CB77 fall flat above 3000 rpm, need input.

[LArzfromarz]

Hey guys, I'm here cause I'm finally stumped on this one, here goes.
1965 CB77 original and stock. PW26, 140 main, 42 pilot, floats set to 22.5 with gasket removed and measured from the mating surface, floats seem high to me btw and not sure if floats aren't the wrong ones. Timing is set and points fire a couple of degrees advanced (or before) LF. Runs well and idles well ( with stock intake tubes and filters installed) and advancer is confirmed to fully advance by 3000 rpm via timing light (but is slow to return to idle the weak spring issue). Needles are in the second clip position from the bottom (per LM recommendation?) and I am at sea level with 80 + degrees ambient temps. Yes slides are in properly.
Throttle application (slow or fast) results in rpm drop to stalling (if you don't let off) from about 3500 rpm.

I have tried the 26 mm float position and different clip settings with not any much better results.

Yes I've checked the passages and cleaned the carbs ( a couple of times).

My assumption is that I should be coming on the needle at this throttle setting (1/8-1/4 throttle and above) but yet are either going very lean or flooded rich (I say lean). Seems to act like a intake leak but I have done a solvent spray test and get no RPM rise from spraying either carb.

So I place this problem in the hands of the group (even though I don't frequent this boards as much as I used to, sorry).

[48lesco]

Suspect a fuel flow issue but - Difficult to give much guidance if the floats might be the wrong ones??? But I will anyway! Is the right side timing right? You only mentioned "LF". I always set the floats from the cut-outs in the lip but that's not much different from the mating surface. Do you have sufficient fuel flow from the petcock? Gas cap vent OK? Fuel should drain continuously from the bowl drains. Couple things to check. Let us know.
-48

[Goodysnap]

I agree with 48. Your symptoms are classic for starving of fuel.

Make sure the bowl vents are open. Normally if the tank vent is the issue it will run good til you get going a mile or so. Then once vacuum generates in the tank and starvation begins. But that's an easy one.

Sometimes the issue can be found in the history of repairs- how long has it been this way?

[LArzfromarz]

Yes guys I agree about lean or starvation possibilities.
WRT right side timing I haven't checked with the timing light but static timing and point gaps are real good. I'm doubtful it could be so far out as to be the source of this trouble.

Typically many runs are with gas cap loose or off and during this phases I only keep minimal fuel in the tank since its been off an on so many times. Been down the closed cap vent road before too.

Petcock flows well and I use clear fuel lines and filters. Also this allows some checking of level of fuel in the bowl, which has been comparable to other threads regarding the subject found here on the forum.

The only reason I speak of possible wrong floats is that I recall sometime, before I sold the bike, putting in a float that I think I remember was cb350 number. At the time (what ever float I used) was compared and matched up to what came out of it. Sadly they aren't marked with part numbers so no telling for sure but again matched up identically (previous had a pin hole).

As an experiment I'll run it from the IV gas bottle but expect no differing results.

BTW jets and emulsifier tubes were cleaned sonically too. Visual verification of open orifi (orficsis?) as well.

Bike did run well when sold about 18 mos ago, new owner let it sit up and now...

I'd post a pic but have been getting odd error messages.

[jambretta]

Keep looking at simple things first. I kept having fuel flow problems and it ended up being the vent in my gascap being clogged. I drilled it clean and drilled a second similar one to boot. No issues after that. There are photos and posts in this forum somewhere.

[GG]

You might want to make sure the problem isn't with an over rich mixture. At sea level I know it isn't likely, but I've had similar symptoms from a rich mixture (I'm at 4500 feet, so that was my problem here). Doing a plug chop isn't easy on the Superhawks, but you should try. Rev it out until it gives you the symptom for a few seconds, then get the motor killed and get yourself off the road without getting killed! Pull a plug. If the condition was fuel starvation, they will be white. If they are black, it's too rich.

If you don't have the right float for this carburator, any measurement could be off. When I pull my floats bowls off with gasoline in them, they are a little over half full of gasoline.

Good luck!

GG