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 reference...ie, 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...