Running rich after complete carb strip and rebuild

Hi Almighty KZ Forum!

im at a loss with this one - I have completely stripped, ultrasonically cleaned and rebuild the carbs on my 83 KZ440 Ltd but it is running rich. 

starts on no choke, both air screws are completely in - to the stops, not the 1 1/4 turns out as mandated in the manual and the filter is clean. 

yet it still starts and runs like a pig - this is the first time the bike has been run I believe in a few years - I bought it as a non runner in July this year - no running due to broken weights on the ATU - replaced the ATU and set about trying to get it running smoothly 

plugs are getting really sooted up with only a ten min run.
compression test is giving me 150 on both cylinders
temp on both cylinders is 149 left and 173 right
tick over is around 850 / 900 rpm

as soon as you put any throttle on it stalls out - super flooding I think 🤔 

these carbs (Keihin CV36) have been on and off this bike three times to make sure everything is sitting where it should and the float levels are down slightly from the usual - deliberately to see if it was a float problem causing it to flood. 

the only think I haven't looked at are the exhausts to see if they are blocked but I'm getting black smoke out of them so don't think that is the issue. 

I am flummoxed- does anyone have ANY ideas where I can go with this?

When you rebuilt the carbs did you use cheap kits or quality parts - if you fitted kits try putting the original brass parts back in if you still have them

Low float height, results in HIGH fuel level.  

Hi Wookie - didn't fall foul of chineski parts, used Keyster KK-0173N for the CV36 carbs and reused what I could of the original brass parts 😉
 

Hi Nessism - when I said low float hight I mean I had used sight glass levels for the actual fuel level in the float bowl and adjusted it so it was lower than the 3mm suggested level in the manual - it's around 4.5mm now
adjusted the tang on the float to shut off sooner that needed. 
Hope that makes sense 👍

If you are on stock jetting with a good air filter and good compression it would suggest that the choke isn't shutting off completely (possibly leaking plungers)

Those Keihin carbs have a choke flap.  No plungers like the Mikuni carbs.

Sometimes the choke flaps hang up, so that's something to check.

A fuel level that's a smig low is no problem.

Hi Utton!

I'm curious if you ever came to a solution here.  I'm having the same issue on my '83 KZ750 with 4 Keihin CV34's!

I've cleaned and rebuilt, synced, adjusted floats to spec and she runs with the the fuel screws fully seated; the screws have almost no effect on idle at all.  Plugs are black and sooty, RPM's hang high after a quick rev..   I'm at a loss.

Can you share any updates on your build?

Thanks!

TCooneyKZ750 wrote:

Hi Utton!

I'm curious if you ever came to a solution here.  I'm having the same issue on my '83 KZ750 with 4 Keihin CV34's!

I've cleaned and rebuilt, synced, adjusted floats to spec and she runs with the the fuel screws fully seated; the screws have almost no effect on idle at all.  Plugs are black and sooty, RPM's hang high after a quick rev..   I'm at a loss.

Can you share any updates on your build?

Thanks!

Hanging idle is generally a sign of air leaks, are you still using the original 43 year old intake rubber manifolds (if so very likely to be a problem)
Are the adjusters definitely fuel and not air screws ? it must be getting fuel somewhere to be rich ?

Hi Wookie58, thanks for your response!

I don't want to be overly confident (I'm no carb expert) but I'm fairly certain they are fuel adjustment screws; sharp point and motor-side of the carb.  All are in good condition: no broken tips, springs, o-rings, washers..

I still have the original rubber manifolds!  I'll order a new set and throw them on to be safe.  

If the manifolds are leaking, and therefore additional unmetered air, wouldn't that contribute to a lean mixture from the affected carb?  I guess that would explain why it might fix my hanging revs.. 

That still leaves me perplexed on how a closed pilot screw would allow the engine to still idle.  This would have to mean one of two things:

1) Pilot screw is either broken or not seating properly.
       - I don't know to check for this other than visually.  The screw top looks fine and visually seems to seat when closed (inspected while carbs were out)

2) Low pressure zone exist above the primary or secondary jet pulling fuel into the system.
       - The CV venturi cross-sectional area would need to increase (i.e. the slide to open up)

Am I thinking about this correctly?

If similar to vm carbs there is a fuel bypass port aswell as the fuel screw. On vm carbs the port outlet is under the slide and supplies fuel even if the fuel mixture screw is fully seated.

TCooneyKZ750 wrote:

Hi Wookie58, thanks for your response!

I don't want to be overly confident (I'm no carb expert) but I'm fairly certain they are fuel adjustment screws; sharp point and motor-side of the carb.  All are in good condition: no broken tips, springs, o-rings, washers..

I still have the original rubber manifolds!  I'll order a new set and throw them on to be safe.  
If the manifolds are leaking, and therefore additional unmetered air, wouldn't that contribute to a lean mixture from the affected carb?  I guess that would explain why it might fix my hanging revs.. Correct it would cause a lean condition, however it's almost a given if the boots are old they will leak so yes it should help the hanging idle. If you can go for OEM (I recently fitted aftermarket and all four split after an hour of running.)

That still leaves me perplexed on how a closed pilot screw would allow the engine to still idle.  This would have to mean one of two things:

1) Pilot screw is either broken or not seating properly.
       - I don't know to check for this other than visually.  The screw top looks fine and visually seems to seat when closed (inspected while carbs were out)

2) Low pressure zone exist above the primary or secondary jet pulling fuel into the system.
       - The CV venturi cross-sectional area would need to increase (i.e. the slide to open up)

Am I thinking about this correctly? Yes, it must be getting fuel from somewhere and a pressure difference is required to draw fuel. I assume you are happy that the soot is fuel related and not burnt oil ? do you get black smoke from the exhaust when you rev it ?