Synchronizing Carburetors on 750 Nighthawk

After installing the Dynojet Jet Kit in the Nighthawk the next step would be to synchronize the carbs. In theory, if they were synchronized before you changed the jets, and you didn’t fool around with any of the adjustments,the carbs still be synchronized. But, with the tank off it is not a big deal to check it before you put the bike back together.

You will need a couple of things.

1) A set of vacuum gauges or a manometer for synchronizing the carburetors.

2) Three feet of 1/8 vacuum hose and about the same of 1/4 inch fuel line.

3) Assortment of vacuum tees to test the vacuum gauges.

4) A hand vacuum pump. (If you don’t have a vacuum pump, you can use the manifold vacuum from the #2 carb, but it will be more difficult to adjust the gauges because of the pulsing of the vacuum.)

carbsync-003.jpg We need something to measure the vacuum in the intakes. I purchased these vacuum gauges from JC Whitney. The first thing I did was to test the vacuum gauges. Using a vacuum pump and several Tee’s, I plumbed all the vacuum gauges to the vacuum pump. The idea here is to make sure that these “bargain” gauges read reasonably close to the same vacuum. If they don’t you can unscrew the covers and adjust them so they read the same at about 15 inches of vacuum.
carbsync-004.jpg The carbs are numbered from 1 to 4, one being on the left side of the engine, 4 on the right. If you look on the top of the manifolds from the top you will see them numbered. Remove the screws from the manifolds and screw in the tubes that were supplied with your gauge set or manometer. Attach the vacuum hoses to the tubes.
carbsync-005.jpg On carb #2, leave the vacuum hose that goes to the petcock, use a vacuum line Tee to split off a line to the petcock and the other line to the vacuum gauge.
carbsync-008.jpg Set the gas tank back on the bike and hook up the fuel line and the vacuum line with the extra tubing you purchased. Set the tank where it will be stable but not in the way of adjusting the carburetor screws.
carbsync-010.jpg Hang the gauges on the handle bars and start the bike up. If the needles are unsteady, pulsing heavily, adjust the restrictor valves to smooth out the gauge needles. Be careful not to completely close the valves as that would cause the reading to be inaccurate. Let the motor warm up so it will idle smoothly with no choke. Adjust the idle to 1000 RPM. The gauges should read in the 8-10 inches range.
carbsync-012.jpg On the Nighthawk, carb #2 is the “base” carb, that means you adjust the other carbs to the same reading as #2. There are three adjustment screws, the screw to the left of #2 adjusts carb #1, the screw between #2 and #3 adjusts carb 3, and the screw between #3 and #4 adjusts carb #4. With the engine warmed up and idling, start at #1 and working toward #4, adjust each carb to the same vacuum reading as #2. These adjustments will interact a bit, so it may take a few passes to get them together. According to the shop manual, the carbs should be adjusted to within 1.2 inches of each other.

Ok, that does it, after adjusting the carbs you can put the bike back together. Be careful not to drop the small plug screws when installing them in the manifolds. I did, it wasn’t pretty!

After installing the Dynojet Jet Kit in my ’93 Nighthawk, I set up the gauges and started the engine and all of the carbs were within about 1″ without any further adjustment. So, again, if you install the jets if you don’t mess around with the adjustments the carbs should be pretty well synchronized.

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