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?


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