4 replies to this topic

[blackbelt1990]

I’m totally at my wits end here 😫 hoping someone can advise.

Got an early 90s 1275 (age unknown) that has an annoying backfire, usually in high gear at low revs (it’s an auto) or decelerating in high gear. But to be honest it can be quite random.

So far this is what I’ve done, with zero changes to the symptoms.

HS4 carb rebuilt with new jet and needle valve, correct needle installed as it was a 1098 one.
Dizzy replaced with A+ electronic ignition from a montego.
Coil replaced, removed the ballast 9v system and replaced with a 12v coil and 12v feed.
HT leads changed for another 2nd hand set (again no changes to symptoms)
Timing Bob on, emissions have been set to 3% as a bench mark before I started changing stuff as it was running super rich when I got it and then someone turned it super lean 🙄

I’ve just whipped the plugs out, 1 and 4 were a nice brown colour, 2 and 3 were sooted, I’m not sure whether that’s related, it runs too well to be a HG issue.

Anyone any ideas? I feel like I’ve changed all the potential issues.

[Cooperman]

Backfiring through the carb can only happen if a plug fires whilst the inlet valve is open on that cylinder.

Thus you need to work out how that is happening.

It sounds as though you have some sort of ignition cross-feed from the correct firing lead to one of the other leads, which is causing two plugs to fire at the same instant.

Check the leads and the distributor cap first of all. One tip for doing this is to start the engine when it is completely dark as that helps to see any cross-feeding. Making the leads slightly damp can also help with the diagnosis (damp, not soaking wet!).

Alternatively, is it possible that one of the inlet valves is not completely closing on an intermittent basis. Could it be a tight valve guide or a problem with the rockers/valve clearances?

[Tornado99]

Compression check? Thinking a leaky valve seat. Perhaps carbon build up giving a dieseling effect where ignition is happening before spark fires and with an inlet valve partly open.

[blackbelt1990]

So... I think I've fixed this!

 

I thought it might have been running lean, other threads suggest that a symptom of running lean is backfiring. Checked the CO2 again and it was a touch over 3. Had a think about it and I guessed that fluctuation in the dashpot could cause lean spots?

 

Binned off the SU oil out of the dashpot, put some random engine oil in... ITS STOPPED BACK FIRING!!!!

 

Why is it always the simplest things?!

 

Was just coming on the delete the thread 

[cal844]

Now Give the car a good long test drive, driving both normal and abusing the car, get it nice and hot and see how it is then