T3 – Pop pop, Bang bang.

So I was shaking down Blue at the weekend and that went OK. Must have done over 150 miles and with some of that cruising 85 for a while (indicated, obviously). But …

By the time I’d got to the rally site on Saturday evening I had a little over 210 miles on the tank so figured an early fill on the Sunday would be sensible. I ended up taking a full load of J Sainsbury’s finest high octane. Not long after I found I was getting a pretty good set of popping and banging on the over run. From the back my pal following on his Veefer reckoned he could see flame outs in the pipes. After a while I pulled over just to check nothing was adrift but all looked OK.

So the bike continues to run just fine – starts on the button, tick over, revs out.

Just a case of dodgy supermarket gasoline or something else I should be looking for? I’ve probably done near on 100 miles on that tank now so I’m thinking I’ll drain another gallon for the mower and then take a refill of proper stuff, with maybe some injector cleaner lobbed in.


I use Sainsburys super all the time. Never had a problem myself. But it does sound like a bit of a coincidence.

On my Spada, the overrun popping was caused by an air leak at the joint between the carb adaptor and the cylinder head.

Popping and banging on the over run is often due to leaks between the head and the exhaust pipe. Caused by air mixing with an amount of unburnt fuel in the exhaust and then igniting on the hot exhaust. This would account for the occasional flame out of the exhaust.
Could it be that you are not getting full combustion of the fuel with the higher octane super grade?

As predicted above (thanks) …

Leak from the LH ex port. It hadn’t felt like it seated right when I first put it back together at the winter rebuild. I then convinced myself that the chuff I could hear was just induction noise through the cone filters. And when I tried a retighten the nuts were still firm. Checking yesterday they were loose, so I pulled it all off and remounted. Hopefully OK now – sounds better running on the table but will need a run out to prove for sure.

Went out for a run last night.

Original problem is fixed as expected. But the slight fluff that I used to experience occasionally from a low speed pick up seems to be maybe now be worserer. Wondering if I’ve burnt the zorst valve on that side with high speed running with the port leak.

Best run a compression test I suppose …

A nice fat 150 psi both sides so all good. Found the plugs a little wide so gapped them down was all.

Can you define “a little wide” please, just for reference? :smiley:

I usually gap by eye to 25 thou. By eye I’d say these were at 30. I got the feelers out for once and tapped them down to just a smidge over 20.

Fairy nuff! Only asked because lately I have been using 25 thou, which is more or less how they come out the box. (Hitherto I always used to set 20.)