GL1100 strange acceleration problem

[77MM99]

Hello all,

I have a GL1100 1980 and a couple of years ago an issue appeared: all of a sudden, during riding the engine got somewhat laggy, it accelerated and revved slower than before, it lost that “oomph” - mainly in lower revs between 2000-4000 RPM. The motorcycle at that time was stock, with stock exhausts, which were leaky and later rotted out completely. Nevertheless, even with those exhausts before then the motorcycle pulled really strong and ran great.

In hope to mend the issue the following things were replaced/serviced:
- aftermarket Marving exhausts
- carburettors overhauled with Keyster kit, whereas the 4 smaller pumps and the accelerator pump are new Honda OEM
- felts in carburettors replaced with custom ones
- carburettors synchronized
- ignition coils replaced (aftermarket)
- pulse generator replaced - NOS Honda OEM
- vacuum advance replaced - new Honda OEM
- mechanical ignition checked and greased – the pin was not worn out
- fuel pump replaced - new Honda OEM
- water pump and hoses replaced - new Honda OEM
- stator replaced (aftermarket)
- heads and valves were overhauled; gaskets, valve seats and seals - new Honda OEM
- valves were adjusted
- spark plugs replaced
- spark plug cables checked
- fuel tank cleaned of rust; fuel filter and hoses regularly replaced
- air filter replaced
- belts regularly changed – new Honda OEM
- the compression is equal on all four cylinders – about 150 psi
- oil in final drive changed, the final drive does not make any noise.

All of these things improved the performance considerably, however the issue is still persistent, although to a lesser extent. The motorcycle starts immediately every time, idles perfectly and at constant RPMs, does not stutter or sputter, neither at idle nor at acceleration and is throttle responsive. It backfires a little bit on higher revs when the throttle is completely closed, which I ascribe to aftermarket exhausts. It does not smoke whatsoever, not even at startup. It does not overheat, it does not consume or leak coolant. It burns a bit of oil, mostly when ridden for longer periods on highway when RPMs are constantly between 5500-6000 – regardless, it burned some oil even before the mentioned issue appeared. I’m using mineral 20W50 motorcycle oil. Fuel consumption is around 40 mpg. It has no problem reaching speeds of 100 mph.

TLDR version:
The weirdest thing is, that the issue is intermittent – it comes and goes. I usually do about 300 miles trips with a couple of stops for a coffee and lunch and it goes as follows:
- when I start the bike in the morning (overnight cold) – the bike runs perfectly, it pulls well in all RPM ranges, the engine runs smooth and the sound is normal
- after about half an hour or so the engine starts to sound a bit different (if normal acceleration sounds “VOOOM”, it starts to sound like “VRRROOM”; when the bike is running at constant revs it is sort of a buzzing sound like “BRRR” – I do not know how to describe it more accurately – but it is not an engine knocking noise or the chain rattle noise), it still runs on all 4 cylinders, however acceleration gets worse and laggy at lower revs, it feels and sounds as if the engine is struggling somewhat and does not seem to run that smooth; the takeoffs from standstill are not problematic and at revs over 4000 RPM the bike still pulls fairly OK – it just seems to lose some power and that “oomph” that I know the motorcycle is otherwise capable of
- now approximately on the second half of the trip the issue all of a sudden completely disappears, the engine sound is back to normal, the engine runs completely smooth, it pulls as strong as ever across the whole RPM range and it stays so for the rest of the trip.
It’s the same every time.

Does anyone have any clues what might be wrong? It would be greatly appreciated.

Thank you