I have always associated popping on the overun with too rich a mixture leaving unburnt fuel in the exhaust.
My dellorto car which is otherwise running well has just started popping a fair bit on the overun after I have weakened the mixture by a tiny amount. My adjustments are so small it should not make a major difference, just a bit of fine tuning. It never popped much on the overun before, just occasionaly.
Coincidently at exactly the same time I have just tried out for the first time Tesco 99 ron superunleaded. I usually run BP 98 ron superunleaded. I am using my usual lead additive in the mix so no change there. I do not use octane boost.
Before I start looking whether my fine tuning has upset it, has anybody here in the UK experienced anything similar when switching to Tesco 99 ron or another supermarket 99 octane fuel?
If you are actually adding TEL ( TeraEthylLead) to the fuel it will raise the octane number of unleaded fuel, if the package has MMT then it will also raise the octane number -
Everything has been rebuilt and it’s virtually new under the bonnet. It’s just strange it suddenly started doing it only after I had slightly weakened off the mixtures and filled up with the 99 octane fuel. Other than the popping on overun she’s running very well.
I filled up with BP Ultimate this morning and noted that it says 97 ron on the pump. Has this changed? I had always assumed that Shell Optimax (Vpower) and BP were the same 98 ron. In the modern it does not seem to make much difference but the Elan does like the Shell 98 ron. Mine will pop on big overruns (hills); I am on webers and I thought they all did that!!