Topic: Okey, back to the synch job

[Lars]

This is my first CB 750, it is a K4 model. Only 10 500 miles. I have done my lesson, timing gap, timing, valves, cam chain. New spark plugs, oil, oil filter and air filter.  Carbs totally down and cleaned up and back on bike. Air screws are all 1 turn out. New 4-4 mufflers. She fires right up and goes pretty well.

Now, today I started the synch job with my Morgan Carbtune. Warm engine, idle at 1200 rpm. After quite  bit of struggling, all 4 carbs settle pretty well at about reading 12 on the meter. I can't make them close to 20-22 without it starts to rev high. Idle screw is only sligthly touching the nob and about 1-1/2 turn from touching point.

Done this several times on my CB 550's, but this one is drilling me. Someone please guide me here?

Lars

[sbparks]

Hi Lars,

 I just performed the carb sync on my 750 K4.  I used the Motion Pro sync tool to do the job.

 I am not sure exatcly what you are having the difficulty with.  Maybe you could elaborate a little more.

 For the Motion Pro, I had to calibrate the unit first... they provided a 'multi tap' so that I can level each of my meter tubes to a single carb.  There are screws on the unit to perform this function.  Once I am calibrated, I take each individual tube and place on each carb.

 Now I started synching the carbs to each other.  I decided to use carb #2 as the 'reference' carb.  As I made adjustments to an individual carb, I would see changes across to the others, so I went across a few times to get them right.

 In the end they were much closer, but not perfect.  I will probably attempt this excersize again.  Here's a shot of my synch setup.

Scott

[Kframe]

If they're all reading similar values (12's) then you are sync'd.
The actually number doesn't matter, just that they all pull the same vacuum.
-K

[Lars]

Nice bike you got there, Scott!

I think my bike had fun with me. After typing my previous post, I went back to go over it one more time. Fired it up, and now the reading was better, after a few minor adjustments it settled down reading 18 on the meter. Locked the nuts up and tank back on

Last issue is the front brake, have to be ready during Sunday as I have inspection time on Monday.

Thanks for the respons, guys1

[Gordon]

For future synch jobs do yourself a favor and put a strip of tape over the numbers.  The actual reading doesn't matter.  All you need to do is set them so they're all the same.  Forget about the numbers. 

[Lars]

Yes, I know Gordon, but why does the Honda Shop Manual say i.e. 20-22? What was in on a bike, when new? I ended up at 18 and it runs nice

[Gordon]

Yes, I know Gordon, but why does the Honda Shop Manual say i.e. 20-22? What was in on a bike, when new? I ended up at 18 and it runs nice

That's the reading for the factory worker or honda shop tech to aim for on a bone stock, brand new engine.  It's a base line so they're all set the same.  It doesn't really apply to the average home mechanic working on his own bike because there are multiple factors that will affect the total vacuum. 

[crazypj]

Yes, I know Gordon, but why does the Honda Shop Manual say i.e. 20-22? What was in on a bike, when new? I ended up at 18 and it runs nice

 Pretty sure you need genuine Honda gauges to get their readings, they are calibrated from factory slightly different for middle cylinders (I know because I've had a set for 30+yrs)
 As mentioned though, the numbers don't matter, just that they are all level.
 If your cam timing is out or you have 'race' cams it will also change vacuum readings
Modern sports 600's only run 10~12 at idle