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MG MGB Technical - Backfiring when shifting and on overrun

I have a '73 MGB with 139,000 miles on original engine. It still has the complete emission controls on it. About a year ago it started to pop (light backfiring on the overrun. This summer I changed the gulp valve, spark plugs, points etc, and adjusted the ignition timing. It seemed a bit better but not perfect. The spark plugs are the correct tan colour so the mixture must be close. Now it has started to backfire between shifts, and in the past this has been the gulp valve. The new one I installed is made in China, so I wonder about the quality of it. Other than this the car runs great, starts immediately, idles nice, and compression on all 4 cylinders is good. Several things: The distributor shaft bushings are worn making point adjustment a little tricky, and it has a Monza free flow exhaust on it whic has been on for six or seven years and may be losing some of the baffles as the exhaust seems to be getting louder, but the outside of the exhaust system is solid. Any ideas? Is it caused by too rich of a mixture when closing the throttle?
Ralph
Ralph

Could be too rich mixture, but your plug color tends to discount that. I would say more likely you have an exhaust leak, as a backfire on overrun is a classic sympton of that. Perhaps your pipe has a small hole, or the clamps holding the sections together have loosened.
Paul K

Replaced a worn distributor . . . tuned great, intermittant backfire . . . finally sucked on the vacuum advance tube --- distributor plate didn't budge = internal rust in vacuum advance. Changed same. Lived hapily ever after.
Dan Robinson

I have that problem with my highly modified Datsun Roadster. I connected an A/F gauge and it confirmed what research on the internet said - it is lean on the over-run/deceleration. All cars have a tendency to do that including a newer car like my Miata. Nothing is going wrong with my Datsun engine because of this. Also caused by holes in the exhaust or a low restriction/loud exhaust. Since your baffles are going you may hear the backfiring more than if your exhaust was new and quieter.

Personally I like it as it keeps drivers of my ass and if they're tailgating I just downshift and get off the gas and listen to the popping and cars back off :) Kinda sounds like a gun going off :)

New York drivers, like those in many other states, don't know how to drive and never leave enough room for braking - everybody is in a rush - not that I drive slowly, but I do leave room between me and the car in front of me.
Mike MaGee

Ralph,

I had the same symptom on my 74 1/2 "B" after installing K&N pancake filters. I richened the mixture up a bit and that cured it.

Cheers, Doug
Doug Keene

Confirm that the gulp valve is working. Disconnect the pipe from the air pump, rev the engine and release the throttle, and you should get some vacuum at the air pipe port. If you have a vacuum gauge and that is connected to the air pump port with no opening left to atmosphere I'd expect to see the gauge to jump from no vacuum to something in excess of 20 in Hg when decelerating, and maybe holding it even when the revs were back down at idle.
Paul Hunt

Ralph,
For what it's worth, any exhaust leak upstream of the muffler can cause backfire on the overrun. Air being introduced into a hot, fuel rich exhaust and all.....
David
David

Woops! Paul already said that. Sorry. Paul, was that your article in Classic Motorsports on polarity changes? If so, good job!
David
David

Not me, that's Paul Kile up in Sack O' Tomatoes, er Sacramento. Strangely enough, that's my hometown.
Paul K

Thanks, guys.
I'll try the above suggestions and post what the problem was when I find the solution.
Ralph
Ralph

This thread was discussed between 29/11/2003 and 04/12/2003

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