Backfiring under load

Took my 3.4 Mk2 out today.

Under heavy load it is backfiring through the carbs. So pulling hard up a hill in 4th at 90kph with my foot flat, around 3000rpm or so.

Smells a bit of petrol, even though I tuned it recently and CO at idle is around 4.5 to 5%.

Otherwise it starts and runs well and idles smoothly. Doesn’t stall or run on.


Similar experience on a XK150 , could smell fuel at exhaust.
Float levels were all over the place so went back to basics and started from scratch.

What’s your points gapped at?

Float level is a good suggestion, or a fuel delivery problem, if so equipped one at the float bowl? I imagine you’ll want to start comparing carbs and then watch for a fuel flow issue since it seems good at idle and low load. Being tuned (=touched) recently something might have gone awry. Or it is fuel delivery (lack thereof). Since backfiring means lean and load means high fuel use that would make sense to me.

Does it happen instantly or after a bit, my understanding is that if there’s a supply problem it will run well under load until the float bowls run low on fuel.

9 out of 10 it’s ignition that’s why I posted the loaded question😃

It’s a gradual thing. The engine subtly starts to lose power and runs slightly rough, then BANG. I too suspect it’s lean, although I’m pretty confident the mixtures are right. I’ll double check the float bowls too, that’s a good suggestion.

I have a Pertronix on this car, but I’m going to replace the plugs and make up a new set of leads as what’s on there looks original. I checked the timing with a dial back light when I first got the car, and don’t recall anything out of the ordinary, but I’ll recheck that too.

Medium term I’m going to replace the whole engine including carbs with a 4.2 with 2" twin SUs so hopefully the problem will become moot anyway.

Andrew you unknowingly answered your own question…
Do you have copper spark plug wires or carbon centers?
3.0 ohms coil?
When it cools off or idles its fine, but back fires after warm up or hitting the pedal?
Then hard to start hot?
If its cold and fine then hot and backfires its a faulty unit.
Feel your coil when up to temp, is it real hot?
I bet its the above, save yourself ALOT of time…
Good luck.

I had a very similar issue with my XJ6 Series III. it would die spit and sputter under load, then smooth out when at an idle… until one day when it didn’t it died at an idle after an engine rev. wouldn’t start. drug it back home, then it fired right up. long story short, the intake timing chain to gained some slack. tightened up the chain and it fires right up on first click; and is the best running XJ6 I have. Too bad it has a host of other issues.

Oh yeah, and I had the same exact issue with my SWB XJ6 Series II 2 freshly rebuilt Strombergs & a points ignition. This was a two fold issue.

  1. The car ran great around town. but when I took it on the motorway and the revs got around 2k, then the sputtering, back firing and loss of power started happening.
    I’d pull over and it would die. Id have to pull the fuel inlet line, let the pumps run until there was steady fuel coming out of the fuel line, hook it up, and we’re off to the races. The S-2 would run fine, as long as the revs didn’t get above 2 grand.
    Long story short, the distributor bushing was completely shot and was spinning in an oval fowling the points regularly. I cheated again, and picked up a complete pertronics distributor and coil. Poof! ignition issue solved. (saved the distributor for rebuilding later).
    Only to discover # 2 problem.
  2. fuel pump wasn’t pushing enough fuel at 2000rpm. (I eluded to this above) I cheated and purchased two electric external fuel pumps (mounted in the boot) and a pressure regulator just before the carbs. problem solved.

I wanted to write “if so equipped maybe a filter at the float bowl is clogged”.
It does sound like the fuel flow can’t keep up with demand.
In the stuttering phase, before the bang, try if the dampers give you enrichment by stabbing the throttle, and cure it for a second, if so it’s definitely lean. I take it you’ve checked the filter already and the line isn’t silted up.

Still betting on ignition…

Added to the list.

To be honest when I buy a new (to me) car I usually just replace plugs points dizzy cap leads etc etc.

For some reason (poor access to the dizzy) I was a bit lazy and just checked the timing with a light.

So the dizzy is coming out, being replaced by a rebuilt spare with points and new cap leads, Red Rotor etc. I’ve never been a fan of Pertronics units either.

I’ll go over the fuel system at the same time.

Stay posted.

It’s a start
At least the bucking and a fire will go away😃