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MG MGB Technical - 74 Runs OK then not OK

My early 74 with stock HIF-4s, stock 25D distributor, aluminum head, and full CA smog equipment compliment suddenly started idling roughly, backfiring through the carbs and dying. These symptoms, which began shortly after passing the CA smog test where all indicators were well within limits, typically kick-in when the engine is completely warmed up.
Since the problem began, I've replaced the ignition (Ignitor) and fuel pump, removed and cleaned the fuel tank, installed a rebuilt short block (the no. 3 cylinder had low compression due to a ring failure), inspected and cleaned the carbs and installed Grose-Jets, replaced the defective distributor vacuum advance, installed a new coil. I've also tested the fuel delivery to assure a continuous flow at the right quantity, isolated the smog control components, and monitored dwell (rock steady). The engine rebuild resulted in all new gaskets and checkout of all hoses and vacuum connections.
The problem persists.
I can start up and warm up the engine and it runs smoothly for several minutes. It is even driveable but seems to surge under load. The engine will then begin idling roughly, backfiring through the carbs (when the throttle is opened) and dying. Upon restart, the engine will often idle smoothly for a few minutes and then revert to the symptoms mentioned.
Any help would be appreciated.
Gary Cash

Gary,
I'm having similar problems with my '71...but I also have fuel venting through the charcoal cannister about the time it starts running really badly (and then quits). See my problem description under 'MGB venting problems', this web page. I still haven't solved it, but noticed in your problem description that you didn't mention checking your fuel tank venting system. I've read that if the tank vent is obstructed, you can create a vacuum in the tank, with resultant fuel starvation...possible backfiring, etc.. If you haven't checked the vent system already, I'd blow some air through the tube, from the cannister under the hood, through the vapor separator in the trunk, to the tank. Good luck...

dave
David

Dave,
I'll give it a try. I'd already tried simply removing the gas cap to see if the problem was a vacuum but it had no effect and I do have good fuel delivery. I read your link. Good luck with your problem.
Have you checked your floats and jets?
Cheers,
Gary
Gary Cash

Gary,
I should have picked up on the fact you said fuel delivery was OK. Since you took the cap off, and it still did the same thing, I don't see how it could be a venting problem. One other idea: have you replaced the distributor cap? Reason I ask, I had a problem with my 83 Honda Prelude that had me seriously stumped, to the point I just started replacing stuff. It was exhibiting similar symptoms as your MG. I'd inspected the cap, and it looked fine...no cracks. After I replaced it (anyway), the car ran great. I put the old cap on my bench, came back 2 days later, and the cap was laying in 2 pieces! I couldn't believe it. Obviously, it was flawed, and I couldn't detect it with my eye. Possibly, your cap might have a minor defect that degrades with thermal expansion.

Regarding my MG problem, yes, I did check jets/seats, and the float valves. My first thought, upon seeing the raw fuel running out the cannister, was that a float valve must be stuck. But on a subsequent occasion of raw fuel leaking, I pulled the vent tubes off the top of the cannister, and it was quite apparent that the gas was coming from the tank...not the carburator bowls. So I'm still perplexed.

Thanks for your thoughts.

dave
Dave

Hi,
I had this problem when I first got my car and was sorting out all of the work the previous owner had done.My car would not idle for more than 30 seconds or so. I found out that the fuel pump was not a LOW pressure SU type, but one meant for an American car and the pump was simply overpowering the floats and jets. When that happened the engine would "load up" and stumble and stop - too rich. Check the condition of the plugs if they are black and sooty I would be looking in that area. Also take a look at how full the fuel bowls are (both of them) as soon as the thing stops. Try running the car with the overflow hoses dumping onto the ground to see if the bowls are over filling. DO NOT RUN the car like this for more than a short while 10 - 15 seconds. A proper fuel pump (Keep the banjo bolts lined up)and a set of Gross jets installed after cleaning the carbs and bowls thoroughly,(some disassembly required)Check for any vacuum loss from rubber hoses being split or disconnected better yet replace them all.Retune carbs and set proper advance.
Alan

Gary;
You mentioned new ignition ignitor. Is yours stock electronic or after market? I have heard of three people having problems with rotors failing when aftermarket electronic ignition systems are used. apparently the stock rotors cannot withstand the higher voltages, and after some time, they short right thru the rotor to the dist shaft. We had one Austin Healey in our group fail this way driving back from Minnesota this past summer. Started out like fuel delivery problems, as a matter of fact he thought he ran out of gas.(twice) It finally died, and was towed back on a trailer. Turned out simply to be the rotor.
I have also had similar problems with the little carbon contact in center of the dist cap.

Good Luck!

Pete Haburt
Pete Haburt

YOu might try the anti-run-on valve. It's on the 2 inch shelf on the front of the firewall in front of the cannister. If it doesnt' open or close right it will starve the engine for fuel, according to the Hayne's manual. When the engine is idling, hold your hand over the bottom of it, and it should cause the engine to start to die, and when you let go it should start runnign again. Also you should hear a loud click from it a few seconds after you shut the engine off. What it does is cut the fuel off to keep the engine from "dieseling" when you shut the switch off. It probably runs off a vacuum line from the intake manifold, so if it's not closing right, it could allow a vacuum leak to the manifold.
Ken Thompson

Some sensible ideas here
I had trouble with a distributor cap which had me going for a while - that and the rotor arm are easy to replace and certainly worht trying.
T Green

This thread was discussed between 04/11/2001 and 09/11/2001

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