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Author Topic: Syncing carbs & adjusting idle mixture  (Read 822 times)
Spurlock
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« on: March 17, 2018, 07:49:19 PM »

I thought I would share my take on synchronizing throttle plates and adjusting idle mixture screws. I described Honda's "idle drop" method for adjusting mixture screws in a previous post here: http://www.hondacb1.org/forum/index.php/topic,5607.msg27986.html#msg27986. But the two adjustments, syncing and mixture, affect one another so they really need to be done in sequence. And I find it best to make multiple passes of sync/ adj. pilots/sync/ adj. pilots for best results since errors in one can affect your perception of the other.

First, to have decent access and visibility of the adjusting screws you need either a remote fuel tank or to set the gas tank off to one side. I find it easiest to just add an extra short length of fuel line and set the bike's tank on a table off to one side. Yeah you can turn the tank around backwards and set it on the tail of the bike but for me that gets in the way of my head when looking for the syncing screws.



Then the sub-air filter/wiring plugs bracket along with hoses need to be pulled out of the way for best access to the syncing adjusters as well as the mixture screws.



Also, an accurate tachometer that can detect small changes in rpm is not necessary for syncing but is essential to be able to adjust mixture screws individually. There are lots of cheap tach/hour meters on ebay that do the job just fine. This one reads in 10 rpm increments and gets its signal from a wire wrapped around any spark plug wire.





Next you need a decent vacuum gauge setup. I use the Motion Pro Sync-Pro tool which works well but has one quirk you need to be aware of, namely that it's calibration changes depending upon vacuum level. It is a four part manometer, and the first step in using it is to connect all four tubes to a single vacuum source, then turn calibration screws so each column reads the same. OK fine, but for whatever reason the gauge columns will read differently at higher or lower vacuum levels than where they were calibrated. So I like to first connect each gauge hose to its own cylinder, start the engine and see where the readings are on the gauge. Here they are about 1/4th of the way up.



Knowing about where the gauge level is going to be, I connect all four gauge hoses to a single manifold port so they receive the same vacuum, temporarily plug the other manifold ports, and adjust the calibration screws on the gauge to equalize the readings. Now it will be totally accurate, right? If only it were so. It will be accurate at this vacuum level so I'm good to go for an initial synchronizing but will need to re-calibrate if gauge level changes. So for now I connect each gauge hose back to its own manifold and start the engine. The readings below are actually plenty close enough since I had done this when I first got the bike. But I will go ahead and get them as perfect as possibly.



Before adjusting sync screws you really need to know what each screw does, and Honda's manual is completely unhelpful in this regard. Sure you can eventually get the carbs synced going from one screw to the other until all four cylinders are either well synchronized or you run out of patience. But it saves a lot of trial and error to understand the following: On the CB-1 (and most Hondas with throttle plates instead of slides with individual adjusters) one screw equalizes vacuum between cylinders 1 & 2, another equalizes 3 & 4. Once those pairs have been synced, a third screw equalizes the 1&2 cylinder pair to the 3&4 pair. A change in any screw changes rpm and so changes all four readings, which can lead a person to jump from screw to screw in a whack-a-mole fashion, wasting time and getting frustrated. And truthfully carb syncing can be fussy work, but understanding what each screw does lets you take the most direct route to success.



So here I have turned the left screw slightly clockwise to equalize 1 & 2. While I make the adjustment I pay no attention to columns 3 & 4.



With 1 & 2 synced, I tweak the right screw clockwise to equalize 3 & 4. I pay no attention to the fact that 1/2 are different than 3/4 because that adjustment is last.



With 1&2 equalized and 3&4 equalized, I can now adjust the center screw to match right pair to left.



At this point I go through the idle mixture screw adjustment as described in the link above. I like to leave both vacuum gauges and tachometer connected and cycle through both sync and mixture adjustment twice or more to best refine the tuning. As mixture gets closer to optimum the idle speed keeps changing and so the idle speed screw needs changing which affects throttle plate position which affects - guess what - both idle mixture and throttle sync. So after setting sync and mixture, the vacuum level is now higher than it was when I did the initial gauge calibration. So now it is time to re-calibrate the vacuum gauge before the second go-around. I find it easiest to use a brake bleeder pump to apply vacuum to the carb sync tool, pumping it up to the current level at idle and recalibrating the gauge for the final run through for both sync and mixture adjustments.



