Carb Synchronizing - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 2 (permalink) Old 01-15-2006, 07:36 PM Thread Starter
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Carb Synchronizing

Here's a simple, very inexpensive, way to find out if it's carb balance:

Q: asked this question a while back, and don' remember if we ever arrived at a concensus. Shops recommend owners synchronize their carbs at regular intervals. Now, I an imagine what might happen if they were grossly out of whack, but otherwise what are the symptoms of carbs that need synchronizing?
A: I just did mine this weekend. I used a TwinMax I got from Adventure Motorcycle Gear. It's a great tool, if a bit pricey. Some of the most obvious symptoms of out of sync carbs would be an increase in vibration while cruising, esp when accelerating. Reduced gas mileage and less snappy throttle response would be good indicators as well...all else (jetting, air cleaners etc) being in good tune. jm

A: I sync'd mine (using a mercury sync stick) in 1987 when I bought the bike.. to be sure it was withing the 2cm spec. It was. I checked it last month (17 years later), and it's still within 2cm. I would say it does not go out often -.. unless the screw is loose somehow.

If you have vibration and acceleration might be a good idea to check the sync. However..3 diffrent wrenches told me the same thing: "Your bike is new..the carbs were sync'd already. Being CV carbs..they should not need to be sync'd till you hit 20k...and that's questionable. Usualy we find the carbs are fine...but if not...90% of the time it's the cable or linkage that got loose. Most cases are from older bikes that someone has messed with and didn't set them properly to begin with." When I asked why it was suggested in the manual to sync them every X miles..the response was more or less...." Well... lets see..shaft chain adjusters..guess they felt we should have something to"
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post #2 of 2 (permalink) Old 01-15-2006, 07:36 PM Thread Starter
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Synching Carbs on your VN750

Carb Synching on the VN750 is a breeze. The vacuum ports are right next to the mixture screws on both carbs...there are excellent pics of the A/F screws in the Yahoo Groups/files section in Jax's folder.

The front cylinder carb has the vacuum port on the left side of the bike. The vacuum port is at the end of the hose connected from the bottom of the petcock into the carb. This is where the vacuum from the carb activates the petcock to send fuel into the lines.

The rear cylinder, on the right side, has a hose connected to it's vac port that goes under the seat and connects to the fresh air vac valve, if you're still stock.

If you've 'coastered' and/or removed the EPA fresh air injection system, this hose should be removed and the vac port plugged with a 5.5mm vacuum cap. Disconnect both hoses at the carb, and hook up your CarbStix or whatever (I use a TwinMax) to the vac ports.

You'll need a 1/4" barbed 'T' hose connector for the petcock side to maintain fuel flow, and (3) 1/4" flexible hoses total. You'll want to keep these hoses as short as possible to maintain equal vacuum and still give you enough slack to operate & read your balancer.

The base of the 'T' inserts into the hose from the petcock. A short hose goes back to the front carb, and a longer one goes to your Balancer. An equally long hose attaches your Balancer to the rear carb on the right side.

On our bikes, the FRONT cylinder is, when you turn the vacuum adjust screw you are increasing/decreasing vacuum on the REAR cylinder. Read the instructions w/ your Balancer as to which side on it should be reference, if any.

The adjuster screw is located directly beneath the carbs, right in the middle. It's a 7mm bolt, probably w/ some yellow or whatever paint on it to prevent movement. You can see and adjust this best from the LEFT side. MAKE VERY SMALL, INCREMENTAL ADJUSTMENTS TO THIS SCREW, IF ANY AT ALL ARE NECESSARY!

Basically...warm up the bike first on the centerstand, shut it down and carefully hook up your instrument.
Loosen the throttle cable adjustment screws at the handlebars to ensure the throttle plates close completely.
Fire up the bike and get your readings as per the Balancer instructions.
Remember...When you turn the adjuster bolt IN (tighten), you are INCREASING vacuum on the REAR cylinder. Loosening the bolt DECREASES vacuum on the REAR cylinder....

I feel that our bikes have a bit of a buzz between 4300 ~ 4800rpms, so I don't just check synch at idle, I rev it and synch at 4500, then check idle again. I don't mess with the adjusters (carb side) on the throttle cables (to fine-tune the throttle plates) either, just loosen the ones on the grips a bit to make sure the valves fully close.
Hope this helps...

Last edited by Vulcan Verses; 01-15-2006 at 07:39 PM.
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