Cold back-fire

[chard]

When the engine is warming up but the choke can be pushed home. Engine idles ok and pulls through the rev range but pops (sounds like a muffled backfire) under light revs/load. This clears as soon as its warm.

1725 Rootes engine, twin strombergs, recently fitted 123 ignition.

Anyone with any ideas??

Chard

[GreenV8S]

I think you may be describing a misfire (which then results in the charge lighting up in the exhaust and/or flashing back into the intake). Does it clear if you pull the choke out again? Maybe you're pushing it in a little bit too soon.

[chard]

I'll try the choke. I wondered if the advance curve on the 123 ignition may be a bit aggressive around 2000 RPM?
Almost sounds like the carb rather than the exhaust back-firing

[GreenV8S]

chard said:

I'll try the choke. I wondered if the advance curve on the 123 ignition may be a bit aggressive around 2000 RPM?
Almost sounds like the carb rather than the exhaust back-firing

If you have a misfire the charge can light up when it hits the exhaust and then blow back into the intake manifold during the overlap period and light that up too. The lower the revs and load the more likely it is to blow back into the intake.

[chard]

Beacause the missfire is intermittant (pops just once then seems ok) I don't know if the choke cures the problem or not.
It happens when the engine is approaching operating temp (a couple of miles) but before the oil etc is properly warm.
I may just monitor it for a while.

[oakdale]

Has it only started doing this since fitting the 123 ignition?
Sounds like insufficient early advance to me.

[chard]

I have only had the car a couple of months and fitted the 123 kit 2 weeks ago. I was replacing a worn out distributor.

The car has always run badly from cold for the 1st couple of min's the car won't idle or pull well, so I have tended to warm the car up on the drive.

It may have always miss-fired when cold but I wouldnt necessarily have noticed.

I am running the car on Shell optimax with the maximum advance @ 2000 & 4500 rpm no pinking and pulls well when warm. I am told it has a stage one cam (which may not help)

Could this be a fueling rather than ignition problem?

[oakdale]

chard said:

I have only had the car a couple of months and fitted the 123 kit 2 weeks ago. I was replacing a worn out distributor.

The car has always run badly from cold for the 1st couple of min's the car won't idle or pull well, so I have tended to warm the car up on the drive.

It may have always miss-fired when cold but I wouldnt necessarily have noticed.

I am running the car on Shell optimax with the maximum advance @ 2000 & 4500 rpm no pinking and pulls well when warm. I am told it has a stage one cam (which may not help)

Could this be a fueling rather than ignition problem?

This is fairly normal for a carb engine of this era, unless it's a tuned engine using a rich mixture.

[chard]

I think I will check the choke and carb balance and leave it at that. The mixture seems OK judging by the plugs & the diaphrams are in good order.

Unless anyone else has any bright ideas?

Thanks guys