Spark plug cables, the bits that sit on the plugs and go to the ignition coil

 

Can you still get these OEM or equivalent? I did a quick Google search with no results. And are they loose at the coil module end or spark plug end?
Also, there are several spark troubleshooting tests outlined in the Clymer manual. I may have to perform them as I have yet to troubleshoot anything spark related, only fuel.

 

My guess would be carbs. If it starts and idles ok, and takes full throttle, then sparks and fuel delivery seem unlikely. You've rebuilt the carbs and cleaned the jets (all I trust). Did you touch the butterflys? It is possble to put them back the wrong way round. How about the diaphragms, manifold rubbers, o-rings? Air leaks will mess up the mid range. Is float level correct? Choke stuck on?

 

The thing is, this issue sratrted even before I touched the carbs. I only then removed them and cleaned them as a preventative measure. They were spotless.
As for the butterflies, I did not remove them. So I can rule this out.
As for the diaphragms, all 4 seemed fine and seal correctly. HOWEVER..... cylinder 1's carb diaphragm cylinder goes up and down by hand a bit easier than the other 3. If I'm not mistaken, it's been this way for years though. I just assumed the plastic diaphragm slide was a tad more worn out than the rest (friction?) This shouldn't affect performance right?

Manifold rubbers are good as well. I've tested for air leaks using carb cleaner and spraying around these areas. No RPM change.
Float levels all seem correct as well.
Choke lever seems properly adjusted as well.
I may tear into the carbs once again while having a motorcycle tech with me through the process to see if he notices anything. Another set of eyes in this case is what I think I really need. Otherwise, I'm at a loss. Again, this issue arose one day and I'm having difficulty tracking it down.
Today I elimated the ignition system as I performed every test in the Clymer manual and all checks out.
As does fuel.

 

Yes, they're called throttle slides. They should move freely and return fully under spring pressure. Provided they do this I wouldn't worry if one seems a tiny little bit 'slower' than the others.

I guess the next question has to be - what happened with the bike leading up to the original problem? Were you riding it regularly and 'sensibly' or had it stood for a long time or been bounced off the rev limiter a lot?

Does petrol flow freely from the tank when you pull the hose off the tap? Might be worth draining the tank and flushing it out. And change the fuel filter. But if it will take full throttle with some revs on then I can't see fuel supply being the issue.

What do those vacuum valves connect to? Does it have a vacuum fuel tap?

 

The bike stood stationary for maybe 2-3 weeks before my next ride. This is when symptoms arose. Nothing changed between my last ride and then. I always use 93 octane.
So I suppose I wasn't riding it "regularly" per say. And I do ride sensibly. I rarely, if ever, bounce off the rev limiter. Maybe only done this 3 times in the 6 years I've owner it. But how could doing so cause these symptoms?
Fuel filter is brand new, as is the fuel pump (both genuine Honda parts)
As for the vacuum valves, they lead to the carbs. I believe they're purpose is only for CARB (California Research Board) compliance and some models came with them and some not. I've freed up the sticking 3 way valves and issue still persisted. In fact, I believe this sticking valve to have been present since I bought it.
The odd thing was, fuel was leaking ever so slightly from that 3 way valve AT THE SAME TIME THESE ISSUES AROSE. I found the leak to be caused by a bad float. I believe the bad float was allowing this vacuum valve to suck in fuel, when it's only supposed to be air. As a result, fuel dripped out this 3 way valve. I replaced the float and NO MORE LEAKS. but the acceleration lag was still present. I thought the 2 were related, but fixing the fuel leak did not fix the acceleration lag.

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It does sound like an air leak. At full throttle (and a decent number of revs) the butterfly valve and slides are fully open, it's on the main jet and running is largely unaffect by small leaks. At idle butterfly and slide are fully closed and carb is running on a different circuit. In the middle, air leaks can mess up the response of the slide to the throttle (butterfly) position because it moves up and down on the diaphragm and spring to balance air pressure on the engine manifold side with pressure on the intake (airbox) side. So if it just went wrong in normal use and you found a problem with those vaccum valves I'd look closely at whether there's a route for excess air to be drawn in. E.g. a cracked hose. Can you remove the California emmissions plumbing?

Had to ask about the bike's history to get a better picture. A bike standing for an extended period might have water in the fuel - ethanol is a pain that way. But 3 weeks is nothing. And if a bike is regularly bounced off the rev limiter you might suspect a valve or valve clearances.

 

Thank you so much @stanthomas for your input on this. It's greatly appreciated.
As for that vacuum system, I do believe it can be removed and the remaining open vacuum lines can be replummed. Here is an owner of a 1990 model asking about removing the CARB vacuum system :

https://cbrforum.com/forum/cbr-600f-15/can-some-one-tell-me-how-remove-my-cali-emissions-1990-cbr600-f1-135769/

Then there's this one:

https://cbrforum.com/forum/cbr-600f-15/california-bike-removing-emissions-system-31593/

My model must be a California model, since it has the charcoal canister on the underside near the kickstand. That, and all the damn vacuum hoses!

I belive these are SMOG vacuum hoses and valves and were likely required by the EPA. I've gotta look more closely into eliminating this system and re routing it
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I'll re inspect all vacuum lines VERY carefully and test by spraying starter fluid or around this area and see if there's any RPM change.

Today though, I completed ALL ignition tests in the Clymer manual and every one checks out. So now I can completely rule out spark as my issue. I can also rule out fuel now, as I've tested flow from the tank and all checks out. Fuel valve is actually only 2 years old and I sealed (POR 15) the tank then also. That only leaves air. So SURELY, I must have a vacuum leak SOMEWHERE.
Off to the parts store for starter fluid.....