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Strange motor behavior at high speed
Posted: Sat May 06, 2023 4:01 am
by Micha
I just got back from a short morning ride on the highway and found that the motorcycle easily reaches 90 km/h in 4th gear but that's it. Not more. When I keep opening the throttle, I feel a kind of "retraction" in the engine's behavior, like it's running out of breath, retreating and losing speed. When I close the throttle a bit again, the motorcycle jumps forward and breathes freely again, cruising along at a leisurely 90 km/h. I hope my strange description is clear. I've never encountered the phenomenon and of course I assume it has to do with the carbs, not sure though.
A vape 12 v alternator & ignition system in installed.
Any ideas?
Re: Strange motor behavior at high speed
Posted: Sat May 06, 2023 9:41 am
by therealkennyboy
Fuel flow issue? Any in-line filters which might be impeding flow (i.e. just enough to starve the engine at full-open throttle)? If so, take them out temporarily and try another speed run and see if that helps. If not, maybe check that the petcock filter is not clogged/dirty?
Good luck.
Re: Strange motor behavior at high speed
Posted: Sat May 06, 2023 9:53 am
by schrader7032
Gas cap not breathing well under high demand?
Re: Strange motor behavior at high speed
Posted: Sat May 06, 2023 10:05 am
by jwonder
Check the timing as well, it could be many small factors.
Re: Strange motor behavior at high speed
Posted: Sat May 06, 2023 10:28 am
by therealkennyboy
Try the next notch on the needle? (The needle is still blocking the main jet a little at full throttle?)
Re: Strange motor behavior at high speed
Posted: Mon May 08, 2023 9:25 pm
by Flx48
Michael-
Verify static timing set at "OT" on the flywheel, with "0" marking on the rotor aligned with the RH edge of the sensor.
Verify advance unit is set with four switches in off position. (for timing of 8 degrees BTDC at idle and 38 degrees BTDC at 3000rpm)
Verify sufficient fuel flow at the carb end of a fuel line.
For me, with 4 gallons in the tank, flow gets around 400ml/min.
More fuel volume in the tank will equal more head/flow, less fuel volume less head/flow.
Best-
George
Re: Strange motor behavior at high speed
Posted: Tue May 09, 2023 1:16 am
by Seek
It could also be too rich mixture.
Re: Strange motor behavior at high speed
Posted: Tue May 09, 2023 10:25 am
by Micha
Thanks guys.
Could it be that the reason is little fuel amount in the tank?
I assume that in a few miles I will hit the reserve mode.
Re: Strange motor behavior at high speed
Posted: Wed May 10, 2023 9:07 am
by Micha
I ask because right now the tank is almost empty, and it is easier to empty it in favor of disassembling the fuel tap.
If there's a chance that's the reason, I'll fill up the tank and go for a test drive.
If the chance is slim, I'll drain it and start disassembling that tap and filters that sit on the rubber lines.
Re: Strange motor behavior at high speed
Posted: Wed May 10, 2023 9:15 am
by schrader7032
It's possible that you have restricted fuel flow. Have you sampled how much fuel flows from the tap with the current low fuel in the tank? Generally, fuel should fill a cup in about a minute or around 225-235 cc/min. You can do smaller amounts over a shorter period of time and scale it up from there. You can just pour the fuel back into the tank.