Series 2a backfiring

My Derek series 2a 1965 has recently been starting to backfire , most often when I put the choke in having driven him for a bit, he splutters and labours then pops. Also when coming up to traffic lights or something, and shifting down gears the engine just dies. When the choke cable is fully out then he is fine, which is what makes me scratch my brain the most because normally backfiring is related to running rich which is normally worse with the choke out. I also have a SU carb which needs tuning and new oil as I have engine oil in it at the moment which isn’t helping with the air flow. Spark plugs also may be loose , something I’ve not checked yet. Thinking it may be just related to the recent cold weather as it hasn’t been an issue before but could be something more sinister

Engine oil in the SU is fine.
Is it on points? Could be a dodgy condenser

kermit_rr said:

Engine oil in the SU is fine.
Is it on points? Could be a dodgy condenser

Click to expand...

I know it prefers 3in1 or SU damper oil. I’ll check the rest

I've only ever used 20/50 mineral

kermit_rr said:

I've only ever used 20/50 mineral

Click to expand...

I think it’s any of them really , i only know 3in1 through a friend who has the same carb in theirs and the 3in1 is thinner which in my case would help

Landy2a said:

I think it’s any of them really , i only know 3in1 through a friend who has the same carb in theirs and the 3in1 is thinner which in my case would help

Click to expand...

Which do you have? Mine came off a rover p6 v8.. if i recall

FFS do not use 3in one in your carb, the stuff is sh1t.
If it is sure ignition system is good then suspect problem is air leak at the carb like worn spindle shaft and or where it passes through carb body.

tottot said:

FFS do not use 3in one in your carb, the stuff is sh1t.
If it is sure ignition system is good then suspect problem is air leak at the carb like worn spindle shaft and or where it passes through carb body.

Click to expand...

I was thinking air leak too, and also check the vacuum advance pipe for leaks that might cause trouble (air to carb and ignition timing).

tottot said:

FFS do not use 3in one in your carb, the stuff is sh1t.
If it is sure ignition system is good then suspect problem is air leak at the carb like worn spindle shaft and or where it passes through carb body.

Click to expand...

I know you can get 'proper' SU damper oil, but I am not sure what it's like. Part of the problem I think for me is the engine oil is so dense its taking much longer for it to heat up requiring it to run richer

have you checked your piston with the drop test to confirm it's not stick ,what type Su is it

Damper oil is just regular single weight oil, but whatever you put in shouldn't cause a backfire
In my opinion, a small vacuum leak shouldn't cause it either, though it wouldn't run right

Sounds like a fuel mixture which is far too lean when the choke is off. Have you set up the mixture when the engine is up to running temperature? If you don’t have access to a CO meter, then you can tune the mixture by ear, but you need to be sure that the carb is clean, the needle or jet is clean, the float level is correct and there are no air leaks. Damper oil should be topped up and when you lift the piston and let it go, it should drop smoothly and consistently and stop at the bottom of its travel with a soft click. If it drops suddenly or judders, you will need to strip it down and investigate further.
Backfiring is common when the fuel mixture is weak or an air leak is present.