Lotus Elan and Lotus Elan Plus 2 Forums

Since I've had my S4 it has exhibited excessive popping and banging on over run. Whilst I do like a little burbling this has always been excessive and has recently got worse, with visible flames from the exhaust at times I'm told.

I have checked:

Ignition timing
Vacuum leaks
Cam timing (Only as far as checking the timing marks and that the cam wheels are on the correct sides)
Carb float levels (DHLA)
Leak down test results all good.

and haven't been able to find anything amiss, any ideas?

I do currently have the head off to change a noisy water pump, and did note the the carb studs appear to be drilled incorrectly leaving a small step between the carb throats and inlet posts (60 thou max) but a little port fettling will resolve this.

Your engine runs a bit rich on initial overrun, so there's unburnt fuel in the exhaust. If there's any possibility of air entering the exhaust system, that can cause the fuel to ignite with results as you describe.
Have you checked the exhaust for leaks?

Sorry, I forgot to mention that I have had the exhaust system and manifold off, replaced the gaskets and Y piece and as far as I can tell it's air tight.

Popping also happens with too lean carburetion (or one spark not giving...) : the mix does not burn in the chamber, but if the exhaust is hot conditions for it to burn there can be met from time to time, hence the occasional bang...

Seems you've checked most obvious things, so it might be worth looking at less likely suspects such as the rotor arm, coil and the points. I change the points first whenever I adjust the timing as a matter of routine. It sounds more like an ignition fault to me based on experience. A failing condenser can also cause the same symptoms.

Nigel F.

I had this problem on over run on my car. I took all the exhaust system off including the SS manifold and reassembled using jointing compound on all the joints as well as the manifold to cylinder head area. This appeared to fix the problem.

Thanks for the input guys. The car runs Luminition but I'm not sure if I have put a proper rotor arm in and will check.

The exhaust manifold gaskets do show some signs of leakage, although no undue noise was evident. The manifold face doesn't look to good so I will try to face this up a bit which will hopefully give a better seal and report back once the cars back together.

I agree I would go for to lean, I had the identical problem.

Update, the frustration continues. All back together and running but the popping and banging persists.

Whilst the head was off I relapped all the valves and reshimmed to spec. Had a brief moment when I thought inhad found the problem when timing the cams. The exhaust cam was retarded 6 degrees. Sorting this and cleaning up the inlet ports has certainly perked the engine up, but the issue remains.

I also rebuilt the dizzy whilst it was apart, but couldn't find anything amiss.

The crackles are quite violent, but dissapear with only a hint of throttle, not much left, carbs?

Old fuel?

Webers? Turn your idle mixture screws out a little, maybe a half turn, or you could pull the choke on slightly. If this helps, you are too lean, richer idle jets could be in order.

Also suggest you're running lean on the idle circuit. As suggested open your idle mixture screws up a little. I use a Colourtune plug (use 2 if you can, it's quicker) to ensure I have the mixtures the same in each cylinder, ish.

Success, Thank you all for the input.

It would appear that the idle jet in the front barrel of the front carb was partially blocked. A little attention with a 0.5mm jet drill and peace and quiet is restored,

Grumbly

Well done a process of elimination you are now a happygrumbly

Thanks, I very nearly smiled.

Now if I can cure the running on in my TR6!