1983 CB750SC nighthawk

[geri]

Hi my dear dudes,

I would like to ask you a question about my bike. I have rebuilt my carb (taking it apart, cleaning reassembly) and now my bike iddles nicely, but when i pull the trothle, it revs up around 4000-6000 rpm and is never coming down. When i adjust the iddle knob than it goues lower and slowly dies. Wont start, only if i pull the trothle, than starts and dies. If i adjust back the iddle speed knob than starts and if i pull the trothle the same again. Anybody have met the same issue? I am a newbie. Maybe bad carb sinc?

 

[geri]

Forget a pic of the beast

 

[brettp]

Air leak in the boots or carb sync are most likely.

 

[geri]

Air leak in the boots or carb sync are most likely.

Thank you for the idea. You think a leak between the airbox and the carb or a leak between tha carb and the engine itself? I mean i had new rubbers for between the carb and engine and the brake fluid test does not show negative results only in between the airbox and the carb

 

[brettp]

Thank you for the idea. You think a leak between the airbox and the carb or a leak between tha carb and the engine itself? I mean i had new rubbers for between the carb and engine and the brake fluid test does not show negative results only in between the airbox and the carb

First, I wouldn't use brake fluid. Or starter fluid. Use carb cleaner. And as the bike idles, spray around all boots.

 

[Travis]

The potential leak you're looking for would be between the engine and carbs. If you don't have a leak there, keep checking for other issues. I had a similar problem once with a Yamaha XS400 from the fuel mixture screws being turn in too much so it was running lean.

 

[geri]

First, I wouldn't use brake fluid. Or starter fluid. Use carb cleaner. And as the bike idles, spray around all boots.

Done and done. I bought this bike last year. Disassembled the carbs, cleaned them (with carb cleaner, and high pressure air) and reassembled them back together. After I have did the leak test (with brake cleaner) and it seems like the carb is leaking from between the airbox and the carb. Did a sync on the carbs too, but maybe it was not correct.

 

[geri]

The potential leak you're looking for would be between the engine and carbs. If you don't have a leak there, keep checking for other issues. I had a similar problem once with a Yamaha XS400 from the fuel mixture screws being turn in too much so it was running lean.

Thank you for the idea. I was thinking the same, the leaking issue is more of a problem between the engine and the carb. Mine second idea was the mixture too. I have unscrewed the mixture screw, maybe even a little too much, because when I took the machine on a test drive I nearly lost one of the mixture screws. So what do you think? Either a wrong carb cleaning ir sync?

 

[brettp]

I'm guessing you haven't yet unscrewed the idle knob to bring the idle down. I would definitely do that before anything else. It seems like we might be chasing simmering that isn't a problem by allowing the idle to remain so high.

 

[geri]

I'm guessing you haven't yet unscrewed the idle knob to bring the idle down. I would definitely do that before anything else. It seems like we might be chasing simmering that isn't a problem by allowing the idle to remain so high.

So basically you are suggesting to adjust only the iddle speed before doing anything. I like the idea, because it is the easiest solution thank you will def. do it before anything. Thank you. I will come back to you after i have tried it.