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MG MGB Technical - Backfiring, bucking & other bad stuff
|AFter several years of my 1972 B being off the road because of mechanical/time issues, I finally got everything sorted out late last fall. Rolled it out this spring and had a few weeks of fun. It ran well. Last week I made a run to a town some 30 miles away at 60 mph speeds. About halfway there it started hesitating, with an occasional backfire. It got worse and worse until it was completely undrivable. I brought it home on a flatbed. My first thought was that the timing was way off. Popped the dizzy cap (Mallory dual-point) and found that one of the wire connectors to one of the breaker points seemed perilously close to the body of the distributor. I thought it might be grounding and arcing. I bent it out of the way. This didn't solve the problem. Now I think it may be running way rich. I'm beginning the process of setting up and balancing the carbs, although everything looks normal. Carb floats are OK. The plan is to set up the carbs, then check the timing.|
|Bill. Check the timing and adjust the carbs. Paul Hunt's website, the Pages of Bee and Vee have a good article on how to tune the carbs. My website, www.custompistols.com/ has an article on checking out the ignition system and setting the timing. If that does not correct the problem, run a compression check. A cracked valve will also cause the problem you describe. Les|
|Hi Bill. I had a very similar problem with my 74 GT. It ran fine when cool or off load but as it warmed up the misfiring became worse. It would tick over or rev off load but let the clutch out and it didnt want to know. After a lot changing parts my garage finaly found that the new coil fitted by the previous owner was the wrong type 9 volt ballasted not 12 volt. This was causing the points to overheat and stick. New coil fitted no problems since. Not saying its your coil but the symptoms seem the same so change the points.|
|Don't think its any of the above. Remember this car was running fine evan after its idle years. So on that basis the ignition, carbs etc are all set at least within acceptable limits so don't mess around with the settings at the moment as there is something more fundamental wrong.|
My guess is fuel shortage which will cause weakness, lack of power, backfiring and all the symptoms you describe. It could be dirt being dredged up in the tank, intermittent fuel pump due to dirty points in the pump, a fuel pump earth (in the trunk) partially choked fuel filter or gummy needle valves.
What ever it is I would advise checking the fuel system perhaps initially by removing a pipe from one carb and checking delivery and then locating the fault. I don't think there is a major fauld and would strongly advise against altering any settings until you are sure that the fuel supply is OK
|Backfiring is caused by unburnt mixture in the exhaust and can be caused by grossly retarded timing but the bucking suggests intermittent firing (which would also cause the backfiring).|
I'd go along with John on this one and suspect an intermittent ignition fault such as coil packing up when hot which is quite common and can happen any time.
Also check the connections on the low tension side of the diz. An electrical service inc changing plugs, points, condenser and resetting the timing would be a good idea too.
|Spitting out the carbs and exhaust can be caused by fuel problems as well as timing. An overly lean engine can have dieseling type problems. An engine that gets an intermittent fuel supply can buck horrendously - I had a 1ton truck that would buck so bad the front wheels would hop up and down off the ground. That turned out to be an internal carb leak - too much fuel. I also had a bad fuel pressure regulator on my B that caused low pressure and spitting and kicking at freeway speeds. |
I agree that it's something bigger than a tune-up issue. How's the dashpot oil? Check the fuel pump and wiring, as well as the ignition system top to bottom. Make certain the primary coil wire is good - they tend to go bad early with some aftermarket ignition systems.
|Change the fuel filter as that causes bucking problems and it's cheap. Second, how old is the gas? Gas goes bad.|
This thread was discussed on 24/06/2005
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