Sounds like you're hitting the rev limiter, a built-in safeguard function 
of the cdi to keep the motor from overrevving.

Engine wasn't designed for sustained WOT operation, as you may have guessed 
from having had to replace the previous one.  




On Saturday, April 7, 2018 at 9:30:16 PM UTC-4, 64alpha2 wrote:

> Hi all,
>
> I ended up having a motor swapped into my 1985 cb650sc after the old one 
> got rod knock. However, there seems to be a problem I'm having at high revs 
> and WOT (same 'problem' with last motor too, and same RPM's/throttle 
> position), so it could be an issue with tuning of carbs, or maybe an issue 
> ALL nighthawks have? However, I'd like to rule out potential valve pumping 
> up issues or whatever the case may be.
>
> So, the engine revs freely to redline in neutral but in gears 1-3 (4,5,6, 
> doesnt seem to have this problem, but at high revs and WOT i'm going pretty 
> fast so I don't notice as much, still pulls hard.) it'll only get to about 
> 8-9k before I hit a wall of revs, and the engine sounds very different and 
> the revs refuse to climb. However, during this WOT in these lower 3 gears, 
> I notice that if i REDUCE my throttle slightly from WOT to lets say 70%, 
> the engine sounds normal again and will climb in RPM slowly. Once I twist 
> the throttle more to the OPEN position, it'll hit this weird wall again. 
>
> It's not clutch slippage because the vehicle doesn't accelerate and revs 
> stay the same. I've experienced clutch slippage before, this isn't that. 
>
>
> What could this be? I'm thinking fuel/air problem as reducing throttle 
> still makes the vehicle accelerate, and then increasing it starts to sound 
> really weird. Almost like combustion isn't occurring inside and the noise 
> is just the engine spinning at 8k RPM without combustion. 
>
> Otherwise, the bike runs great and pulls very hard. This seems to be an 
> issue at only 8-10k RPM and at WOT.
>
> Any ideas?
>
> Thanks,
> Chris
>

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