-Bill
« Last Edit: March 18, 2018, 02:12:19 PM by Spurlock » Logged

1975 Honda CB125S2, 1989 Honda NX250, 1989 Honda GB500, 1989 Honda CB-1
CapitanBOD
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« Reply #1 on: March 18, 2018, 01:53:15 PM »

Good read thanks🖒
I have been quoted 60 to balance my carbs by Tim Blakemore racing in bristol. He says he knows the nc27 but I shall study this and ask to be present when he does it.
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Efreeman55
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« Reply #2 on: March 18, 2018, 02:23:51 PM »

Very nice writeup and pics that are sure to help lots of CB1 owners who do their own work.  This is on my list of things to do this spring when I take my CB1 out of winter hibernation.  I use a set of calibrated vacuum gauges to set the initial synchronization of the carbs to get them close and then use a 30" water tube manometer for final adjustment.  Very accurate but tricky to use.  I've been using this method for years to synch the twin Weber DCOE's on my '75 VW Scirocco so I'm accustomed to the technique.

Eric
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Sugs
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« Reply #3 on: March 18, 2018, 03:15:31 PM »

Nice writeup Spurlock!  Now if you please, finish by detailing Honda's very hard to grasp(to my mind anyway) idle drop procedure and this will be a perfect set of instructions.  Grin
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Spurlock
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« Reply #4 on: March 18, 2018, 04:02:38 PM »

Thanks for the comments all.

Nice writeup Spurlock!  Now if you please, finish by detailing Honda's very hard to grasp(to my mind anyway) idle drop procedure and this will be a perfect set of instructions.  Grin

Sugs, the link at the top of my post listed the steps involved in the idle drop procedure, but I'll try to put it in my own words. Basically you are using changes in idle speed to tell you whether idle mixture is optimum or not. First step is to set them all to the book setting (2-1/2 turns out). Then you begin backing out all four screws at once, 1/2 turn at a time to see whether rpm increases. If so, reset idle speed and back them all out another 1/2 turn, etc. Stop when you no longer get a speed increase. You have now done a gross adjustment of all screws to get them in the ballpark, but you do not yet know whether each individual screw is in its best position.

So the final step is to apply the test to each carb individually. Reset idle speed, then turn #1 mixture screw back in 1/2 turn at a time, pausing between each turn to see if rpm changes. keep turning in (leaner) until you see a 50 rpm drop in idle. Then back the screw out one full turn, which is what the manual calls the "final setting". Repeat this process with carb #2, then #3, then #4. In this way you are using the feedback of idle drop to achieve a consistent mixture setting on all carbs.

At this point I would want to reset idle speed and confirm that carbs are synced since throttle plates might be in a different position than when they were synced. For instance, if mixture was off enough to cause poor idling, then the idle speed knob would have needed to be turned in to achieve desired idle speed. But after correcting mixture, the idle would have been too fast and the knob turned back down, closing the throttle plates a bit to get back to the correct idle speed. And I've noticed that on many bikes the carb sync becomes uneven as the throttles begin to open, probably due to slight misalignment in the linkages. So, I would want to tweak the syncing again if needed.

Then this refining of sync will likely lead to smoother idling and allow you to refine the mixture adjustments further by making it easier to zero in on the 50 rpm drop point. So I would run through just the last stage of individual mixture screw adjustments, not the all at once first steps. Confused yet? Grin

Last but not least is to confirm with the old "butt dyno" that the bike idles well and responds smoothly to initial throttle opening. If it seems to stumble a bit when the throttle is cracked slightly, try backing out each mixture screw a half turn to richen slightly. On my bike, after blipping the throttle the engine would often die when dropping back to idle. Turning each screw back in 1/2 turn corrected the problem, so my bike likes a "final setting" of 1/2 turn, not one turn. Your mileage may vary.

-Bill
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Spurlock
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« Reply #5 on: March 18, 2018, 04:20:10 PM »

I forgot to add that mixture screw adjustment is only really possible if the limiter stops have been ground off the float bowls or the flag caps removed from the screws.

-Bill
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« Reply #6 on: March 19, 2018, 08:17:59 AM »

Excellent guide, thanks for posting it! I plan to have a go at mine in the spring, still haven't got round to trying to remove a stuck mixture screw on carb 4
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« Reply #7 on: March 19, 2018, 09:23:36 PM »

Thanks for spelling it out in your own words Spurlock, very helpful!
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