1976 kz 900a #2 carb air screw inoperative?

originally posted a #2carb running rich. I have four into one pipes with a k/n filter. I rebuilt carbs and have 17.5 pilots and 117 mains. Tried adjusting the #2 carb after adjusting the float setting using clear plastic tube method. While the bike is idling I can screw it all the way in and no effect on the carb idle? Took out air screw and sprayed carb cleaner down it and then removed bowl and took out the pilot jet and spayed carb cleaner in the pilot jet hole and then blew some compressed air in the hole. Put everything back and tried adjusting it again and nothing? Any ideas?

Turn the screw out to 1 3/8 turns and see if the spark plug on that cylinder burns sooty. If it does, turn it out another 1/2 turn. If you still have soot, then you can conclude your cleaning efforts were not adequate or there is another reason for the soot (electrical, timing or valve related).

Post edited by: wiredgeorge, at: 2007/08/31 10:08

Tried turning screw out to 1 3/8 and then to another 1/2 turn and still sooty? I though I read in another post that when you go to higher main jets (117)you need to take the adjusting screw out about 2 1/2 turns? Anyways i went 2 1/2 turns out and still sooty although the bike seem to run better. Noticed the #1 carb doesn't respond to the screw adjustmen either? Turned both all the way in and no effect on idle? Checked the #1 spark plug and it looks wet but no soot? Pulled the screw out and sprayed some cleaner in it and the rpm jumped but when i put the screw back in and tried to adjust it, still nothing. Is there something in the air feed system to the pilot system that is common to both of these carbs? Any ideas will be greatly appreciated. I am the original owner of this scooter (30 years) and have never had any problem with it before. It has always been dependable and when every one else had their bikes in the shop i was riding, not so this year.

gman911 wrote:

Tried turning screw out to 1 3/8 and then to another 1/2 turn and still sooty? I though I read in another post that when you go to higher main jets (117)you need to take the adjusting screw out about 2 1/2 turns? Anyways i went 2 1/2 turns out and still sooty although the bike seem to run better. Noticed the #1 carb doesn't respond to the screw adjustmen either? Turned both all the way in and no effect on idle? Checked the #1 spark plug and it looks wet but no soot? Pulled the screw out and sprayed some cleaner in it and the rpm jumped but when i put the screw back in and tried to adjust it, still nothing. Is there something in the air feed system to the pilot system that is common to both of these carbs? Any ideas will be greatly appreciated. I am the original owner of this scooter (30 years) and have never had any problem with it before. It has always been dependable and when every one else had their bikes in the shop i was riding, not so this year.

As earlier suggested, all other components and systems should be spot-on before addressing the carbs (i.e., compression; valve clearances; good property timed spark via ignition system, timing, wiring, plugs; no vacuum leaks; adequate clean fuel supply; clear air filter; etc., etc.)

Main jet size is immaterial to pilot circuit.

IMO, having a perfect carb pilot circuit is returning it to basically as-new condition.

Freshly filtered air goes into a small entrance hole on the airbox side of the carb. Sometimes there's an air jet, but oftentimes not. This air continues along a passageway until reaching the pointed tip of the pilot air adjusting screw which allows some of the air to continue on to the pilot jet.

Meanwhile the pilot jet has picked up raw fuel from the float bowl. The air reaches the end of the air passage at the pilot jet where the air goes through the lateral holes in the pilot jet creating an air/fuel mixture which mixture then travels from the top of the pilot jet on through the pilot passage and out the pilot outlet, from where the mixture is sucked through the intake valve into the combustion chamber.

All this is accomplished while the throttle slides are positioned completely down with no additional mixture contributed from the needle jet. And the pilot circuit continues this function of providing fuel mixture until about 1/8th to 1/4th throttle opening position. As the throttle is opened, the slides are raised, which allows increasingly more mixture from the needle jet and lessening effect of mixture from the pilot circuit.

The above is my over-simplified understanding of the pilot circuit system.

IMO, the pilot circuit is to blame for many of the sooty plug woes (and sometimes a too high float bowl fuel level causes sooty plugs regardless of an otherwise perfect carb).

Now, toward a perfect carb pilot circuit --- And there are undoubtedly other and perhaps better methods, maybe even far superior methods, but here's how I do it, until learning a better way.



Pilot Air Passage

The pilot air passage throughout its entire length from entrance hole at airbox side of carb throat to the pilot jet should be clear and ultra-clean.



Pilot Passage

The pilot passage from top of pilot jet to and through the pilot outlet should be clear and ultra-clean.



Pilot Jet

The pilot jet central hole and lateral holes should be clear and ultra-clean. The pilot jet itself should be in pristine condition and the proper size. Remember, possible damage from previous cleaning efforts (with wire or otherwise) may be visually imperceptible. Holes appearing perfectly round and clean may be faulty enough to disallow the absolute precision needed to properly meter fuel intake from float bowl and infusion of air from the air passage. As new pilot jets are relatively inexpensive, I prefer to replace them. Opinions may differ, but new pilot jets worked for me. And I have yet to see any visual defect in the old original pilot jets.


