Hooked up the Carbtune to do a carb sync today.

Set the idle to 2000rpm and balanced all the carbs fine.

But just to check things I reved it to 3,000 and it was all over the place. The same happened when I dropped the revs down to 1000.

Am I doing something wrong. or is that just the way it is?

 

I just synced my cars two days ago.
What I do is make sure that the RPM is around 1,000 to 1,200. Once adjusted close I drop down to 1,000 to 800 RPM.

To balance properly, since you are checking the vacuum, you need to create the most vacuum. With the butterflies open you can't get good vacuum, so you need to close them down as much as possible without rough or uneven idle.

Yeah! Cracking the throttle will get them reading funny. The important thing is to get them in at lower engine speed. Over the highway you have throttle on and things smooth out because one cylinder is not pulling harder that the others or one is not being dragged by the others either.

Get them in at idle and see how she runs when you go for a Fosters.

 

+1 to what TBT said. Your idle was set to high. Adjust it at the lower idle and all will be well.

 

This is wot confuses me.

I did a lot of reading on the net before doing this.

Some mechanics say the carbs should be adjusted at Idle.

Some say it should be set just above idle at ~2000rpm to reduce fluctuations.

Some mechanics even say you should sync the carbs at 1/4 to 2/3 od peak torque!

Sometimes I wish for a EFI conversion.

Also who was the f**king genius at Honda/Keihin who decided the tops of the carbies was too convenient a place to have the Vacuum plugs and that they should stick them underneath the carbs for the P models onwards???

Not that I have one, but even with the Honda special tool I fail to see how you can adjust the mixtures while the bike is running.

 

The Honda Common Service manual states that syncronization is to be done at idle speed with the engine warm, transmission in neutral and bike on the center stand. It's possible that you see variations in vacuum readings between cylinders at higher rpm because of differences in intake run from the air box to carb, differences in cylinder conditions, or differences in exhaust runs. On mine I saw lower vacuum readings on cylinders 1 & 4 than 2 & 3 at 3,000 rpm after syncronization (or was it the other way around?) before doing a valve job. Didn't think to check it at higher rpm when synchronizing after the valve job, so don't know if that made a difference.

Note also that carb synchronization is the final step in a tune up. Plugs, valve settings, carb setup, etc. all need to be done first to insure each cylinder is running at peak level.