Carb Syncing - For What It's Worth... - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 9 (permalink) Old 09-28-2006, 09:41 AM Thread Starter
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Carb Syncing - For What It's Worth...

For what it is worth, I thought I would share my recent carb syncing experience….

I built my own manometer by taping clear plastic tubing (1/4” id) to a yard stick and using automatic transmission fluid. I let it hang on the garage wall for several days to make sure all the A/T fluid had settled.

When it came time to do the sync, I ran the bike to let it warm up. I then shut it down and hooked up the manometer to the carbs. The bike was on its center stand. Since I was in my garage, I set a box fan in front of the bike to help with cooling.

My pre-determined approach was to run the bike, read the manometer, shut the bike down to make the adjustment, and then start it up again to see the effect of the adjustment. I used a small 9/32 wrench (I didn’t have a 7mm) to turn the adjusting screw/bolt. I had decided to make the adjustment with the bike off because I found that opening the throttle moved the adjusting screw/bolt to a better position for turning the bolt from the right side of the bike. I hooked the vacuum hose on the petcock to an external vacuum source to keep the fuel flowing.

Starting the bike with the manometer attached for the first time indicated that the carbs were very much out of sync. Before the A/T fluid got sucked out, I shut the bike down and made my first adjustment turning the bolt in the appropriate direction as indicated by the Clymer’s manual. I restarted and found that I had improved the sync, but not enough. I don’t know how many times I repeated this procedure because I tried to make small incremental adjustments and usually did something like this: not enough…..not enough….Oops! Too much! I would then adjust the bolt in the opposite direction and repeat the ”not enough…..not enough….Oops! Too much!” scenario multiple times.

The end result was that the fluid in the tube would stay at the same level on each side of the manometer at idle. I did run the throttle up to see what it would do and one side would be higher than the other depending on the throttle position. First, one side would be higher. I would then open the throttle a little more and then the other side would be higher. It was interesting to watch, but as long as they were even at idle I considered it good enough. After disconnecting the manometer and reconnecting the vacuum hoses to the carbs, I found that the RPMs at idle had increased. I adjusted the idle back down and the bike seems to run fine. At least I don’t think I hurt anything. For me, that’s a job well done…
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post #2 of 9 (permalink) Old 09-28-2006, 10:11 AM
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Can you give a seat-of-the-pants ride report? Any noticeable differences after syncing the carbs? If you experience extra vibration - at what rpm's?

Thanks! Mark
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post #3 of 9 (permalink) Old 09-28-2006, 01:09 PM Thread Starter
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"Seat of the pants" impression is that the engine seems to run smoother from idle up through 5000 RPM. I haven't had it above 5000 RPM since the sync. The engine seems to be a little more balanced...
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post #4 of 9 (permalink) Old 09-28-2006, 01:13 PM
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Thanks for the update. I have the carb sync tool but I'm a little intimidated by it so I haven't hooked it up yet. I feel a little vibration around 4000 rpm - which seems to be normal - and I'd like to smooth that out if possible.
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post #5 of 9 (permalink) Old 09-28-2006, 07:48 PM
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I was just working on a Suzuki Katana 750 today, and we hooked up the carb sync tool... It was cake to interpret and use. I also used a valve compression tester on the same bike... that was amazing! (I hang out at a Ducati dealership and watch the mechs work... sometimes they let me play along.) For me it is demystifying...once I see something like that in action and see how it works and why I am not afraid to use it anymore.

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post #6 of 9 (permalink) Old 09-28-2006, 08:11 PM
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Everytime I hook a tube of fluid up to my vacuum line, it gets sucked in. I must be doing something wrong.

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post #7 of 9 (permalink) Old 09-28-2006, 08:45 PM
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It is all that electricity around you... that is freaking me out Ernie...

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Mods and other stuff:
nat'l cycle deflector shield
final drive splines lubed 8/06
replaced broken speedometer cable (5194 miles) 9/06

Vehicles:
95 S-10 Blazer 187K miles (Black and Silver)
01 Kawasaki VN750 3300+ my miles, 5300+ total miles (Black and Silver)
04 John Deere L118SE Riding Mower w/ snow plow (John Deere green and yellow)
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post #8 of 9 (permalink) Old 09-28-2006, 10:42 PM
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That's one of my old avatars from when I was involved with A Treasure's Trove. I was fifth to the site of the snail ($12K). Could have possibly been first or second had my car (or bike) been driveable. It was all the way up at Lake Anita, Iowa.

The Secret's of Alchemist Dar came out Tuesday. This one is worldwide, but there is some kind of submission that you do to locate the ring. 100 rings total, at least that's what I hear.

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Drilled baffles
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Last edited by cegodsey; 09-29-2006 at 01:30 PM.
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post #9 of 9 (permalink) Old 09-29-2006, 09:00 AM Thread Starter
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Mark,
I have owned my 1992 VN750 for three months now, and only been a rider for three months - so I don't have much experience. However, I do know that the bike "feels" different under different circumstances. You mentioned a vibration at 4000 RPM. I have noticed that the smoothness of my bike varies not only with the RPM of the engine, but also with the Dow Industrial average, the price of Pork Bellies, brightness of the Sun, wind direction, relative stability of the Middle East, the Consumer Price Index, rate of inflation, 30 year fixed mortgage rates, and the price of Hot Wheels cars at Target stores. In other words, sometimes it runs great, and sometimes just a little less than great for no apparent reason. I had dinner with some friends the other night, one of whom races motorcycles. He told me the way a bike runs is affected by ambient temperature, atmospheric pressure, and who knows what else. I just try to enjoy the new experience of riding. I do believe the carb sync is a good idea and does affect performance. If you haven't tried it, make small adjustments and see what happens.
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