Cleaning The Passages

With carb on the bench, remove float bowl, float and pilot jet. Wear protective goggles. Plug threaded area where pilot jet was removed so as to block access into float bowl (using finger is okay). If pilot air screw has been removed, block its hole with finger. Direct carb cleaner spray into air passage entrance and blast away until spray freely exits from pilot outlet.


Pilot Air Screw

Remove pilot air screw and examine pointed tip to assure it isn't damaged. Carb cleaner spray directed into the (1) pilot air screw hole has three places where it may exit, (2) Air passage entrance, (3) into float bowl through pilot jet location, (4) pilot outlet. Any two of these may be purposefully blocked so the spray flows between the other two. Spray every combination until assured all the passages are clear and clean as new.


And please don't rely on the upside down work-bench method of measuring float level. The clear plastic tube method of verifying correct service float bowl fuel level is much more accurate and works just as well with carbs on the bench as it does with carbs on the bike. And remember WG's time saving tip, only one float bowl screw is needed during the fuel level testing. Hold the carbs level. It won't leak. But possibility of a little seepage from the gasket area won't invalidate the test because correct level will be below the gasket.

Good Luck with the repairs!

The baseline setting for the airscrews on a 76 are 1 3/8 turns out. If you turned the #2 airscrew out an addition half turn and the plug is still black, it is very likely that:
1. air filter or pod dirty
2. ignition problem
3. timing problem
4. valve clearance issue

I suggest you quit messing with the carburetors to try and rectify this issue as the issue is poor combustion. Poor combustion can be caused by any of the four issues I noted. If you get a Kaw Factory Service Manual and go through tune up steps, this will likely fix the problem. The only exception is the ignition situation which will require more troubleshooting. If you have points on your bike, replace the plugs, points, condensors and wires and check coil voltages. We have a good "Coil Repowering" tech tip on our website under "wg's Tech Stuff Index" which explains this particular issue as well as a Valve Clearances" how-to. The reason? Both these things cause problems with sooty plugs and customer's carburetors get blamed. If you don't have the know how or time to go through the bike yourself, take care finding someone who can do the job correctly as this site has horror story after horror story about shops that work on older bikes. The work I described is about a half day's work so don't expect to get off real cheap. Most shops won't have the correct valve tool to change the shims nor will they have the shims. Check if they have BOTH before letting them take the work on.

THANKS Patton!! That was just what the doctor ordered. Good break down. I'll start from scratch again and recheck everything else (i.e., compression; valve clearances; good property timed spark via ignition system, timing, wiring, plugs; no vacuum leaks and try and do some pilot circuit system cleaning pilot while the carbs are still on the bike.

Trust me i didn't want to jump back into the carbs but I figured it was running good before I rebuilt them, no sooty plugs before the rebuilt? So when I started I checked the easy stuff;
1. Didn't know how to use my old car timing light so
I just statically adjusted them, but thanks to some
other posts i think i can use the timing light now.
2. Reset the points. (they were clean, no pitting)
3. Replaced plugs (ngk).
4. Cleaned and recoated my K/N air filter.
5. Cleaned my pods when i rebuilt the carbs
6. After rebuilt had the carbs synchronized.
7. After sooty #2 plug Adjusted float level (clear
tube method) cheap trick and worked good
8. Replaced #2 plug with hotter plug (9es)
9. Pulled float off and removed pilot jet and air
mixure screw and sprayed carb cleaner and
compressed air into their orifices.
My new plan is to start over and go back and recheck everythng and use the the info you guys have provided me and do the compression check and valve clearance check. Thanks again for all your help and i wish i had known about this site 10 years ago (thanks Z-1 enterprises). I love tinkering but man summer is almost over.... Let the good times roll!

Trust me i didn't want to jump back into the carbs but I figured it was running good before I rebuilt them, no sooty plugs before the rebuilt? So when I started I checked the easy stuff;
1. Didn't know how to use my old car timing light so
I just statically adjusted them, but thanks to some
other posts i think i can use the timing light now.
2. Reset the points. (they were clean, no pitting)
3. Replaced plugs (ngk).
4. Cleaned and recoated my K/N air filter.
5. Cleaned my pods when i rebuilt the carbs
6. After rebuilt had the carbs synchronized.
7. After sooty #2 plug Adjusted float level (clear
tube method) cheap trick and worked good
8. Replaced #2 plug with hotter plug (9es)
9. Pulled float off and removed pilot jet and air
mixure screw and sprayed carb cleaner and
compressed air into their orifices.
My new plan is to start over and go back and recheck everythng and use the the info you guys have provided me and do the compression check and valve clearance check. Thanks again for all your help and i wish i had known about this site 10 years ago (thanks Z-1 enterprises). I love tinkering but man summer is almost over.... Let the good times roll!