuel filter, fuel pump, emissions

cheers

Gary
79 MGB
gary hansen

First thing is as Gary says i.e. look at the tach as soon as the problem occurs, and that is before you switch off, press the clutch pedal down, or anything else i.e. when the momentum of the car is still spinning the engine.

Has it dropped to zero/bouncing around? Or is it still showing the engine is spinning.

If zero or bouncing then it is an ignition LT problem. If still registering the it's HT, fuel, or if you still had points could be the capacitor.

If it's at zero is the ignition warning light on or off?

If it's off while the engine is still spinning, and comes on when the engine stops, then you have lost the feed from the ignition relay to the coil, cooling fan and fused ignition circuits. If it's off when the engine is spinning and stays off when it is stopped but the ignition switch is still on, then you have lost the feed from the ignition switch.

If the tach still registered until the engine stopped spinning, what does the fuel pump do when you turn on the ignition immediately before the engine restarts? If it chatters away like billy-oh then it's likely the pump had cut out.
paulh4

As above, what the tacho does and whether the pump goes silent, OR noisy can point to ignition or fuel supply problems. Also a failure in the tank vent (the cap)will soon create a partial vacuum in the tank, the pump will hammer away but the engine will be starved of fuel. If this is happening and causing your problems removing the filler cap will result in an obvious in flow of air.
Are these cut-out situations in hot weather too? What condition is the carb heat shield in? Your levels of ethanol, in fuel, in the States make makes it more volatile and the heat soak from the engine and manifold can cause the fuel in the float chambers to start vaporising.
Allan Reeling

Thank you so much everyone. I checked the Tach and it did not flicker or jump when the engine began missing again. What did happen, I started the car and drove it for about 1/2 hour with no problems with the engine missing. I pulled in the drive way and turned off the engine. I remembered that the last time I drove the car, I had gone to the doctor and when I started the car after the appointment is when the car started missing. I started the car in the driveway and sure enough, the engine began missing again really bad. Any thoughts? Michael
MVB Brothers

Put some dry gas in the tank. Maybe there's a little bit of water in the tank? Just a little bit of water in the tank will cause the car to run badly...I always use high test gas. Dry gas is cheap so put it in and you can rule out the water issue.
S

Put some dry gas in the tank. Maybe there's a little bit of water in the tank? Just a little bit of water i will cause the car to run badly...I always use high test gas. Dry gas is cheap so put it in and you can rule out the water issue.
S

How are the spark plugs? As you did not mention them in your post. Also the coil could be breaking down. Is it hot to the touch when the break down occurs??

Cheers

Gary
79 MGB
gary hansen

Sounds like heat soak. Anyone remember the tale whereby someone's MGB had this problem when buying choc-chip ice-cream but not vanilla? One of the was at the front of the store and the other the back, so different lengths of time in the shop, and different heat-soaks. Next time you get the opportunity repeat your half-hour drive then park with the bonnet up, and try a restart as before. If no misfire then it's probably heat affecting one or more components.

Any water in the tank will be removed all the time you are driving as the pickup is in a recess right at the bottom. In any case unless the temps are very high and the car is left for some time the float chambers will be full or nearly so of the fuel you last ran cleanly with.

Misfiring with no tach flicker points (pun not intended) to HT, fuel or possibly condenser (but electronic ignition noted). If you are getting backfiring in the exhaust with the misfire then that implies intermittent ignition, if not then more likely to be fuel.

Repeat your test to get the misfire and try and gauge the temps of the coil and carb. If the coil is hot to the touch wrap cloth soaked in cold water around the body (not the contacts) and see what that does. Similarly round the carb.

paulh4

Thanks s,d, Gary and Paul for your insight, I just put in the dry gas additive and filled up. The car ran with no misfiring. I am going to let it sit overnight and redo the test again tomorrow. The spark plugs were checked a few weeks ago and they were fine. I appreciate you all sticking with me in this adventure. I will follow-up to let you know how it goes tomorrow. Michael
MVB Brothers

Paul. I do recall the ice cream story. It was a Pontiac Catalina (1964-65??) 389 C.I.D. V-8. The owner wrote to General Motors explaining the non start issue when he went to the grocery store to buy ice cream. When he bought vanilla, the car ran. When he bought chocolate the car did'nt start.

Normally, a GM tech would send a letter to owners' complaints with solutions to their problems. But this one really got him. He had to see it to believe it. So he made a personal visit to the owner's home.

As it turned out, the vanilla ice cream was located at the very front of the freezer nearest the entrance to the store. It only took x-minutes to buy it and return to the car. The chocolate ice cream was located in the rear freezer of the store, further from the entrance, and took longer to extract before returning to the car.

That time difference is when the vapor lock took place resulting the car's failure to start.

That story ran through GM for years.

Cheers

Gary
79MGB
gary hansen

Thanks for the clarification, Gary. It was on an MGB board or list, so that was how I remembered it.

The MGB has its own odd stories, like the one in Australia that packed up at the same point on a commuter bridge every time making diagnosis tricky to say the least, but nowhere else. Never heard the outcome of that, but there was a more recent one in Ireland that did the same on a particular hill, which happened to be a narrow lane so equally difficult to diagnose with other traffic passing both ways. As I recall there were a number of things done that seemed to improve it, rather than one clear thing that completely fixed it.
paulh4

This thread was discussed between 04/09/2016 and 16/09/2016

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