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MG MGA - cronic backfire

The MGA didn't make Goodwood at the weekend as it developed a cronic backfire. The engine was rebuilt last winter and was running very well for 40 miles when had to pull over to put the hood up. Started off again and 3 miles down the road developed a continuous backfire and almost total loss of power. Pull over again and after 10 minutes set off with engine running very well for three miles when the same problem reoccured. Where do I start? Peter
P B Chappell

Peter,
Backfiring through the carbs or the exhaust? If you have standard contact points ignition maybe the points gap has closed. This greatly retards the ignition timing, unburned fuel will gather in the exhaust and cause backfiring. I was following a '70s midget in a cruise one day that was running poorly and backfiring. The points gap had closed and we set them by eye and it ran fine after that.

Ralph
Ralph

Anything that causes intermittent spark can do that. If it misses few sparks, then fires once, it can ignite unburned fuel and air in the exhaust system. A condenser going bad can do that, also points that barely open at all, or a broken flex wire inside the distributor (2 of them there). Loose clamp at base of the distributor can do it too if it breaks the ground connection sometimes.
Barney Gaylord

Check that your condenser is tight; I've had a condenser loosen up and the symptoms are the same.
Mike Parker

Or maybe try changing the condenser as it's common for new ones to be faulty - try putting the old one back maybe?
Graham M V

I replaced the condensor at the roadside. This improved it slightly but the backfire still continued especially under load going up hill. I will check the points tomorrow. Thanks Peter
P B Chappell

Cracked distributor cap?
Steve Gyles

My monies with Steve on this one. Don't be surprised if there's no sign of any visual defect. Run the engine in the dark and see if you can spot any short circuits.
J H Cole

The distributor cap doesn't even have to be cracked. Look for carbon tracking (thin black lines) inside the cap that are evidence of cross firing. But checking it in the dark is cool cause you sometimes see the "fire dancing" across the cap or even from the wires!
Cheers,
GTF
G T Foster

Hi, check points, condensor distributor cap, spark plugs, and rotor. Defects in any of these parts can cause the symptom you describe. Glenn
Glenn Hedrich

If its backfiring back through the carbies - try cleaning all the grit out of the dashpots. This can be helped in future by not overfilling the dashpots with 'sticky' oil, also regularly oil the standard airfilters to catch the dirt.
I found this dirt in the dashpots can hold the pistons up causing very lean running whioh caused the backfiring and would barely make 40mph.
Pete
Pete Tipping

My first thought as others is a bad distributor.
However, what are the chances you have a blown head gasket? Have you tried a compression test? Could it be the fire from one cylinder is going across to another cylinder via a bad head gasket, causing back fire in the carbs or out the tailpipe.
Ray Ammeter

This thread was discussed between 24/09/2010 and 04/10/2010

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