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With a 1981 motorcycle specially the GL1100 I'm going to speculate that if you don't have an exhaust manifold leak at the header then what you are experiencing is pre-ignition. In short I'll bet you are using regular 87 octane gas where as you should not be using anything lower then 89 octane gas (mid grade) in that engine. There are additives on the market that will boost octane and I've used then with my 1987 GL1000 but only when traveling and mid grade was not available. I had several little bottles pre-measured with the correct ratio of octane booster tucked inside the pocket of the fairing. It's a practice you should not make as routine as the mid grade is the best way to go without doing harm to the vales, pistons and head(s)
There is alot of discussion on this matter, ill give you mine, lol.
The recomended fuel to use is the high octane pump gas.
The proper fuel to use is actually the octane just above what will make your engine knock. Now no pump gas out there will make a harley engine knock exept like 83 or something, I would use the 91 or 93 is ok. but the 95 or higher in some areas will not give you any extra, just soak up your pocket book. and actually a really high octane is bad for your stock engine, so no octane boosters either.
What does the manufacture say to use? if they say to use it there must be a reason. It won;t hurt to try a lower octane but listen carefully for engine knock, if you hear it get back to the highere octane fuel. stokes make it real hard to hear the knock but the performance will be deminished. I personally use the higher octane when ever I can get it but I use mid grade and if only regular is available I use it with no problem.
I don't know what you have in New Zealand, but you probabley will want to use the premium (best grade) fuel. You can fix the current tank full by adding an octane booster available from a local auto parts store.
You could do some math and figure out if it's worth useing octane booster with cheaper fuel..