Identifying carbs? (Pilot screw adjustment, please!)

Mcdroid wrote:

gas wrote:

If anyone has a Factory Service Manual that covers the 1977 KZ 650 B1, please tell me what it lists for carbs. Thanks.

Chris: My KHI manual lists VM24SS for the KZ650B1 model...making the Clymer correct:lol:

Thanks buddy. Sometimes I almost know what I'm doing.:laugh: I appreciate the confirmation.

When I disassembled the carbs I carefully counted the turns of each pilot screw until seated. Here is the list:

#1: 2 3/4
#2: 3 1/8
#3: 2 5/8
#4: 3 1/8

Judging from the condition of the O-rings, I would say that #1 and #3 had been adjusted previously. So, what is the consensus on a good starting point for these screws? The service manual only says to count the turns in until seated and then use that count on reassembly. The manual assumes that the turn limiter cap has not been previously removed. This set came with no limiter caps so I really have no confidence that any of these are correct. It's interesting that #2 and #4 were at the same position and those O-rings were in the best condition.

All ideas are appreciated. I did about an hour's worth of searches, but didn't find a value for the pilot screws. I did find a value for the air screws on my SR that matches the value in the manual.

- Nate >>

N0NB wrote:

When I disassembled the carbs I carefully counted the turns of each pilot screw until seated. Here is the list:

#1: 2 3/4
#2: 3 1/8
#3: 2 5/8
#4: 3 1/8

Judging from the condition of the O-rings, I would say that #1 and #3 had been adjusted previously. So, what is the consensus on a good starting point for these screws? The service manual only says to count the turns in until seated and then use that count on reassembly. The manual assumes that the turn limiter cap has not been previously removed. This set came with no limiter caps so I really have no confidence that any of these are correct. It's interesting that #2 and #4 were at the same position and those O-rings were in the best condition.

All ideas are appreciated. I did about an hour's worth of searches, but didn't find a value for the pilot screws. I did find a value for the air screws on my SR that matches the value in the manual.

- Nate >>

I thought that it was always 1.5 turns out, from a gentle seating. There is also some finer tuning of each individual carb pilot screw affecting idle, that I have never done. I know that Wiregeorge spoke of it before and I meant to do it, but never got around to it. I'll dig in the archives and see what I can find.

kzrider.com/forum/viewthread.php?tid=18321

Post edited by: gas, at: 2006/05/04 00:34

I believe those are not really "air screws", but would really be called "mixture screws".

Air screws are usually found on the air-horn side of the carbs. They only control air into the pilot system.

Mixture screws are usually on the outlet side of the carbs. They control the air AND fuel (mixture) as it exits the pilot system into the engine.

The main thing is to know which way to turn the screws to make it lean or rich. Air-screws make the idle lean as you unscrew them. Mixture-screws make the idle richer as you unscrew them. This is important to know when you use the "lean-drop" method of idle tuning.

loudhvx wrote:

I believe those are not really "air screws", but would really be called "mixture screws".

Air screws are usually found on the air-horn side of the carbs. They only control air into the pilot system.

Mixture screws are usually on the outlet side of the carbs. They control the air AND fuel (mixture) as it exits the pilot system into the engine.

The main thing is to know which way to turn the screws to make it lean or rich. Air-screws make the idle lean as you unscrew them. Mixture-screws make the idle richer as you unscrew them. This is important to know when you use the "lean-drop" method of idle tuning.

I can't speak for Nate, but myself, I often mix the terms up because I forget. I think that I have it all together now. So..... My 77 KZ 650 B1 has VM24SS carbs. These carbs have a (pilot fuel mixture screw) sticking out of the bottom/front of the float bowl. I just read in the archives that Wiredgeorge recommended one turn out from gently seated for 17.5 pilots, thus 15's should be 1.5 turns out from seated. So now I have to experiment. My plugs are showing a bit of a rich mixture (dry and black), I want to lean the carbs out just a bit. So your advice is the same as Wiredgeorg's basicly. I'm a little rich so I should turn my screws in half a turn to lean them out (which coincides with WG). So.........., to boil this down. The carbs with the brass screw at the bottom have (pilot fuel mixture screws) When screwed in they lean the carb out, when unscrewed they make the carb richer. Correct? Also, what is the "lean drop" method of carb tuning? I suppose it's obviously what I would be doing, but could you please explain in "big picture" terms. I wish to learn and maybe this will help others here also, thanks.

Post edited by: gas, at: 2006/05/04 13:47

The lean drop method is to find the strongest idle.
It is done only after you check compression, valve clearance, valve timing, ignition timing, clean carbs, set fuel level in the carbs, have jetting correct, carbs are balanced (sync'd). It is truly the last step in tuning.


You need a very accurate tach. This is not the dashboard tach. It is an electronic, analog, tach incorporated into an engine analyzer. They're pretty cheap. You can get a decent Sunpro in the $30 to $40 range if I recall. Dashboard tachs of any kind are not precise enough.

There are many versions of the lean-drop, here's the one I developed. It's not the "official" one, but it works like a charm for me as long as everything else, above, is right.

You run the bike to normal operating temp. Put the bike on the centerstand. Attach tach. Adjust idle speed to normal.

Turn pilot screw to get it LEAN. The idle will DROP. Slowly turn the screw richer to get the RPM as high as it will go. I use 1/8th turn increments. Keep very close track of the number of turns! And keep close track of the average RPM. It will fluctuate, but you need to eyeball the average. As you go richer, the idle will drop again. Make note of where the highest RPM was first attained and how many turns back it was. Set it back to where you first attained the highest RPM. That is the leanest position that will give you a strong idle.

Now, with the motor running or not, turn the screw all the way in counting the turns to see how far out it was. That's how many turns your base setting is at. You want to go up to a half turn richer if it doesn't drop the idle too much. This is because the bike probably warmed up quite a bit during your adjustments. When it's cooler, it will want to be richer.

Do the same for each carb, but re-adjust the idle speed if it starts to climb.

All four should be within a 1/2 turn of each other if everything else is right on (I usually end up within a 1/4).

If one is off by a significant amount, check fuel level and synchronization.

If you know all four are setup the same, you can try to do all 4 carbs at once. It's faster, but won't show up a problem if one cylinder is off.


The reason you don't do a "rich-drop" method is because when you go rich, excess fuel can collect during the rich position. You want to start lean so there is no excess fuel messing you up after you start.

Post edited by: loudhvx, at: 2006/05/04 14:32

attack of the post clones

Post edited by: loudhvx, at: 2006/05/04 14:29

Wow, thanks for the detailed info. This is pretty sophisticated fine tuning for an individual like myself, a carb Neanderthal slowly evolving into a Cromagnon. Looks like I am close to fullfilling all of the tune up prerequisits to attain this level. I'll save this info for the time when I'm ready to take a run at it. Thanks again.

Thanks Lou. This is most helpful.

The carb set I am working on definitely has mixture screws while my SR has air screws (which I'm not working on ATM). I now have a starting point and an excuse to get an electronic tach! :laugh:

Of course, the factory service manual assumes that the turn limiters are in place and doesn't say how far from seated is a good starting point. EPA regs. Sigh...

- Nate >>

I find the starting point, regardless of screw type or carb type is always around 1 to 2 turns out. If it won't run at all, you probably have some other issues.