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Discussion Starter · #1 ·
I know I'm going to get "it" handed to me on this one, but I've watched and listened to two different videos now that show how to sync carbs.
At the end of the demonstration, they say "That's how it's done" and roll the credits.

Before you can successfully balance your carburetors, EVERYTHING else must be correct. Ignition, compression, etc.

Everyone wants to treat this like it's a procedure. But it's not. It's a process. You don't go 1,2,3 close enough. 1,2,3, all done.
When you sync your carbs, you are opening the throttle plates to match a vacuum reading on a "sister" carb.
As it pertains to the 1200...........
When you adjust the left side, you're changing the relationship of the #2 carb throttle plate to match #4 vacuum reading..
When you adjust the right side, you're changing the relationship of the #1 carb throttle plate to match #3 vacuum.
When you adjust right to left, you are adjusting #3 throttle plate to match the vacuum of #2/4.
Since the carbs are mechanically linked, when you adjust #3, #1 has to follow. They're linked.
Here it comes.........
When you adjust the base idle screw, you are adjusting the throttle plate for #4 carb. Look at your linkage.
Since the carbs are mechanically linked, when you adjust #4, numbers 2, 3, and 1 have to follow.
The number four carb throttle plates are just as adjustable as #1,#2, and #3. That's why most people in the trade call #4 the BASE carburetor, not the "non adjustable" carb. Because by adjusting the throttle plate on #4, you're adjusting all the rest.
Where the videos fall short, is although they have adjusted the carbs for idle, they haven't inspected carburetor "function".
I know a lot of people think you raise the rpm to 3,000 and let it off to settle the linkage. Some, more correctly, say you rev it to clear the cylinders. But what you are suppose to be doing, is comparing the carbs that are balanced at idle, to make sure they are still balanced at cruise.
Our carbs have an idle circuit, and a high speed circuit (simplified). By raising the rpm's to three grand, you ensure that you are out of the idle portion, and the slides are now pulling fuel on the mains. This is important. The carbs should still be in sync at 3,000 rpm. If not, you have a problem.
You could have a partially plugged idle circuit that required opening the throttle plate a little wider to achieve the same vacuum as the rest, or you could have slides that are sticking, or diaphragms that are leaking. Whatever the reason, it needs to be addressed.
A lot of guys sync their carbs at 3,000, because "that's where I do all my cruising", only to put up with a lumpy idle. You should have a smooth idle, and a balanced top end.
Get out your sync tools and "mess around" with them. It can be a very valuable "diagnostic" tool as well.
 

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I spent over 5 hours using the sync tool today. Even if my motor has a lot of miles and some issues. I'm sure the carbs need a rebuild the sync helped a lot. The bike off idle pulls smoother and stronger, at 3000rpm all the vibration is all.It idles smoother, So I agree to spend a lot of time with this tool but I also say it can help even less then prefect motors run better.
 

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glhonda, Don't know why you would get blasted over anything you said there.

As you have observed and stated those 15 minute videos leave a lot out with creative editing. Mostly time....

a Carb Sync tool can and will tell you a lot if you know what is "normal" for your machine or the machine type in general.

One thing the gets me and this may get the thread blasted. general advertising says you can do a thorough job with 2 gauges on a 4 or 6 cyl engine. Well that is partially true. You can fairly well sync the multiple carbs with just two gauges. However, when you are going into diagnostics or observing the overall function, with two gauges you are only seeing two parts of the equation. and since this is a mechanical operation every time you open and close the throttle you have the potential of getting different readings.

Funny it has been that way for as long as I can remember even back with my old merc sticks in glass tubes. Plain and simple a good carb sync requires patience...

If you tear the carbs off the bike, then plan on bench setting the plates, idle screws and using like a vacuum cleaner to at least check the operation before you put it back on and hit the button. It will help your patience factor a lot.

Good Write-up!
The is my .02 and of course ymmv
 

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To your point, I synced the carbs yesterday and took it for a short ride. All was good, this morning it was colder out then yesterday. Bike was rough starting idled like a harley. Once warm it wouldnt hold idle it would just keep dropping if I didnt rev it. But from a stop it has a lot more power and pulls strong. When I opened the trottle in 3 rd gear and rev'd it through 4th. The power was stronger and more linaear then it was before. The motor while cruising or accelerating is much less noisey. It no longer sounds like a VW motor. While at idle it sounds like a harley?
So to you point this is a process :)
 

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Maybe reading the vacuum at 3k rpm can be used as a diagnostic tool, but trying to adjust the synch screws at anything other than idle speed is futile. Carb synch can improve smooth idle...and I'll buy it may help off idle, allow rpms to drop cleanly when you 'blip' the throttle, and might help eliminate some decel backfiring if they're way off. But once the butterflies are open any significant amount, with the much larger quantity of air, carb synch doesn't matter that much.
 

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Discussion Starter · #6 ·
Maybe reading the vacuum at 3k rpm can be used as a diagnostic tool, but trying to adjust the synch screws at anything other than idle speed is futile.
That was one of the points I was trying to make. Carb sync at 3,000 is feel good.
But if you're balancing your carbs, you've got the sync tool all hooked up, it only makes sense to check for proper operation at higher speeds.
If it's good at idle, and it's good at 3,000 rpm, ....it's good.
 

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Nice post Mike!!!


But then again..... I expect no less from you!!!!!!!!!! :grin: :coollep: :coollep:
 

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For those of you who are wondering, Don is on the payroll...:ROFL:
So he gets $0.001 every time he agrees with you?

If I got on the payroll I could have like a penny every two or three years?:claps:
 
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