Topic: 82cb650 boggs down around 4500 rpm.

[BomberMann650]

A flex head inspection camera should be easy to snake into the airbox and plenum.

Vacuum gauges wouldn't give any usable data for this function.

[MJL]

You said it's all stock, but have the exhaust pipes been changed? I had a similar issue with my 81 650, rejetted the carbs, put a K&N filter in, had the coils tested, everything you've done.  I know you said you took the air filter out, but did you put the airbox cover back on?  What I found on mine was the airbox intake cover was too restrictive for the 4-1 pipes I had on. Try taking to cover off and go for a ride and see how it runs.

[Pappy]

Let me chime in.
My wife's 1982 CB650SC is doing exactly the same thing. Exactly! Still is not fixed. There is a multi page thread on it as well in here.
No need in doing a leak down or looking at weak valve springs. Weak valve springs would float whether or not there is a high load on the engine. Failed leak down test shows up more quickly in starting ability and idle quality as well as just a reduction in overall power output, not a surge then flat spot.
As load increases so does the need for a robust ignition system as dynamic compression requires more spark output from the ignition system. Mine sounds more fuel related but try this and let us know.
As load increases so does the need to supply additional fuel to make the additional power needed. A surge or flat spot then power drop off indicates lack of fuel in most cases. Good place to look at may be the needles , slides and diaphragms........
This is where I stopped on my wife's bike. I am going to remove the carbs again, clean and go into the diaphragms and needles that lift and supply fuel at high load/rpm. Try yours as well and let us know and as soon as I can I will do the same.
Here is one of the two threads I started. The other one was on the slides.
http://forums.sohc4.net/index.php/topic,186403.0.html
And the one on the slides
http://forums.sohc4.net/index.php/topic,187361.msg2172773.html#msg2172773
I have yet to look for the insert mentioned in the thread on the slides. Easy to see though and you may already remember if it was there or not.

[scottly]

Pappy, just to add to the discussion, a leaner mixture requires a stronger spark than a rich mixture, which can contribute to confusion when trying to determine if it's an ignition problem or a carburation problem.   

[Pappy]

Scottly -  I would agree but the chances of both the ignition and fuel supply being issues at the same time are pretty slim.
If his bike is doing the same as ours is then it is pretty drastic and load related. Bike will rev in neutral and in the lower gears but as the load of each higher gear comes into play the engine falls flat at lower and lower RPMs until a slight reduction in throttle angle happens then it picks up again and will run steady. You can play with it and run it right at the threshold  of the flat spot all day long.
To the OP.....correct me if I am incorrect on the symptoms.
Oh.....and hello Fizzle....LTNS!

[fizzlebottom]

Hey Pappy!

There's a dude over on the /r/HondaCB sub that fixed his no-start issue by cleaning up a corroded advancer & weights on his '82 CB650 Nighthawk. If you haven't done so already, maybe check to see if yours is properly greased and if it needs any attention. I suppose it could cause an issue at higher RPMs.

Common Motor has a good video showing the detailed operation of these units and general instructions on how to service them.

[scunny]

now that.s a good point.

[MJL]

Hey Pappy!

There's a dude over on the /r/HondaCB sub that fixed his no-start issue by cleaning up a corroded advancer & weights on his '82 CB650 Nighthawk. If you haven't done so already, maybe check to see if yours is properly greased and if it needs any attention. I suppose it could cause an issue at higher RPMs.

Common Motor has a good video showing the detailed operation of these units and general instructions on how to service them.

It's been awhile since I had the cover off my 650, but IIRC they have a hall effect sensor instead of points, do they have an advance mechanism?

[scunny]

Yes, they have the same advance mechanism as other sohc

[Pappy]

yep - checked mine quite a while back and it works well. Popped a timing light on it and watched it advance. Also checked by hand to make sure everything was free and easy.

[fizzlebottom]

Man, I wish I had more knowledge to throw your way to help resolve this issue. Last time I had a significant problem with my bike I ended up replacing the whole damn engine. I guess when adjustment and spec confirmation options run out, replacing parts is the next step. But I don't know where to suggest starting, or even if you want to approach that. I mean, it could be some $.10 spring or it could be something completely unavailable for this bike.

Either way, let us know what else you're doing at least so we can add it to the tome of knowledge.

[Pappy]

Can do! I have not given up on it. Lesa and I are in the middle of remodeling both bathrooms and other stuff. I will get back on it and take a look again.
In the meantime we purchased a brand new Kawasaki Z650RS in silver. FI bike so no carbs to worry about. We love our retro bikes !!