HOW TO: Bench-synch your carbs

This might help, but it goes, spring,washer, 'O' ring.

 

thanks.... that link was PERFECT! Amazing amount of detail on some insanely clean carbs. Thanks chacal

 

After performing this procedure as described, my center sync screw is still almost bottomed out. I remember reading in another thread that this should not be the case. My butterflies are adjusted and all seem pretty equal, though, so I'm at least starting off close...I think. I just don't have any experience to draw upon as far as whether or not I might need to be able to tighten that screw further once it's on the bike.

Thanks.

 

The difference between a good bench sync and a running sync should be no more than a half turn of a screw.

It does sound like something is wrong. like carb alignment or the re-installation of a butterfly not fully "square".

 

Look at the position of Gamaru's middle screw in the initial post, though. The picture between instructions #6 and #7 show that screw cranked all the way in, too.

 

I've used Gamuru's technique every time and can tell you
it works great.

Rick....thanks for the pic and advise. I can still remember
reading your carb cleaning write-up a couple of years ago
and the part about the pilot screws getting stuck and needing
drilling out gave me the chills (I was such a newbe).
Thankfully it all went well.

 

To get the details right, ... I had to take a carb apart while I was writing the piece.

Family and visitors using the computer for what-not were shocked to find carb parts all-over the computer desk-top.

 

Is this only for xj650 or can this be done on a xj750??

 

i was lost after the main post.

 

alright so this sounds like a stellar write up! Im not sure if my browser is blocking pics or are they are no longer hosted wherever they were originally hosted? All I see is an icon labeled IMG...would love to see the pics....anyways. Is this as accurate as using a manometer or the "2 bottle system"?

 

A bench synch just gets you close (sometimes spot on if all the cylinders and valves have equal wear). You will still need to do a running synch with your preferred vacuum comparison device.

 

the pics were lost when the forum made a recent migration to new software and the op hasn't been around in a couple years.

FU

 

Thank the gods almighty

The pics have returned to us

 

awesome! Great pics too!

 

I was cleaning and adjusting the carbs of my 1982 XJ 750 today and was also fiddling around with paperclips like described in the How-to. After a few minutes I just tossed them away out of misery and I think I've found a much quicker and way more accurate way to calibrate the carbs.

I adjusted carb 3 with the idler screw so that the top of the valve aligned with the (correct me if i'm wrong) canal for the idle gas you see in the top of your carb when you look into it from the engine side. I took a screw driver and adjusted the rest of the carbs in the same way with the synch screws and I was done in under a minute.
Mounted the carbs back on and oh boy what a difference!

 

That is method #3. There is a thread around here somewhere detailing that method. It's the one I use.

 

I just did this very thing. Quick and easy. No fussing with paper clips or drill bits.

 

I missed this comment. Throttle linkage was bent. I bent it back to vertical, or close. The idle speed 1000 after closing the choke. at operating temp, it races up to 2000 RPM. Is there an easy fix?

 

An easy fix? Yes, do all your settings again - carb synch, mixture, idle speed, carb sync, mixture. Until you get it right.
If you can’t get it right (assuming valve clearances correct?), then strip the carbs to find the issue. Then do the above again.
So potentially not so easy a fix, which is why most posts on here concern carb issues. You have to be methodical and somewhat clinical. Any one failure can cause an issue, if you have two or more it becomes impossible to diagnose.
You have to become a carb expert.

 

P.O. had all work done by shops. That had me taking for granted most adjustments were reasonably done correctly. WRONG!!! I had to dis regard the conventional means of bench synching and use the visual method to uncover the holes evenly. Then use the idle knob so they were all were half open. Reinstalling carb bank today and test. Possibly the last good day of the year before we get the 1st snow. I'm not rushing to beat the weather, just don't want to wait till spring to button this up.

 

P.O. had all work done by shops. That had me taking for granted most adjustments were reasonably done correctly. WRONG!!! I had to dis regard the conventional means of bench synching and use the visual method to uncover the holes evenly. Then use the idle knob so they were all were half open. Reinstalling carb bank today and test. Possibly the last good day of the year before we get the 1st snow. I'm not rushing to beat the weather, just don't want to wait till spring to button this up. Thank you.

 

I don't know about conventional, but I've only ever done a bench synch by lining up the hole edges. And my colourtune went in the bin years ago, long before I bought the first Yamaha.

 

Regarding "conventional", I'm only referring to the start of this thread. IE: Paperclips, strips of business cards and such. This Bench synch was nearly spot on! Only had to back off the idle knob 1/4 turn to get 1000 RPM, and it stayed there after riding up to operating temp. Noticeable horsepower and torque increase! Mixture screws are set at 2 and 3/4 turns out from bottom. It's a bit rich. Going to give it more air, by turning out the mixture screws 3 full turns. I have a Colortune but it's not newbie friendly. Doesn't seem very precise I guess. I'll go by smell, sound and feel. I gotta trust my instincts more. This is my 1st attempt at tuning a motorcycle. She's a real beauty. Totally worth the effort. Thanks for replying. This site has some pretty old forums but it has helped a lot!

 

Wouldn't you be better off turning the screws in a bit to make less rich?

 

Yes, in is weaker.....
Only do it when the engine is good and hot. If it speeds up chances are it was rich, keep going, all equal amounts, untill is starts to slow down. Then back out until it speeds back up.
If you have a good ear you can do each one individually, but you can't do running synch by ear, you have to have a gauge/gauges.

 

Thanks. Not knowing how fuel travels through the carb bank, I thought turning the mixture screws IN = Less air. So, please clarify:...IN = rich and OUT = lean. Correct?

 

In = lean, out = rich. As you turn the screw in it plugs the tiny fuel hole more and more, therefore making a more lean mixture.

 

Sorry Huntchuks, that's what I meant to type and that's what I did. 1/4 turn IN equally. It's only 40 degrees so not sure how "good and hot" it got, but the brief ride after adjustments smelled less rich, idle is on point and, compared to this summer, It is scary fast and smooth running. Come spring, I'll take it in to the shop for checking vales and pro carb synched. I had a friend who works at the local Yamaha dealer help me do it at home with his own garage made stuff. Hard to believe it will have even more to give after that! Thanks again.

 

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.