synchronizing those 2-bore cv carbs ????

[Guest]

If you have 4 cylinders, 4 mixture screws, and 4 carb venturies, how the heck do you synch all 4 with only 2 vacuum holes???????

[GQROD]

If you have 4 cylinders, 4 mixture screws, and 4 carb venturies, how the heck do you synch all 4 with only 2 vacuum holes???????

It's actually easy

On each carb you only have one screw ( flat head ) on the outside that you remove to attach the sync adapter. Don't lose the copper washer when you remove the screws.

Because the throttle shafts connect both butterflies in both venturi's you can sync them with only one vac port ( screw ) on each carb. It is a 5mm thread.

Hope this helps.

[lrburke]

answer

I just did a 1983 GS550L and how these carbs work is both butterfly valves on each carb open and close simultaneously. One carb is the main and the other has to be synced to it, so you only need the one sync screw in between the two carbs. Please reply back if this is still unclear to you. Hope this helped! Larry

[don]

I should have made myself more clear. Sory guys. When I say synch, I mean with the idle air screws. The butterfly thing is quite obvious, you have no choice with those. Sory for the confusion.

[KennyJ]

Synching the idle mixture screws is done with a tachometer and a calibrated ear. They don't make enough consistant idle vacuum to set them with a synch tool.

With it warm and idling, you start at one end and work to the other, in/out maybe a 1/2 turn and see if it makes a difference. You get the hang of it quickly...small changes can make big differences (for example, my GS550EF took 1/2 turn to bring cylinder 3 up to operation and the RPMs jumps 500+). I also have a good temp gun and you can measure the exhaust temp at the head pipe to see if they are close to equal (rich and they are cool, lean and they are hot, really lean and they are cold, really rich and they are hot...)

Good luck! I am currently trying to track down a horrible metallic "clacking" noise in mine. Its not there upon cold start butonce I ride away it starts within a mile and comes and goes. If it happens at a stop sign I rev it quick and it goes away (sounds like a really loose rocker, but it doesn't always match engine rpm and when you blip it, it buzzes before it stops). I am at a loss at the moment figuring it out.

Kenny

[GQROD]

I should have made myself more clear. Sory guys. When I say synch, I mean with the idle air screws. The butterfly thing is quite obvious, you have no choice with those. Sory for the confusion.

Colortune spark plug will get you in the ball park and show when each cylinders mixture is proper.

Plug Chops will confirm and further fine tune the mix until you can get a light gray, tan or brownish color telling you the mix is right for each cylinder.

Hope this helps.

[Steve]

If you have 4 cylinders, 4 mixture screws, and 4 carb venturies, how the heck do you synch all 4 with only 2 vacuum holes???????

I should have made myself more clear. Sory guys. When I say synch, I mean with the idle air screws. The butterfly thing is quite obvious, you have no choice with those. Sory for the confusion.

I think we need a refresher in terminology. "Sync" will adjust the carbs so they open the same amount at the same time. It is a mechanical adjustment between the carbs.
The "idle air screws" don't do anything to affect "sync". They do have an affect on how well the engine runs at lower rpm, but they don't affect "sync".

With the 2-barrel carbs, both barrels should have the same amount of vacuum, so you only need one port to measure it. There might be a very small difference between the two barrels, but that would be due to a difference in how the cylinders are working, most likely due to a difference in valve adjustment.

I am currently trying to track down a horrible metallic "clacking" noise in mine. Its not there upon cold start butonce I ride away it starts within a mile and comes and goes. If it happens at a stop sign I rev it quick and it goes away (sounds like a really loose rocker, but it doesn't always match engine rpm and when you blip it, it buzzes before it stops). I am at a loss at the moment figuring it out.

Kenny, your problems sounds like cam end float. Try this. When you hear the sound, don't blip the throttle, just slowly increase engine speed to about 1500, see if it goes away. If it does, it's likely cam end float. The thrust bushings that keep the cam centered have worn a bit, allowing the cam to move sideways. Increasing engine speed increases tension on the cam chain, pulling back to the middle, eliminating the noise. Although there are machining methods to eliminate the problem, most of us just live with it. There are apparently no problems caused by this, except for the noise.

.

[don]

Thanks Steve. That was the answer I think I was looking for. At least it makes the most sence to me. I just remember my Dad's cousun tweaking in the carbs on his rail dragster back in the 60's & 70's. He always used the vacuum tubes to dial in the mixture screws a bit tighter (all adjusted the same) than just by the plugs and his ears. I have diald in lots of carbs with the, turn the screw, hit the throttle to clear any fuel " settling " in the bores, listen and smell that one exhaust tube, wach the tach, then start all ovee till it's right. It works well.

Anyway, thanks for the help on this one guys.

Happy riding.