MGB & GT Forum

Hesitation and Backfiring

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Mark Jones Avatar
Close to Sarnia, Ontario, Canada   CAN
1995 MG MGF "Barney"
1996 Land Rover Discovery
I've been struggling with my MGB GT all summer, trying to find the cause of the hesitation and backfiring that developes while driving along at any speed, even when just idling.

I've replace the fuel filter that I have installed ahead of the fuel pump and have checked for good fuel flow to the carburetors. The distributor cap, rotor and spark plugs are in good condition. The points and gap is good and I have also replaced the condensor with a known good one, but this makes no difference. Two nights ago a mechanic friend suggested checking the timing. I haven't had a chance yet, but the distributor's orientation hasn't changed. I've poured half a can of Seafoam directly into the carburetors and the remainder into the gas tank.

One thing I did notice the other night after changing the condenser was that the fuel flow to the clear plastic fuel filter I have installed just before the carburetors (for diagonstic purposes) had only a trickle of gas going into it, even though when I had disconnected the hose going into the carburetor I had a nice stream of fuel going into an ice cream container. Normally the clear filter is about half full of gas, but the other night it would never get more than a quarter full. And when I started the car I didn't here the electronic SU fuel pump ticking prior to turning the key, where as before it always ticks bringing up fuel pressure

Could the problem be gummed up carburetor floats, incorrect timing or my 2 year old electronic SU fuel pump working intermittently? Or could it be something else I haven't thought of? The backfiring really has me stumped.

Going through MGB withdrawal in Nova Scotia

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manning John Craven
Charleston South Carolina, USA   USA

Checking the timing should be high on your list. Not sure what you mean when you say orientation has not changed, But I would not assume that it is correct. Lots of possibilities for the correct setting depending on your year and distributor.

Mark Jones Avatar
Close to Sarnia, Ontario, Canada   CAN
1995 MG MGF "Barney"
1996 Land Rover Discovery
What I mean is that the orientation of the distributor has not changed since the last time I set the timing, which was early last year, long before the problems I am having now. Easy to tell because I put a make on the distributor and engine just in case I had to remove the distributor while on the road for some reason.

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Ken Lessig Avatar
Nevada, Tx., USA   USA
Along with thew timing, take a look at your plugs. I'm thinking they may have a whitish look to them, indicating a lean condition. If so, with the engine idling, spray a bit of carb cleaner around the bases of your carbs and around the intake manifold. If you hear a sudden increase in RPMs you've found a vacuum leak.

Vern Avatar
Vern Vern Smith
Athol, Idaho, USA   USA
A timing light can tell you a great deal besides timing. You can see if each cylinder is firing consistently, or clamp it on the coil wire to see if you have an intermittent miss (mine did, and a new Pertronix ignition made it run like a top.) A good old vacuum gauge is also a good tool to have. It will tell you valve condition, timing problems, mixture settings, vacuum leaks, etc. Just be sure all is well with the ignition before you start chasing fuel problems.

johnbamford John Bamford
Colorado, USA   USA
1969 MG MGB
Yeah, I'm with Vern on checking each basic system and eliminating possibilities. The old "Occam's Razor"....whatever's left, however apparently illogical, is the problem. What year car? Go to the Teglerizer site (Paul Tegler), I think) and look up your proper timing to set initially. It depends on your car year and distributor number which you ought to know as much depends on it. I found that Haynes gives a general setting that was flat wrong for my 69 B Roadster. They show 14 degrees and mine is supposed to be 20 degrees which, whatever it is for yours, is a good place to start then advance till it pings and back off. Great idea on marking the cizzy and engine once you get it set. Have to do that one myself. Be sure to check mechanical advance and vacuum advance to be sure both work properly per the chart.

Had some missing problems and found: 1. Slow vacuum advance leak so replaced vacuum unit. 2. Bad sparkplug and 3. Timing set wrong. Good luck.


John Bamford
69 B Roadster Pale Primrose Yellow
100K, HS-4 Carbs, Rebuilt Head
Owned for 8 years
Denver, CO

Victoria, BC, Canada   CAN
I'm thinking lean too!! Maybe a intakeleak, but probably just lean!!

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