Engine not rev'ing down, and shutting off when turning throttle fast

[alexdecker]

Hey all

So having some more trouble with this fickle beast of mine. The model is a CB550F2 1977.

When I rev up the engine, the revs hang in the highs, even though Im idling at around 1100. It does this almost every time I rev up. Also, if I let it fall down completely, and then turn the throttle fast, the engine turns out. Lastly, I got the occasional popping and small explosions in my exhaust.

I have checked that there is no carb leak with some carb spray, and I've also vacuum synced the carbs. The air mixture screw is turned out about two full turns. I tried 1,5 turns as well, but with same result. When I go one turn only on the air mixture screw, the bike doesn't idle, but just goes out. Also, the carbs got cleaned half a year ago.

Lastly, my choker is acting up. Out of nowhere, its sorta stuck in a certain position, and I have to sorta "click" it out of it. I have checked if there is anything physical in the way of it turning freely, but there is not. What could this be?

Here is links to videos illustration all the problems. Somehow I have a feeling they are all interconnected…

Choker not turning freely: https://youtu.be/9xfIopnMSzU

Engine stops when turning throttle fast: https://youtu.be/NTRt5yrF5UQ

Revs doesn't come back down: https://youtu.be/DTc0kpwH6r4

Any help is greatly appreciated!

[McCabe-Thiele (Ted)]

I'm an amateur at this but the usual culprit for this type of behaviour (assuming the throttle cable is mechanically closing off the carbs) has to be too much air getting in somewhere.

Have you a fuel filter between the tank & the carbs to keep the crap out of the carbs?

Sounds like a carb strip down is going to be the answer.

[alexdecker]

Yes, but for the life of me, I can’t figure out where the air is coming in. It ran perfectly fine before I replaced the top engine seals (where I also cleaned up and restored the carburetors), but after putting it back together, it’s just not running right. Feel like I have checked everything.

[Laverda Dave]

When you stripped and rebuilt the carbs did you use new O-rings and gaskets?
Did you remove the floats and re-check the float height at reassembly. These carbs are a pig (as are the 400/4's) if the idle jets get blocked which they frequently do if left with old petrol in the bowls. I agree with Ted, another carb strip and clean is likely, use an ultrasonic cleaner if you have access to one with the correct carb cleaner added. Remove all the brassware and the floats and put these in the cleaner. Renew all the o-rings with viton replacements (Nurse Julie sells these and they are of excellent quality). Make sure the felts are still in place on the choke spindles (replacements are available from Mike Nixon of the Mototrcycle Project in the USA, if you google his site he has a lot of very useful info). Spray carb cleaner through every jet and airway of the carbs after blaster air through following removal from the ultrasonic cleaner (do the same with all the brassware. Keep the original brassware as well, the pattern parts are of questionable quality sometimes.
With regards to the sticking choke, is the very small ball bearing and spring still in place, they sit directly behind the choke lever in the carb body, they have a habit of dropping out when the carbs are taken apart as they are hard to spot. If not that check the choke spindle adjuster screws and levers are not fouling on one of the metal bands between the carb to air filter trumpet.
All the above refers to a 400/4 but carbs are carbs and the priciples apply to all the sohc carbs. If you have PD carbs throw them away and get a set of non-PD's and save yourself from a life of misery

[K2-K6]

This setup routine may be of help http://www.sohc.co.uk/index.php/topic,17769.0.html as it prevents you building a conflict into the carb link adjustments.