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Discussion Starter · #1 · (Edited)
It's time to fine tune the carbs. I have other mufflers, UNI pods, about 3" stacks, 90/125 jets, 1 shim and it runs pretty good. Carbs VB21/31 from 1979.

Air/fuel mixture has to be 3.5-4 turns out. Isn't that a bit much?
What can I do to decrease the turns?

Maybe better stacks or slightly more restriction in the pod filter or anything else I could try?
I don't really understand what does what in the RPM range. I believe secondary jet is full throttle but that's it.
 

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Getting CV carbs to work with pods is tricky, best way I know to do it is covered here:

https://www.hondatwins.net/forums/6...4-cb400-cv-carbs-pods-how-make-them-work.html

The mixture screws are for adjusting the mixture at idle. Their effect diminishes rapidly as the throttle is opened but trying to correct other problems with them will mess up the idle mixture quickly.

Here's an explanation of what the circuits do:

https://www.hondatwins.net/forums/63-fuel-supply-carburation/16943-vbulletin-carb-circuits.html
 

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Discussion Starter · #3 ·
I've read that pod thread so I have a good start. I will have to check the plugs later.
Thanks for the links, interesting to see the circuits.

Right now it's about the mixture scews. I tuned them as described in the manual but I remember that I've seen a bike builder video where he said he doesn't like to do more than 3 turns out on the mixture screws.
So if you're above that, how come and what do you do about it?

The slow/idle jet is the one I can't change on my carbs, then it should mean that I need more resistance in the filter or different stacks so the air velocity increases but not so much the engine can't breathe at higher rpm.
 

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3 turns out would be unusual but, again, they are only for adjusting the mixture at idle. They are best adjusted along with a dynamic carb sync. With the bike fully warmed up and idling on the center stand adjust the idle to 1200 rpm. Adjust one mixture screw to achieve highest idle rpm. Re-set idle speed to 1200 rpm. Adjust opposite mixture screw to achieve highest rpm. Re-set idle speed and perform a dynamic carb sync. Repeat mixture adjustment. That's where the mixture screws belong. Sometimes hesitation just coming off idle can be reduced by opening both mixture screws slightly but, as you said, 3 turns seems to me to be approaching excessive. No amount of mixture screw tweaking will fix problems at mid-range or WOT.

If you have performance issues at higher throttle settings the correct fix is in the jetting or other components. Fuel level in the bowls can make a difference if you still have the adjustable (black) floats. You've gone up a long ways from the stock 70/112 jetting but with your mods they may be correct or at least in the ball park. Since it's running "pretty good" you should be able to do a series of plug chops to nail down just where you are at.
 

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Discussion Starter · #5 ·
I will try it at higher speed above 100 km/h (62 mph) as soon as I'm allowed to ride it on the streets, most likely it'll need tuning. I've only been testing it outside my house but it went up to 100 km/h uphill and I didn't notice any hesitations but I need more testing. It's just that the mixture screw has to be about 3.5-4 turns out.

Ok, I had done exactly how you describe it. Then I guess 3.5-4 full rotations out is what it wants. Yes, I have the black floats so I'll read how an adjustment will affect the idling.
 
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