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MG MGB Technical - Still backfiring - What to check?

Need the help of this board to diagnose my MGB problem. I have a 80 LE with twin HIF carbs and all pollution equipment except the cat converter (obviously). The car had a very serious backfiring problem during my trip from St. Louis to New Jersey. When I let off the gas I had such a loud backfire that I actually blew a hole in the front muffler. Since then I found that the check valve in the air injection rail was broken letting air into the system. I replaced the check valve, torqued the head, adjusted the valves and the the serious backfire problem did stop. I also installed a complete new SS exhaust system.

However on the trip to and from Grand Rapids I noticed that I still had a slight backfire (something like a burble) when letting off the gas. I didn't think much of it until my friends driving behind me let me know that at night they could see flames coming out of the tailpipe when I let off the gas.

For general information the car ran very well on the trip and got about 23 miles to the gallon. I did seem to use a little more gas than the MGs I was travelling with but chalked that up to differences caused by my slightly hotter cam. (Elgin 289 grind).

Any ideas of what to check now? I plan to look over the new exhaust system tomorrow to insure that there are no loose joints letting air enter. All suggestions will be appreciated as this is getting very old and I can't afford to ruin any more exhaust systems.
Rich Berger

My car backfires slightly under deceleration, in fact all my old cars do. How loud depends on the exhaust system. Nothing is wrong.

BTW, since I changed to a HP exhaust on my MIata I can hear that car backfire slightly too.
Mike MaGee

I thought a slight backfire noise was typical when 'engine braking.' Basically it's the mixture - too much fuel at the intake, not all of it consumed inthe engine, occasionally some ignites inthe exhaust. Blowing holes inthe muffler is another matter, but excess oxygen from your air rail might explain that.

If you're really concerned, you could check that the piston in the carb is dropping properly, and maybe replace the springs or lighten the damper oil.

Steve




Steve Aichele

Rich,
Popping while de-accelerating is as Steve stated usually caused by a rich fuel mixture. Lean out the carbs a couple of flats and see it that makes a difference.
Roy
Roy Dougherty

Backfiring in my experience is not normal in most cars, old or new, MG or otherwise. My 69 would pop through the exhaust on decel when I first acquired it, and the cure was as simple as tightening the exhaust flange nuts where the twin front pipes meet the exhaust manifold. You may also need to replace the exhaust flange "donuts".
I also removed the smog pump, valve, and air rail and did a comprehensive tuning. Runs great with no misbehavior.
Andy Blackley

I don't believe you can easily alter the mixture on HIF carbs. I think the gulp valve is the problem.
George M. Badger III

Rich,

My 79 is backfiring now, and will continue to until we take the emissions gear off and give the junk back to Robert. Ours has been diagnosed as the gulp valve too.

Have you checked your timing?

The Miz

Gulp valve gone = backfire gone.

:-)
The Miz

Whilst the HIF may have had an 'anti-tamper' device when new AFAIK this is just an alloy cup in the adjuster hole, and once picked out allows a wide adjustment of mixture.

Even non-emissions cars burble and crackle, more so if the middle box isn't giving much sound deadening, on the overrun on a long downhill stretch although I have never noticed it on the flat.
Paul Hunt

This thread was discussed between 16/08/2004 and 17/08/2004

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