hi guys, gutted missed out on a good weekends ride out, i,ve got a cbr600f2 94plate. changed the engine over for a used unit. original engine was fine but tried to change for s/s pipes and 5 of the 6 studs sheared, so i got this used unit for good money, straight swap starts fine and is great in the low gears accelerates smooth and clean but when you get to 6th there seems to be no more power, like a flat spot. Put new plugs in and still the same???? do the carbs need to be balanced with the used engine as they were set to the other one, please excuse me if i seem a bit wet behind the ears. i have never balanced carbs before, assuming this is the fault of course

 

out of sync carbs will usually run fine at higher throttle inputs, but run really bad at lower throttle inputs. I don't think if they were out of ballance it would cause your issue.

Since you said your wet behind the ears, I'll ask a few questions.
How fast are you going when your 6th gear?
Have you driven the other bike to compare how it reacts to 6th gear?
Are you running the same sprocket on the new new engine?
Are you running an F2 engine?
Did you put on an aftermarket exhaust without jetting the carbs for that pipe?

These bikes really need their rpms before they bring the power on. If your running too slow and in too tall of a gear, it will flatspot and won't have any power. Just make sure your speeds are high enough to warrant 6th gear.

 

Thanks for the reply, the bike is a 1994 CBR 600 it was running sweet through all the gears high and low rpm,s, but the down pipes were holed and needed to be replaced to pass the MOT. So i purchased a s/steel set, came to take my old set from the bike and all but one of the exhaust studs snapped (****). I knew that i would have to take it to the machine shop to get it done, hele coiled or thread insert etc, and i would also have to supply the head, ( new head gasket, head bolts bla bla bla ) so i got a used complete engine for less money than it would have cost for the work to be done on my old one, tested and guarenteed. Installed it and it started no problem at all, steady tick over 1500 rpm, took it on a little run round the block, checked for leaks, all fine. Next day started again no problem took it on a longer run where i could push it a bit, going through the gears fine but in 5/6 it was missing power, like i would change into 6 about 80 mph throttle and delayed response and then struggling. with the old engine i used to back off at 120 (got to scarey). I did not mess with the intake manifold and carbs i just secured it straight onto the replacement engine expecting the same performance. It is the same engine PC25 by the way

 

I would suggest doing a compression test on the engine if hasn't been done already. Also verify your carbs are fully opening when your full throttle. Also make sure your carbs are fully seated to the intake boots and are secured.

Its very hard to diagnosis over the internet, especially when its only lacking power at the top end.

Is there a possibility you overfilled the oil? Extra oil in the crankcase can actually rob power if the crank it having to run through it.

Maybe someone else has an idea.

Oh, since you have your other engine out of the bike and the studs are snapped off. As long as they are flush or a little sticking out, you can probably still get the studs out if you hav access to a welder. If the studs are flush, you can go buy some steel nuts and hold them over the broken studs. Then weld the nut to the broken stud through the hole of the nut. This will also create the stud to get really hot, which will break up the corrosion and help you to remove the studs.

 

Have you got the direct air induction set up? Any change to air filter & with the replacement motor, did you end up putting on the SS exhausts?