Revs well until 7-8k rpm then stutters like its running out of gas. Fuel line is not kinked. Vacuum hose is not kinked. If I back off throttle then it comes back just like fuel starvation . New bike to me. Plugs good. Starts perfectly. Idle is great. LOts of power off the line. Revs highest in low gears. Top speed is 75mph (130kph) at 6000. Apparently carbs were cleaned but maybe main jets are clogged ?
Engine is good. No oily pistons, no excessive noise. Compression is fair at 150 lbs.
I hoped that this gismo was the culprit. What is it ? Bike runs exactly the same with or without it as far as I can tell. It sits under the tank into 2 prong female
Having the engine stop sputtering when you roll off the throttle a bit seems to suggest that there's too much fuel and too little oxygen to burn efficiently. Do you happen to notice a gas smell when this is happening? And is the exhaust coming out a weird color, like blue?
but it is not very restrictive. Sponge Filter has been cleaned but air input into airbox is not great since it has a small input under the seat. I put an endoscope camera into airbox & carb but no blockage.
I also cleaned the fuel tap and PO declared that he reached 160kph....
Gas smell? Not very obvious smell. To make sure enough air getting into gastank I left the filler cap loose for a test -- no change.
No blue or other weird colour exhaust that I noticed.
Maybe bad cam chain? Chain does rattle a bit at 800rpm. looking to replace spring cam cahin adjuster with a solid mechanically adjustable one
Is it an oiled air filter? You said its been cleaned, presumably re-oiled?
I had same issue with my ZXR750 H1 - '89 model. Wouldn't rev past 8500.
It was the air filter, I had over oiled it.
I spent days testing stuff, replacing fuel lines, carbs off, ultrasonic cleaned emulsion tubes, needles, floats, I removed and refitted the tank, air box and carbs so many times, I cant tell you how frustrated I was. Each time trying something different, putting it back together, take it out for a test only to cut at 8500 on the dot.
I didn't immediately suspect the air filter because I knew it was clean, I had recently cleaned it during a service so it didn't enter my mind that it could be the problem, I don't rev the bike to 8500+ every time I ride it, so it may have been weeks or more between over oiling the filter and discovering the problem. If I had just oiled the filter the found that it would rev past 8500 the next day, it would have been more obvious.
Good luck with it anyway.
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