Topic: Full throttle issue - CB 500

[Norwegian CB-rider]

Hi guys!

Please bear with my english, it´s a little rusty!

I have got a little problem with my CB 500K from 1977. If the bike is idling and I snap the throttle wide open the bike stalls immediately. The bike revs just fine until about 50% of full throttle at idle. On the road the bike behaves like normal until I suddenly snap the throttle open. The bike does not pick up speed immediately, and starts to hesitate. Eventually it picks up speed after a couple of seconds of coughing. If I open the throttle gradually the bike goes like a little rocket, even when it reaches wide open throttle. Seems like the mixture isn't right, to lean - right?

I guess a little more info about the will come in handy. I completely rebuilt the engine during wintertime, and had a new 4-4 exhaust (replica) hooked up. I swapped out the new air filter with a new one, and I run the original air filter box. Yesterday I removed all the plugs, one of them was a little black (cyl. 2). The other ones seemed okay. I cleaned off the black plug with a brush and screwed it back in.

I also removed the carbs, popped them open and removed the needle jets. Cleaned them out with a little needle and compressed air and made sure nothing was trapped inside the little holes. I often hear people talk about crap inside the bowls, well I wouldn't say my ones were messy at all. Some minor white and green contaminants on the needle jet bodies tho... The name of the carburetors I believe is Keihin PD46A. The little mixture screws found on the engine side of the carb body were not adjusted with the same amount of turns on the carbs, so I screwed the snug, then i opened them one and a half turn just to start. I probably should have taken the carbs further apart, because the problem is still there.

So what should I do next? The bike is definitely rideable, so it is not a major problem. But it would be lovely if like the bike responded to wide open throttle without any hesitation.

[Airborne 82nd]

Do you have a accelerator pump on those carb?

[calj737]

Do you have a accelerator pump on those carb?

No.

Your issue is typical with older carburetor. They are not very tolerant of "snapping" the throttle open because you are switching from the idle to main jet and waiting n the engines vacuum pressure to pull the requisite amount of fuel into the engine.

If you ride with a smooth throttle action, you should be just fine.

[Norwegian CB-rider]

Do you have a accelerator pump on those carb?

No.

Your issue is typical with older carburetor. They are not very tolerant of "snapping" the throttle open because you are switching from the idle to main jet and waiting n the engines vacuum pressure to pull the requisite amount of fuel into the engine.

If you ride with a smooth throttle action, you should be just fine.

Like you´re saying there is no accelerator pump, so I came to the same conclusion. The bike goes just fine as long as I handle it right. It is a little annoying, but I can live with it. I added a bottle of a fuel additive to clean carburetors and injectors, but it is a little early to tell if it made any difference. If anyone have got any tips and tricks let me know. Thanks for your help so far!

[calj737]

You have to realize this is not a modern, fuel injected, drive-by-wire variable vale timing machine. Its a 40 year old, vacuum inducing, gravity fed fueled SOHC. Treat it kindly and ride it wisely  You'll have decades of enjoyment once you become accustomed to how it behaves best.