Carbtuning The Carbs - Vacuum Line?? - Kawasaki Motorcycle Forums
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post #1 of 20 (permalink) Old 11-06-2018, 05:29 PM Thread Starter
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Carbtuning The Carbs - Vacuum Line??

I was trying to fine-tune the carbs on my '78 KZ650 with a Carbtune, but what do I do with the vacuum tube coming off the No. 2 carb? The manual and Carbtune instructions just say to hook up the lines, but that #2 carb is being pesky...can I just block off the vacuum line - stick a golf tee in it or something, and then adjust to That carb, or...?


I'm pulling 16 bars Mg off carbs 1, 3, and 4, but #2 is just not behaving. I also put new carb holders on and did a spray test, and that was fine. The idle is set currently at about 1500, which got rid of that rattle I mentioned in a previous post, so it was probably set too low.
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post #2 of 20 (permalink) Old 11-06-2018, 05:53 PM
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I suggest you check the compression to make sure #2 isn't lower than the others. If it is adjust the valves. I don't know what you have done to the bike already.

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post #3 of 20 (permalink) Old 11-06-2018, 06:20 PM
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Yes. Plug the vacuum line. Balance all 4 to as close to all the same reading. The reading itself is not important. Just get all the same. Changing one will make others change too. All the same, or close, is what you want.

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post #4 of 20 (permalink) Old 11-06-2018, 06:26 PM Thread Starter
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Compression runs 155 - 165 each cylinder, so that should be okay...and thanks Mike! Is that what it should be pulling, about 22.5 Hg? What idle?

For what it's worth, I have the idle adjust all the way back off the stop, and it's still running about 1500 rpm... turn each adjuster IN to richen, and OUT to lean, yes...?

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post #5 of 20 (permalink) Old 11-06-2018, 06:57 PM
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The specific reading is NOT important. Just get them balanced. When syncing increase your idle rpm high enough so it runs smoothly, then adjust each carb. Turn the adj screw and see which way the gauge moves.. It's an art in itself. Some say black magic.. Do it on a running temp engine. If its air cooled.. Put a fan on it so you don't overheat.

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post #6 of 20 (permalink) Old 11-07-2018, 06:14 AM Thread Starter
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Funny, that's what one of my biker friends says too, that carb tuning is a Dark Art...I'm starting to believe it ...
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post #7 of 20 (permalink) Old 11-07-2018, 09:39 AM
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And you should be tuning the idle mixture screws at the same time and readjusting as you go to get it as close to right as possible.
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post #8 of 20 (permalink) Old 11-08-2018, 05:58 AM Thread Starter
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Yes, this is my first time, and it's a little trickier than I thought - I set the idle at around 1500, but then couldn't get them all in line - I would adjust one, and doing so would cause another channel to rise, or lower.

I adjust #1, and #2 and #4 don't show anything. I get #'s 1 and 2 equal, and 3 and 4 are flat. Adjust those, and #1 and 2 go down, or Waaay up, and 3 and 4 barely show anything...tighten 3 and 4, and 1 and 2 collapse - or 1 and 3 go up, and 2 and 4 go down - - I'm all over the place !

Adjust one, the idle picks Waay up; screw that one back down, all four drop out...it's like playing carburetor Whack-A-Mole! I did wind up taking the carbs back out and bench-synching them again, and tightened down the allenheads on the carb holders, just to be sure I was on a 'baseline', and I was...the carb is really sensitive.

Technical note, though - the #3 carb appeared to overflow, Just Once - a big puddle of gas came out of the #3 overflow tube - I assume because I probably inadvertently closed the slide and it backed up - but what do you think?
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post #9 of 20 (permalink) Old 11-08-2018, 06:54 AM
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Your description sounds correct for a 1st timer! Turn the idle up a bit and the others moving when adjusting one will decrease. 2000 or so rpm is ok. Do this all at opreating temp. Put a fan on the motor. Loosen the lock nuts just enough to move the adj screw. SMALL adjustments. Think 2 moves ahead. If #1 moves up when you move 3 up, adj #1 down then adj #3. And presto.. #1 moved up to match where you moved #3 to! Lightly snug the lock nuts during the process. Lightly blip the throttle also to settle in the slides after adjusting one. If the idle gets too high as it tends to climb as you get balanced, just turn it basc down some. A special carb tool is also very helpful. It lets you work the lock nut and adj screw without putting pressure on the slides.

https://www.z1enterprises.com/carb-a...-tool-set.html

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post #10 of 20 (permalink) Old 11-08-2018, 07:47 AM Thread Starter
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Sure; easy for You to say...! So do you do #1 and 3 first, and then 2 and 4? Or does it matter?

And I do have that tool you included. I used to use it on my Virago and CX500, but this is the first time I've ever tried to do a whole BANK of them! It's amazing how much fuel it uses...


I think it's amazing how such little adjustments can affect the flow characteristics of the fuel - doing this gives you a real appreciation of just how finely machined carburetors are. Just like life, sometimes a little adjustment can effect a big change in performance...or, conversely, cover you in fuel and burn you to death. Either/or...
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