AMGBA website home page or AMGBA Photo Gallery or AMGBA Club Blog
Subscribe in a reader

Join or renew today and receive a free t-shirt or tech CD, see details in the join the club section at!

Author Topic: '78 B Backfiring On Acceleration  (Read 2712 times)

0 Members and 1 Guest are viewing this topic.


  • Message Board Member not current AMGBA Club Member
  • **
  • Posts: 11
'78 B Backfiring On Acceleration
« on: June 29, 2014, 02:15:10 PM »
I have a 78 B with the standard factory configuration (Zenith carburetor, etc.). I recently fixed a problem with backfiring during deceleration by replacing the gulp valve, but I still have a backfiring problem during acceleration.  It doesn't happen when the engine is cold, but after it warms up it happens quite frequently (but not all the time) during hard acceleration.  It definitely takes the fun out of driving. I recently replaced the exhaust system (the old one was a sieve), not sure if that has any bearing on this. The engine also runs on after shutting off the ignition. I'm looking for help on troubleshooting this problem, any suggestions would be appreciated.


Carl Reinert
« Last Edit: July 11, 2014, 11:21:29 PM by amgba »


  • chfwrench
  • AMGBA Club Tech Staff
  • ****
  • Posts: 260
  • Memberhsip Number (if known):: 91-10014
  • MG information: '73 red B roadster
Re: 78 B backfiring on acceleration
« Reply #1 on: July 11, 2014, 10:21:30 PM »
Hi Carl,
There could be any number ofthings that cause your car to backfire.  
Given what you've replaced and the experiences you are you are listing, I'd first check the timing idle speed.  Both can cause run-on and backfire, particularly when the engine is hot.
Now the new exhaust system may have increased back pressure over the perforated mess of your old system (I've seen enough of them to know what you mean by a seive), but I do not think this a major contributor.
How about the running temperature?  Higher temperatures can cause backfire and run-on and may be due to several factors, includng advanced timing, thermostat malfunction, radiator and/or fan issues.
Being a 78, it has the dual electric fan set-up and they notoriously do not work as they should.  If you see the temaperature climbing and backfire starting, turn the heater on high.  That should reduce the running temperature (though cook the occupants) and if it reduces or eliminates tha backfire, may show you were best to look for the root cause of the problem.
The larger aftermarket fans do work much better.  And a change of thermostat and coolant flush, if not having been done for a while, would be a good thing as well.
I hope that gives at least a start on finding the problem.  Let me know what you find.
Safety Fast!
Art Isaacs
AMGBA Tech Staff


   AMGBA website home page or AMGBA Photo Gallery or AMGBA Club Blog