Pops and crackles

[Alan_M]

I had something like this - there was an internal leak in the thermal valve, so none of the vacuum was getting to the distributor.
Would get some popping on overrun and hesitation on acceleration.

Sounds like an issue I'm currently having Alan, How did you sort it in the end?

Simples - replaced the faulty thermal valve fitted to the middle of the inlet manifold.

[pistolpete]

So you have an SE, which means you have the dizzy which provides a lot of vacuum advance on the overrun. If you retard the ignition on the overrun you get pops and bangs on the overrun.

Eg the SE vac advance provides about 20 degrees of aditional advance at full vacuum. That 20, plus the max mechanical advance of 26 or so gives you ~46 degrees of advance on the overrun above 3500 rpm. (Numbers are from memory, I don't have the Service Notes to hand to check actual numbers).

You only have to retard below 40 degs on the overrun to get pops and bangs. So a vacuum leak in the engine management /vac advance system could be your issue. Also a rich mixture will add to the pops and bangs even if the ignition is the primary cause, as will warm intake air temperatures.

The pops and bangs is caused by unburnt fuel in the exhaust manifold/downpipe. A retarded ignition on a closed throttle at highish rpm above 3k or whatever max advance arrives at, could be your issue.

Worth excluding this before messing with manifold, anyhow!

I think some of this has a correct ring to it , I will try this first thanks to everyone as always for your help