DDK - Die Deutschen Klassiker

Hi all, I've finally got my slightly modified Canadian 68 911s registered and on the road! As the car had 1/4 a tank of fuel in I took it out for a run. I then stopped and filled up with
95 ron unleaded and continued on my run. The car has since developed a fair amount of popping and the odd louder pop when off throttle.
I'm just looking for a bit of advice as to where to start first. My initial thoughts were that the original fuel was Canadian and a lower octane rating than our UK stuff.
Although the mild popping is quite enjoyable I'm concerned I might give someone a hard attack with the louder ones!
Any help would be appreciated.
Alfie.

Check the exhaust system for blown gaskets, pinholes etc as these will cause popping.

Chris

What is the engine/exhaust set up?

How old was the fuel in the car? You may well have stirred up a load of muck in the tank when you filled it.

Rich lower rpm causing backfires on overun ? ( I know mine suffers a bit from this)

Thanks for the replies folks.
The engine spec is as follows, the guy who did the work made some notes on what he did. This all has to be confirmed but it's a 2.0s case with 2.7 rs barrels an pistons.
Short stroke to give 2.5l with 8.0 to 1 Compression ratio.
Later large port heads with no exhaust valve seals.
Msd elec ignition.
Webers with k and n filters.
Sports exhaust with headers.
Extra oil cooler

I'm pretty sure the fuel wasnt that old but you never know.
The previous owner had spent a fair bit of time setting up the weber carbs and it does run really well.
I suppose I wondered if anyone else had brought a car back from the states and noticed a difference in running between us fuel and uk fuel?
Thanks
Alfie