Topic: Power band won't go over 5000 rpm????

[Steve F]

I'm wondering what you folks tink about this...Bike has about 25000 miles on it, new rings and hone job 1-1/2 years ago( you could still see the factory hone cross-hatch!!!), Dyna-s this spring.  K&N filter in stock air box Dyna 3 ohm coils with Accel wires. Starts right up and idles great, blip the throttle, great,  runs great up to 5k or a little above if I "nurse" it.  Has plenty of low-end grunt. Then it starts to cut out around 5k and above.  Also, I played around with the jetting, and the stock jets seemed the best so far.  I'm thinking that the cam didn't get "spot-on" after I did the rings.    The Dyna igy helped loads with the idle and response, but the power loss is still there.  Plugs look a little "dark" but not fouled or tending to become fouled. Wuzzup?    
Steve F

[TwoTired]

 How does it behave with the air filter temporarily removed?

Can you make it worse or better by adding a bit of choke?

Mark your throttle so you can tell what position your slides are in when the "limit" is hit.  1/4 1/3, 1/2 etc.

Have you checked the timing advance?

Do a plug chop on clean plugs after 20-30 second of "cut-out" operation.  What color are they then at the center electrode insulator?

Cheers,

[Sam Green Racing]

Further to lloyds coments, when did this problem start ?
Has it been there since the motor was rebuilt, after the bore?
When did it start?

Sam.

[Steve F]

Lloyd, I'll have to try the things you suggested, but I know for sure, it's made worse with the choke.  I roll the throttle on to about half-way (guess) when it starts acting up.  Advance is working fine, put the timing light on to make sure, timing set right. I might add that I suspect the advancer springs are weak, as the advance kicks in (I think) way too early.
Sam, After I took it apart and re-ringed it was when the problems showed up.  Had an oil leak at the head was why I took it apart in the first place.  Like I said before, I'm suspecting the cam is a bit off. Gawd I hate the thought of pulling that motor again.  But if that's what it takes to get this thing to fly...........

[heffay]

it seems to me like you do know what you're trying to accomplish and you almost have except for that last little glitch.

check all your intake and exhaust mounts... it sounds like a mixture problem to me (air leak or something of the like).  when was the last time you checked the fuel filter. 

cheer up buckeroo... you've got all summer to fiddle 

[Sam Green Racing]

Two things come to mind.
The cam a tooth out, and fuel starvation, but more than likley the cam.

Sam.

[HondaMan]

Steve, it sounds like the cam might be "late". Other things can do this, too, like a weak battery, but here's a peace-of-mind (or proof-of-problem before re-teardown) test for you:

Try this "rough" test, using your timing advancer's timing marks for (1,4) as a reference: the stock marks are TDC, 6 degrees and 40 degrees BTDC. The exhaust cam timing is set to close the exhaust valve completely at 5 degrees ATDC.

So, slip a .002" feeler gauge under the rocker for exhaust on #1 or #4, then turn the engine over manually with a wrench to TDC. Then, keep turning slowly until you can slide the feeler gauge out (that should be .0025" clearance point).  At this point, the timing indicator should be as far ATDC as the timing mark for 6 degrees is BTDC (other side of the timing plate). REMEMBER: it takes 2 crank revs for each full cam rev.

It's only a rough estimate, but the teeth on the cam are approximately 11 degrees, so one tooth's difference will really show up.

The clue to me is the idling. CB750s don't idle all that well. If yours does, something is up...late cam timing makes them idle like a CB350-4, smooth and quiet.

Hope it's not the cam timing....