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General Category => Tech Corner => Topic started by: rrrr on January 23, 2022, 10:21:44 AM

Title: Carbs are working well???
Post by: rrrr on January 23, 2022, 10:21:44 AM
I am here for the first time today and of course I have a question:-)
My son bought an '89 CB1 on which we did a complete engine overhaul.
Now we have rebuilt the bike so that it is actually ready to run and it starts immediately, runs smoothly and doesn't make any strange noises.
But when I look into the carburettors without the air filter box and with the engine running, I am surprised that the vacuum-controlled slides go up quite slowly when I open the throttle and never make it to the top position. I don't know this from my other machines with carburettors....
How fast should the slides go up?
Do they open all the way?
Could it be that there is not enough vacuum to pull the slides all the way open?

The carburettors have been cleaned, adjusted and checked twice that the diaphragms are not damaged!

Is there any good advice for me on what else I could check?

Many thanks for your efforts

Greetings from Germany


Title: Re: Carbs are working well???
Post by: a_morti on January 24, 2022, 02:57:02 AM
The carbs are CV type carbs. If you're used to older type slide carbs, these work in a different way.
Instead of your throttle cable directly controlling how far the carb slides open, the cable opens a butterfly behind the slide. The engine's vacuum then controls how far the slide opens, allowing more accurate metering of fuel.
It doesn't need more than one or two PS when you rev it on the stand, and the slide opens accordingly..
They will get fully open, at that moment the bike needs all of its ca. 45PS.

Title: Re: Carbs are working well???
Post by: rrrr on January 24, 2022, 11:05:21 AM
Hello a_morti

Thank you for your quick reply.
I understand the function of the carburettors, but have no experience with the carburettors on this machine.
Therefore, my question is how fast the vacuum-controlled slides should lift and whether they open all the way up.
Our slides are smooth, but they don't slide all the way up, they look about 1cm into the throttle path.
Even with the throttle wide open, they open slowly, taking about 5 seconds to open.

Can anyone tell me which hole the vacuum is coming through up there???

Thanks for your help!



Title: Re: Carbs are working well???
Post by: a_morti on January 26, 2022, 11:40:24 AM
If they would open much faster, the engine would cut out from lean running condition. You can test this by sticking your fingers in and forcing the slides to open as fast as you'd like them to.

I couldn't tell you which of the tiny drilled passages does what, exactly.

If it really is sluggish (you will perhaps notice this when actually riding) then you might start with the sub air filter, there should be a small piece of foam in the plastic box on the back of the main air filter but it will be dust by now, which won't cause a problem but if its remains have blocked the hose that could be a problem.

Title: Re: Carbs are working well???
Post by: Spurlock on February 24, 2022, 11:53:55 AM
As morti says the slides will not open all the way unless the bike is under load. Running the engine in neutral it will rev to redline but there will without load there will not be enough vacuum to raise the slides. And if they did raise all the way under those conditions the engine would stall. That's the beauty of CV carbs, the slides only open enough to maintain "constant velocity" (optimum velocity) of the air flowing through them.