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MG MGA - Backfire after slow running

After being in slow traffic for a few minutes, my car starts to backfire through the carbs. Once I give it a few revs and drive it a mile or so, it clears and runs ok. There's also a smell of petrol.

I feel sure it's the carbs, but I don't want to take them apart until I know what I'm looking for.

Any helpful suggestions appreciated!!

D J Hill

That sounds to me like weak mixture caused by fuel vapourization

Is the car remaining cool?
Has the carbs got the heat shield in place?
do the manifolds have the insulators fitted?

Difficult to resolve although you might want to ensure the mixture is not weak in normal use? were the carbs set up correctly?
Robert (Bob) Midget Turbo

Is this a new problem and how well does it run once cleared!

You can check the float levels and if it's running too rich (stuck choke). But it might not be (just) the carbs. Also worth checking the ignition system; points gap/condition, timing and HT leads etc. If the backfire is caused by fuel failing to (always) ignite due to a weak spark it is not unusual for there to be a petrol smell.

Check the colour of the plugs too, that's a good indicator of the carb tuning.

Neil McGurk

In my scenario the smell of petrol occurs because the backfire is actually a spitback through the carb which always smells of fuel :)
Robert (Bob) Midget Turbo

Whip off the dashpots, clean them and the piston and put them back. Dirt can get stuck between the pistons and the bore causing them to hang-up which can lean the mixture and cause that backfire. Improved air filters and not over-oiling the dashpots can reduce this tendency. Cured a back-fire I had anyway
Pete Tipping

This thread was discussed between 16/10/2010 and 17/10/2010

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