Supermarket 99 octane - anyone started popping on overun?

[M.J.S]

Just a quickie folks before I delve deeper.

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?

Cheers.

Mark.

[nebogipfel]

FWIW I doubt it's fuel.

I'd check for other things. I had dramatic poppping on the over-run when I had a vacuum leak on one of the headlamp pipes.

Are your "O" rings OK?

[cabc26b]

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 -

G

[M.J.S]

FWIW I doubt it's fuel.

I'd check for other things. I had dramatic poppping on the over-run when I had a vacuum leak on one of the headlamp pipes.

Are your "O" rings OK?

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.

Mark.

[elansprint71]

Only EVER change one thing at once.

[ppnelan]

And I wouldn't buy anything from Tesco - remember the problems with fuel a while back?

Matthew

[hatman]

Do the next fill with BP and see if it still pops.

[Frank Howard]

Mark,

Mine pops on overrun all the time. I think it sounds kinda cool. I believe it's caused by advanced timing.

[Jas]

Mark,

I don't know how little your fine tuning was, but half a turn leaner om my Dellortos changed it from popping to not popping on overrun.

Cheers

[M.J.S]

[quote="Frank Howard"]Mark,

Mine pops on overrun all the time. I think it sounds kinda cool.


I think it rather cool too , but even if you have to live with it it's nice to know what the cause is.

Mark.

[ppnelan]

A burnt valve sounds even more 'cool' - just like a Subaru, but without much power...

Matthew

[steveww]

Mine was popping quite a bit on the over run but when I advanced the timing a tad it was much reduced.

[powellsmail]

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!!

Regards

Mark