TOPIC:

I have my bike all back together now, and it is running pretty well. Next step is to get the carbs "dialed in".

'78 KZ1000-B2 with VM26 carbs. Based on a previous recommendation from WG, I rejetted with 107.5 mains and 17.5 pilots. Needles clipped at #3.

I had the mixture screws at 1.5 turns out. But I was getting some popping out the exhaust of the 1/2 side during idle. So I opened the #1&2 screws up another 1/4-1/2 turn, and it seemed to improve some.

I am preparing to sync the carbs using my homemade vinyl tube & oil manometer. My question is should I set the mixture screws back to 1.5 before adjusting the sync, or should I leave them where they are now, and re-adjust after syncing?

Is there a good procedure for adjusting the mixture screws other than just fiddling with them until the engine sounds better?

Here's an excerpt from a recent post by loudhvx --

Re:Carb syncing question - 2007/02/12 02:48
There is no "sweet spot" that will maximize the RPMs, when syncing. The "sweet-spot" comes into play when you set the mixture screws.


I would first get the carbs synched -- go by vacuum readings, not by smoothness or rpm -- as the synch is to assure matching slide positions. (All mixture screws set at 1.5)

When synched, then go for the "sweet spot" by adjusting the mixture screws. The manual says highest rpm is the target mixture (lowering throttle idle screw as needed during the process).

Suggest using a floor fan for engine cooling during the synching and mixture screw adjustimg.

Post edited by: Patton, at: 2007/02/13 17:52

Thanks, Patton.

Last night, I reset my #1&2 screws back to 1.5, then did my sync procedure. After I finished, it didn't sound like I needed to fiddle with the screws anymore. It seemed like the popping went away, and seemed to run smoothly.

More testing and riding will tell me more.

Appreciate the report.

If you later decide to fine tune the mixture screws, remember screwing "in" richens the mixture by reducing air and screwing "out" leans the mixture by increasing air.

Likely you already knew this -- just wanted to be sure.

Patton wrote:

If you later decide to fine tune the mixture screws, remember screwing "in" richens the mixture by reducing air and screwing "out" leans the mixture by increasing air.

Patton, this is the opposite of what I thought. This is my understanding, tell me if I am wrong.

My carbs have fuel screws on the bottom, not air screws on the sides. So screwing out should richen the mixture and screwing in should lean it.

Am I wrong?

Skyman wrote:

Patton wrote:

If you later decide to fine tune the mixture screws, remember screwing "in" richens the mixture by reducing air and screwing "out" leans the mixture by increasing air.

Patton, this is the opposite of what I thought. This is my understanding, tell me if I am wrong.

My carbs have fuel screws on the bottom, not air screws on the sides. So screwing out should richen the mixture and screwing in should lean it.

Am I wrong?

Thank you for noting the screw position on the bottom. Believe I am wrong and you are correct. Am checking another source also and will re-post. Thanks

My carb idle screw experience has been: If it has a point it meters fuel or fuel/air foam; If it has a flat blunt end, it meters air or wedges something like the slide open.

The KZ1000 FSM (covers A1, A2, A3, D1 and D2) shows 1977A1 and 1978A2 having VM26 carbs with pilot mixture screw on bottom (same as your B2 LTD). This was a change from 1976 model carbs with side located pilot air screws. The 1978D1 (Z1R cafe racer model) used VM28 with side located pilot air screws. 1979 A3 returned to side located pilot air screw.

FSM says initial adjustment of pilot mixture screws for A1 is 1 and 1/2 turns out, but for A2 is just one turn out. Replacing the limiter then allows about 1/8 turn either way for adjustment. The FSM identifies these bottom located mixture screws as "pilot screws" but sometimes as "air screws."


Here's further info from an earlier thread.

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.

gas wrote:

... I often mix the terms up because I forget. I think that I have it all together now.... These carbs have a (pilot fuel mixture screw) sticking out of the bottom/front of the float bowl. I'm a little rich so I should turn my screws in half a turn to lean them out.... When screwed in they lean the carb out, when unscrewed they make the carb richer.... what is the "lean drop" method of carb tuning?


loudhvx wrote:

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.


Back to current post ---
Have never adjusted bottom located mixture screws, but appear more difficult to access.

Sorry about my earlier mistaking which carb model was involved. Glad to hear your bike's up and running.

Post edited by: Patton, at: 2007/02/15 08:02