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MG MGA - Exhaust Back Firing on de-acceleration..help

I am done scratching my head and cant figure it out.I have a 1962 MGA MK II that I just finished rebuilding or replacing everything. The car accelerates great but when I take my foot off the gas at say 50mph I hear alot of back firing from the exhaust. What should I re-check and check again??
thanks for any ideas.
Steve
L. Muzekari

Hi Steve. A couple of thoughts come to mind. You might want to check your ignition point gap, the ignition timing, valve adjustment, and carbutettor fuel mixture. Checking the plugs for proper gap might be a good idea also. If the problem persists, I would think about doing a compression test, to see if you have a leaky engine valve. Cheers! Glenn
Glenn

Glenn,
thanks for your ideas. I checked the points -ok, the timing-ok, i will check the valves, Carbs seem ok and are totally rebuilt and set at 0.70 from the bridge.Plugs are new and set correctly.
Steve
L. Muzekari

You'll have to excuse my inexperience with cars, but on motorcycles this is often attributable to an intake manifold leak. If you spray something like WD-40 around the inlet manifold (carb conection)and the motor stutters you'll know you have a leak.

Please someone correct if this sounds idiotic.
C L Barrett

The normal cause for backfiring when you let off the throttle is a lean idle mixture. Like CL said, this can be caused by a vacuum leak somewhere. I would double check the vacuum advance connections to make sure they are all tight, and there are no cracks in the tubing. Worn throttle shafts are another common cause. I know you said your carbs were rebuilt, but it can't hurt to double check the throttle shafts. It may be just the idle mixture adjustment. Setting the jets by distance is just a starting point, and you need to go through the whole procedure of balancing the carbs and adjusting the mixture.

Jeff Schultz

Have you sealed the exhaust front pipe correctly, a small leak at the front near or on the manifold will result in backfiring (poping ) from the exhaust. If you are getting big backfires (loud), this, as suggested is probably ignition related. Have you got a standard exhaust? Let the backfiring jokes begin!
Terry Drinkwater

Just one thing to add to the thread. The backfiring upon letting up on the throttle is almost always associated with an intake vacuum leak as others have posted. However, it is spray starter (it used to be ether in these cans) that is usually sprayed around the intake to check for leaks. If the engine races in certain areas, then it has a vacuum leak. Carburetor cleaner (toluene) will also work, but in reverse - i.e., with will cause the engine to stumble if it is sucked into the intake and it requires a fairly large leak for it to show up using this route.

WD 40 is a "W"ater "D"isplacement lubricant that might plug a small leak temporarily or get sucked into the engine if the leak is large, but like toluene, it not as sensitive as ether and will leave a film on the outside of what you are spraying . The film may or may not be a desired byproduct of the fault finding.

Steve
Steve Brandt

If you use ether(spray starter) to look for the leak, insure beforehand that your wiring is good, else you may find a 'backfire' in the engine compartment. Been there, done that.
mike parker

I always thought loud exh back firing was caused mostly by running rich?
WCW

When an engine backfires because it is running rich, it is because the plugs and combustion chambers have built up a thick coating of carbon that holds heat efficiently. Because of the excess heat, as the gas/air mixture is drawn into the chamber, it detonates prematurely, causing a backfire. That type of backfire usually happens upon acceleration, not when letting off on the gas.

Steve
Steve Brandt

Some very good ideas. I will spray and check for vaccum leakst this weekend.Terry,the exhaust I am using is the Moss falcon stainless.Jeff, I feel I did set up the carbs correctly and used the unisyn to set the idle between carbs.Also,changing the mixture on the carbs made no difference in the backfiring.The throttle shafts are new.I will report back.
thanks for all your input.


Steve
L. Muzekari

I alsways use an unlit propane toch to seek out vaccuum leaks. Engine will speed up when drawn in through the leak and there's no liquid mess.
R. L Carleen

The point of using WD40 to find leaks is to temporarily seal small gaps. Old timers used to squirt oil in potential leak areas for the same purpose.

Propane works well by introducing a flammable fuel into the intake stream. But should be used with care.

Spraying ether on a running engine is an invitation for trouble.
Bill Boorse

Bill,
Thanks for the advice. Where were you in 1982 when I needed it? ;) My old pickup had the distributor wire from the coil stripped and twisted together(not the high tension lead). Actually, I never would have found that had I not used ether. A big poof and a flame quickly located the problem.
mike parker

This thread was discussed between 18/10/2004 and 19/10/2004

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