HOW TO: Bench-synch your carbs

Just used this to great write-up to re-sync my carbs after jacking up the sync screws the first time I tried to tune it with vacuum gauges. I'm getting really good at pulling off my carbs tweaking them and putting them back on the bike.

 

My Haynes Manual doesn't even show my XJ750MK as an available model, except for some small side bar paragraphs. All I have found is... it's a fantastic piece of craftmanship, it's just a Bitch to tune, only worth the effort or expense to buy the correct tuning equipment, or have it done "professionally"...if you can afford it! Alas! YOU CAN'T AFFORD NOT TO!!!

 

What position should the idle adjustment screw be when I start the sync process?

Totally clockwise? Totally counter-clockwise? Or right in the middle?

 

I read somewhere in this forum to start at 3 turns out from completely in, then after a 10 min ride adjust another half a turn out [This is my last adjustment]

 

Well now, firstly let’s establish the part in question. “Idle adjustment screw” - I take this to be the common idle speed control. So how could this be fully clockwise (ie all the way in)? The throttles would be far to wide open.
Totally counter clockwise - ie doing nothing, would mean no stating position for the “master carb”.
So the answer has to be in between. In fact, it depends if you want to bench synch or on bike synch. On bike synch with vacuum gauges you set the Idle speed to about 1000rpm to start with, keep adjusting it as you go along.
However, if you’re referring to the “idle mixture” screw(s), I would start at 2 and three quarters from soft bottom.

 

Thank you both for your helpful replies. I posted this question a few months back, and I’m proud to report that I have successfully completed my first bench sync in the meantime

I believe I did start about three turns out from completely clockwise.


As far as the terminology goes: I was using the term gamuru used in his first post tutorial



BUT, for the heck of it I looked up the part. On my mikuni BS30 carbs ( part- 31A-14103-00-00) is called a “throttle screw”

(* - All of the above is light hearted banter and I hope it’s taken as such)

Thanks again for the support!

 

I think the idle adjuster and idle mixture screws are the most mixed up of all things mentioned on here, hence my reply.
Good on you for binning the paper clip method of bench synching, lining up the edges of the hole is the way to do it.

 

That's because people don't understand the different aspects of a carb, and therefore (sometimes randomly, it seems) throw words together in an attempt to sound "impressive" to others.


YOU'RE SCREWED:

In three different ways if you don't understand the basics:

There is an IDLE SPEED CONTROL SCREW at the bottom of the carb rack (that's the big one with the castellated head) that controls the idle speed.....basically, turning it in or out opens or closes all 4 butterfly valves equally, increasing or decreasing the amount of air/fuel into the engine, and thus raising or lowering the engine speed (rpm's). In the above post, the big pink arrow is pointing to this IDLE SPEED CONTROL SCREW, and is labelled as an "Idle Adjustment Screw", and, as chris123 points out, is (ridiculously) referred to as a "throttle screw" by Yamaha.......well Yamaha, there's quite a few screws on a carb rack that directly or indirectly influence the "throttle".....but in your defense, it's just a parts manual that lists it as such, and it's just a guide for ordering parts, rather than a guide to adjusting such parts or understanding their purpose. The service manual (both OEM and aftermarket) tend to be a little bit more explanatory as to their purpose........sometimes. Maybe.

Then there is a PILOT FUEL CIRCUIT MIXTURE SCREW that controls the air/fuel mixture ratio thru the pilot circuit....this is located at the very top/front of each carb, and if the mixture screw has never been disturbed previously, will be hidden under a brass "cap" the seals off access to the screw (Yamaha, and the EPA, didn't want owners messing with the air/fuel ratio mixture since it effects emissions). Adjustment of this screw(s) may (will) also affect idle speed (rpm's), but only as a result of richening or leaning out the air/fuel mixture ratio. Adjusting this screw does not directly affect any mechanical parts associated with the butterfly valves (throttle opening), it is only the fuel mixture (air vs. fuel ratio) that may (will) affect engine rpm's.

Finally, we come to the most mis-understood of all screws: the random, late-night, drunken bar hook-up.....whoops, wait, I meant the ENGINE SYNCH SCREWS. Couple of things to take note of here: first, these screws (and there are 3 of them) allow the synchronization.....the equalization....of power output (horsepower, or more technically, torque) from each cylinder and even more important, between each (all) cylinders. You can read a little bit more about this subject at:

https://www.xjbikes.com/forums/threads/82-xj-650-startng-problem-surpise.46358/page-2#post-414262

Second, these spring-loaded screws operate on adjoining pairs of throttle shaft brackets......so the movement of each single screw adjusts the throttle openings (butterfly valves) of two adjacent carbs, moving them in opposite directions from each other (closing one and opening the other) in order to adjust the power output of those two cylinders relative to each other. It takes 3 screws to accomplish this task across the entire engine, thus 1 screw for the carbs #1/2 adjustment, another screw for the #3/4 carbs adjustment, and a final screw for the #2/3 adjustment (this last screw can also be though of as adjusting carbs 1/2 to carbs 3/4, but this is picking nits).

And yes, the entire synch process may (will) affect engine speed (rpm's) because, once again, we are mechanically opening or closing the throttle butterfly valves during this procedure.


So take note: all 3 screws can (will) affect engine rpm's, by via different methods, different mechanical (or not) modes of action, and for different (but all useful and necessary. because of..........reasons) purposes.