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MG MGB Technical - rough idling

I have a 74 MGB GT with HIF carbs. Over the past few years, the car runs rough and backfires, usually within minutes of filling the tank, to the point that it will stall without giving it a lot of gas. The car runs fine when the choke is pulled and after a few more fillups, it runs fine for a while. I originally thought it was a case of bad gas, but it seems more persisant.

I've tried adjusting the carbs, and it will run fine, then the problem returns. Any suggestions?

Tom
TLT Thompson

Tom - The next time the car starts running rough, get out and remove the gas cap. It sounds very much like you have a clogged vent line to the tank. The tank in your car vents through the separator in the trunk, via a line into the charcoal canister in the engine compartment. The line from the separator in the trunk to the charcoal canister has a fitting that often rusts closed, blocking the vent line. cheers - Dave
David DuBois

Dave,

I have tried that and replaced the hose at the charcoal canister canister to the valve cover, still without an improvement. Dry gas doesn't seem to work either or octane booster. Willing to try anything and will try the gas cap again this spring.

Tom
TLT Thompson

HIF carbs have a bimetallic strip in the float chamber which supports the jet. As the fuel warms up in the chamber, this lifts the jet up to lean out the mixture. When tuning HIF carbs, you start the engine and run it for 30 seconds ataround 3000rpm, then spend no longer than around 2 minutes makign adjustments, then hold the revs again for another 30 seconds. This will ensure you're not trying to adjust the carbs with the bimetallic strip lifting the jet up, because any adjustments you make will be futile. Have you tried stripping and cleaning the carbs?
Ross Kelly

Thanks for the comments Ross and Dave,

I replaced the bimetallic strips last year when the problem was much worse and it seemed like the car was working on 3 cylinders. At that time, I cleaned the carbs as well. There isn't a difference whether the car is cold or warm, the problem still persists.

Tom
TLT Thompson

Next thing to check is the condenser in the distributor (when the SU fuel system is not working, check the Lucas ignition). Strange as that may seem, I fought a similar problem in our daughter's GT some years ago and it turned out to be a condenser that had become flaky. Cheers - Dave
David DuBois

"start the engine and run it for 30 seconds ataround 3000rpm, then spend no longer than around 2 minutes makign adjustments, then hold the revs again for another 30 seconds"

FWIW this is actually more applicable to HS carbs than HIF, since the HIF has a bypass port to prevent fuel pooling in the carb body. My HSs gradually choke themselves to death in a long idle, whereas the HIFs don't.

Backfiring in the exhaust, other than on the overrun, is usually a symptom of intermittent ignition, or possibly exhaust valves sticking open. Presumably after refueling it is a hot start, what's it like on other hot starts?
Paul Hunt 2

Thanks Dave and Paul,

I repalced the condensor and rotor and then installed a pertronics elctronic ignition 2 or 3 years ago.

I'll try a few more starts. So far, I haven't had too many chances at a hot start since the car is in the stalling period unless the choke is pulled out about an inch, to which it purrs like a kitten. I'll try adding new fuel and then see what happens to test next.

Thanks for the suggestions.

Tom
TLT Thompson

What does the tach do when this is happening? If it flicks about but otherwise is steady it points to problems with the pertronix or its connections. If it is steady, and you can reproduce the problem at a standstill, compare the dwell reading with when it is running well. Also clip a timing light onto the coil lead and each plug lead and compare the flashes. When on 1 or 4 point it at the crank pulley and compare the timing. There comes a point when you have to ignore what seems logical i.e. it only happens after filling up, and go back to first principles. Like the bloke whose car wouldn't restart after buying vanilla ice-cream, but would after chocolate-chip.
Paul Hunt 2

Thanks Paul, I'll try these at our next MG club tech session.

Tom
TLT Thompson

Tom,

I notice that vacuum issues were not mentioned. I had a few B's in which the manifold nuts loosened. Also, check the valve settings, even though you may be sure of them. Just solved the same problem in my TD this way.

Tom
Tom Balutis

After firing it up this weekend, the car ran perfect. No stalling or backfiring, a steady tach ready with no fluctuations. So I figure this roles out the bad gas symptom. I ran the car and filled er up with fresh gas, and it remained running perfect. Back to checkingthe other items previously mentioned during the next tech session in a few weeks.

Any other ideas are welcome.
TLT Thompson

This thread was discussed between 09/04/2008 and 21/04/2008

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