Bench syncing carbs baseline question

wrote:

 I'm not sure about the smoothbore VM's though.   

No! Non of the VMss or smoothbores have a register carb and pretty much all slide carbs are the same.
It's mostly CV's that you will find it on and usually the one directly attached to the throttle cable reel.

I am not trying to start anything, but it has always been my understanding that the early vm28sc carbs which have external sync did have the #4 carb screw blocked off and one was supposed to set the other 3 to it.  Am I wrong in my understanding?  

This quote from wiredgeorge site "1973-74 Carburetor Four Fixed Synchronizer" 

 " As mentioned previously, the VM28SC assembly uses external syncrhonizing mechanisms.  The 1973-74 versions used number four synchronizer in a fixed positioni by capping it with a hex fitting so it would become stationary and the other three carburetors would need to be syncrhonized relative to the number four carburetor.  In 1975, this changed and the number four carburetor was given a normal synchronizing mechanism." 

wrote:

I am not trying to start anything, but it has always been my understanding that the early vm28sc carbs which have external sync did have the #4 carb screw blocked off and one was supposed to set the other 3 to it.  Am I wrong in my understanding?  


 

Yes! That's correct for the very early VM sc carbs which is why i said "pretty much" and didn't make that statement absolute.
Obviously as with anything those are one of the exceptions but they aren't the general rule.
 

My Mikuni VM 33's have a "lead carb."
Steve

All 4 adjustable. I have spare set from 79 with pump and those too adjustable all four 

Another stupid question. What are 3 outlets on carb tops? I know one upfront on left is for vacuum petcock. What other 2 for? Some kind of vents? Where do they go?

wrote:

All 4 adjustable. I have spare set from 79 with pump and those too adjustable all four 

Another stupid question. What are 3 outlets on carb tops? I know one upfront on left is for vacuum petcock. What other 2 for? Some kind of vents? Where do they go?

Outlets on 2 and 4 are float bowl vents to atmosphere and must not be capped off . They should have at least a short hose attached or on stock bikes hoses that run over the air box or thereabouts.
The one on 3 (not present on every set if not needed ) is the vac port for a vacuum operated fuel tap.
If a manual tap is used then it must be capped off otherwise number 3 will run lean.

I know it's a little bit of an old topic, With older (worn) carbs (and an engine with wear on it).  the sync at idle doesn't mean the slides are even.  Going back a couple of years, I vacuum synced a couple of my Z1 carb racks with gauges - got them perfect.  When I looked at the gaps under the slides, they were all over the place - which means you get uneven fuel delivery though the needle/needle jet. at 3,000-5,000 RPM - where you spend a decent amount of your time riding  What this really highlights is the wear on the carbs (or on the engine)  If a cylinder has slightly lower compression, it's not going to suck as much air/fuel through the carbs... plus, you should always check/adjust valve clearances before syncing carbs.

I want my carbs synced for smooth running while I'm riding and will put up with a little bit of a rough idle.

By all means use the sync gauges, but visually inspect the slide heights - I think some of you will be surprised by how far off the slides are from 'synced'.


The other thing with slides, make sure at wide open throttle, all the slides clear the bore.  When you mess with the individual carb adjustments, you can stop slides from fully clearing the bore..

Assuming sliders are all free moving and settle at different heights when balanced at low revs, is this not setting a fixed relation to each cylinder, taking up the variances in the engine, to compensate for unequal vacuum across the cylinders?If so, is it fair to assume the slide relationship would be the same/similar across most of the rev range? - I’m not too sure on this.In terms of performance, at higher revs different carb circuits come into play and I would assume these circuits are worth tuning if poor performance.