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MG MGB Technical - Backfires when cold

1800. Twin HIFs (with black alcohol resistant floats and new needle valves) electronic ignition. Backfires while cold with the choke out but runs normally when warm.
The only other fault has been two recent occasions when fuel ran out of the for'ard carb overflow after the car had been standing for a few days. This stopped without intervention after the engine warmed.

I am obviously aware of the fundamental reasons for a backfire. This fault may be heavy condensation around the distributor but would be interested in your thoughts.

Thanks
Roger Walker

The usual question, does the tach (I assume as it has HIFs) flick or jump with the backfire other than simply from the very small change in engine speed?

Any evidence of stray HT sparks when it's doing it in the dark?

If you have a timing light clip it onto the coil lead and each plug lead in turn and watch to see if there are any missing flashes that coincide with the backfire.

Is there any reason why your distributor should have heavy condensation? If it was going to affect things at all I'd expect it to not even start. Is it visibly wet inside or out? If so is the heater tap dripping on it?

Does varying the choke vary how often the backfire occurs?

How often it is used, and how long for each time?

I can see this becoming another 'Back to basics: What causes a miss'.
paulh4

Is the backfire thur the carbs or the exhauste

Pual...if youbget alot of moisture around the dissy cap it can cause backfiring but it s alot of water like from a carwash rather then tempeture change condenstion.... cold humudity at 9am and then hot hummid wearher at 9:40am

Like right now...8am it was 22* F. Now 10:30am and it 68*F
1 Paper

You may have blown out a gasket, at one of the carburetors, allowing for a lean mixture. RAY
rjm RAY

It's a family car and clearly we need some more answers.
Thanks folks !
Roger Walker

Roger
Is it a backfire or missfire
As Prop mentioned, is it coughing back through the carbs or is it missing in general and puffing in the exhaust
A simple method of checking to see if the choke itself is coming on enough is to pull the choke on the engine when it is warm and it should choke it up and run rough with black smoke out the exhaust
If you think it is a missfire and you have dampness around the distributor etc, there is a chance that IF your car has cabon core plug leeds and the leeds are damp it could be cross firing accross the leeds until they get warm / dry

willy
William Revit

Hey voodue willy

Are you thinking about tacos??? ...i am

Thats just uncanny

Prop
1 Paper

"hot hummid wearher at 9:40am"

You ain't going to get that in the UK in December!
paulh4

Pualh4

I was still channeling willy...haha
1 Paper

Runs fine when you put the choke back in, or warm in your words? More like backfires because you are not putting the choke back in soon enough. These cars often only need choke to start the engine, and then maybe for a few seconds after it starts, then you can reduce the amount of choke. Some cars you can reduce to 1/2 1/4 or none, others you reduce to less rapidly.
Car not designed to run on choke, it just for starting.
Ben Stevens

Agree with Ben. This is some pretty strange talk about someone leaving the choke out until it backfires. Sounds like some mgexp tripe nonsense garbage a-la skyenazi and his 50 regulars (49 aliases for himself who knows not much on any auto topic, but will agree with himself until all opposition has gone away sandbagged. Poor SOT sits around all day and night making up inane conversation and advice with very little grounding in reality.
S Cole

Never had backfiring with the choke out when it didn't need it. If it is left out longer than it needs to be then it does start the rhythmic beat, but nothing like backfiring.

Need some answers to the questions a few days ago.
paulh4

Change the condenser Roger, it is failing. probably from all that humidity inside the distributor cap
S Cole

More important is the still unanswered question of why there should be so much humidity/condensation inside the cap.
paulh4

Yes, I tend to only ever notice condenstation on the out-side of the dist cap. If inside the cap it just another indicator of a bad condenser. And when you go to get one be sure and pick up a couple. Like one brand X and another brand Z, as these r notoriously poorly made the last 20 years or so and not at all uncommon to get ratty ones or ones that seem just fine, but only last for 20 or 600 miles before exhibiting strange behavior in the engine again.
S Cole

Well thanks everyone - here's the latest; she started normally this afternoon but it was immediately clear that firing was uneven. A strobo then showed that she was missing intermittently on cylinder 2 and not firing at all on three. The light was failing and we had to call a halt but there were no signs of external tracking.
Standard dizzi with points (new) but electronic device cuts down current. Dizzi cap due a change. Rustfree RB lives outside under 'canvas'. London temp today 6 Humidity 100%.
More soon.
R
Roger Walker

#3 eh? Could be a corroded wire or something. Is it getting spark through the wire on 3?
S Cole

Has it got carbon leeds---how old

You could try--remove #3 from the plug and cap
Remove #1 from the plug and cap
Fit #1 where #3 was and #3 where #1 was
Start it up again and see if it's still #3 playing up or has the fault followed the leed to #1
William Revit

"no signs of external tracking."

Had the same in the past myself with different cars. No visible tracking, but strobe indicated it was breaking down, and replacement cured it.

If only one plug is showing intermittent firing then it is the cap that is the problem, unless the plug lead has gone massively high-resistance, or the plug gap is massively over-size, or is otherwise faulty.

"but electronic device cuts down current."

If you mean the points control an electronic module that fires the coil, then that does reduce the current through the points, but doesn't really alter the HT voltage or chance of tracking.

Living outside IS a candidate for condensation with our fluctuating temperatures and humidity. However my V8 lived outside when used as a daily driver and didn't suffer. It's only relatively recently distributors (for HT distribution) stopped being the norm, and I'd say it has been the norm for a long time now for cars to live outside. In itself it shouldn't be a problem. Even coolant dripping from a leaking heat valve directly onto the distributor doesn't guarantee HT problems, BT, DT as well.
paulh4

"no signs of external tracking."

Had the same in the past myself with different cars. No visible tracking, but strobe indicated it was breaking down, and replacement cured it.

If only one plug is showing intermittent firing then it is the cap that is the problem, unless the plug lead has gone massively high-resistance, or the plug gap is massively over-size, or is otherwise faulty.

"but electronic device cuts down current."

If you mean the points control an electronic module that fires the coil, then that does reduce the current through the points, but doesn't really alter the HT voltage or chance of tracking.

Living outside IS a candidate for condensation with our fluctuating temperatures and humidity. However my V8 lived outside when used as a daily driver and didn't suffer. It's only relatively recently distributors (for HT distribution) stopped being the norm, and I'd say it has been the norm for a long time now for cars to live outside. In itself it shouldn't be a problem. Even coolant dripping from a leaking heat valve directly onto the distributor doesn't guarantee HT problems, BT, DT as well.
paulh4

Yes carbon leads - new last Summer like the plugs. Weather yesterday was wet so no progress. Thanks for your interest.
Roger Walker

Ah, carbon, ditched those in the 70s for silicone when I started getting running problems on what had been a new car and found the resistances were all over the place. The silicone on the V8 have been there for 100k now. I have replaced the set on the roadster (once in 26 years) when the brass end that pushed into the coil developed a curious blue coating which seemed to be blocking the HT and could not be scraped off.

Not saying that is the problem, but next time I'd go for silicone, even if you keep a set of carbon for show! I was also taken aback by how much more expensive carbon were over silicone when I replaced the V8s.
paulh4

Repaired yesterday. New contact set that does not short on the dizzi casing and one new replacement for carbon plug lead. Brass monkey weather - glad the job is done.
Thanks for all your help.
Roger Walker

This thread was discussed between 10/01/2017 and 22/01/2017

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