HOW TO: Bench-synch your carbs

This is a "How-To" on bench-synching carbs. I've noticed quite a few new members (me being one of them) searching for clear instructions on how to do this. At some point recently I came upon them, but I can't seem to find where the forum post has gone. So, I decided to add my own How-To to the Forums with the hope that future XJers will find this guide an asset.

The carbs we'll be working on today are a "junk" set that came on my wife's '82 XJ650J Maxim. They may look okay on the outside, but they've got cancer on the inside. We'll need two Paperclips and a Phillips-Headed Screwdriver for this tutorial. Let's get started.

Here are the steps...
1. Turn all the Synchronization Screws clockwise until they bottom out on their springs.



2. Take two Paperclips of the same gauge and bend them to match the illustration.



3. Rock the Throttle Lever open with your right hand and slide the Paperclip (the one with the short bend) between the Venturi and the Butterfly of Carburetor No. 3 (see first picture above for Carburetor positions) with your left hand. Gently let loose of the Throttle Lever so that it pinches the Paperclip. You will want the Paperclip positioned as close as possible to bottom dead center of the Venturi.

Note: if the Butterfly does not pinch the Paperclip securely, you will need to turn the Idle Adjustment Screw counterclockwise until it does. Refer to Step 9 for the location of the Idle Adjustment Screw.



4. With your Phillips-Headed Screwdriver, turn the No. 4 Synchronizing Screw counter-clockwise while sliding the other Paperclip in and out between the Venturi and the Butterfly. Do this until the Paperclip is "grabbed" by the Butterfly. This Butterfly should now match the Butterfly of Carburetor No. 3.



5. Moving to the No. 2 Synchronizing Screw, repeat Step 4 on Carburetor No. 2's Butterfly.



6. Lastly, adjust the last Synchronizing Screw (No. 1) so that Carburetor No. 1's Butterfly matches Carburetor No. 3's Butterfly.



7. Once you're satisfied that all four Butterflies are set approximately the same, hold them up to a light source and compare with your eye. Adjust as needed.



8. Again, with your right hand, rock the Throttle Lever open and remove the Paperclip from Carburetor No. 3.
9. From the backside of the Carburetor Set, turn the Idle Adjustment Screw so that a sliver of light can be seen between the Venturi and the Butterfly of Carburetor No. 3. If you've set everything up correctly, the same amount of light should be visible looking through the other three Carburetors.





That's it! It's time to re-install your carbs and finish fine-tuning... But that's a whole other "How-To".

Ed. Note: I've decided to rename this post to avoid confusing this procedure with the actual procedure of synching carbs while they're installed on a bike.

 

Re: HOW TO: Synch you carbs

I would like to add a personal note.

These "Show and Tell" Posts are GREAT!
I need to learn how to resize a photograph and Post it; too!

With regard to Bench Syncing:

I have had great success using 1/4-Inch wide strips of Business Card.
The Card bends to get a good "Feel" under the Butterfly.
On Carbs I Bench Sync and send back to Mail Order Clients ... I leave the Card Sections IN so that they can verify the Fly's are Mechanically in Sync.

 

Re: HOW TO: Synch you carbs

Rick, Thanks for the compliment. BTW, that elusive post on synching carbs... I think it was written by you. I'll be damned if I can't find it, though!

 

Great article!

Wish we had a place where these articles were listed in alphabetical order so I could go in and look up Carb-Sync-onbike/offbike.

Or Brakes- Front- Pad replacement

1st by classification like Engine, or Brakes, Transmission, Clutch...ETC.

I am no good at this, I just know how people like me tend to look stuff up.

Search engines while thorough are a pain in the butt!

 

Great job Gamuru.

A few edit suggestions though:

1) in step 3 make it clear that that 1st paper clip goes in carb 3.

2) explain for the noobies which carb is #3.

3) in step 3 might add something about the idle screw if #3 doesn't close on the paper clip.

I wonder also if it's ever possible when bottoming all the sync screws as you describe in step 1 to get another butterfly holding #3 open too far.

 

Hvnbnd, I was thinking the same thing. On the computers at work, we look up parts for customers in two stages. First by general classification, then more specific. For example you'll have:
1. IGNITION & ENGINE FILTERS
2. BELTS & COOLING
3. FUEL & EMISSION
4. (ETC... You get the point)

Let's take the category IGNITION & ENGINE FILTERS. Under this category, you'll find sub-categories:
1. ENGINE FILTERS & PCV
2. SPARK PLUGS
3. TUNE-UP IGNITION
4. COILS, MODULES & OTHER IGNITION
5. IGNITION WIRES
6. SPECIFICATIONS
If you're looking for spark plugs, it's pretty clear where you'd go.

Here's my thought on this. Why couldn't there by made a similar structure here that you could look up repair procedures on? Let's say you wanted to find out how to replace you rear brake shoes. You'd look up BRAKES & WHEEL BEARINGS ---> REAR BRAKE PADS & SHOES, ROTORS & DRUMS. Obviously, we'd edit the listing to match our needs, but you get the point.

MiCarl, I'll make those changes you've suggested. Thanks for the feedback!

 

if you look at the top of the bore where the butterfly's go in, you see a small hole that gets closed off when the butterfly closes all the way
use the hole as a reference, since thats what your adjusting
idle speed screw to open them all up then balance screws to make them all even and just uncover the little hole, then idle speed screw to cover half the hole
take off a set of balanced carbs and thats about what you'll find

 

I did about 10 years + of my life as a partsman as well.

 

Sorry to hear that! :wink:

 

If I were grading the presentation for a Course that has a Grade of Pass / Fail what do you suppose I'd have to do?

I have already written to the Author about the one Glaring Error and I'm surprised that nobody else has had anything to say about it.

The #-3 Carb does not have and adjustment other than the Idle Rod. The Sync Screws are on: 1, 2, & 4.

Once the text is edited it PASSES.
A most worthy addition to the other resources we have, here, to make this Site Number One.

 

Contribution to the Presentation:

Immediately after Bench Syncing is done, secure the Carb Linkage Throttle Cable Attachment to the Enrichment System Operating Rod with some string or a small Plastic Tie Wrap.

If the Throttle Cable Attachment is loose and gets caught up under the Cylinder Head when the Carbs are reinstalled, the Throttles will be forced open and the Engine will race as a result.

Do not try to pry or force the Throttle Linkage Attachment out from under the Head if you have made this mistake.
The Attachment Point will break-off or the Linkages will become bent.

 

I must admit that I pondered whether to call this No. 3 synchronization screw or No. 4. synchronization screw. I'm looking at my Yamaha Factory Service Manual right now and while it labels the carbs 1 through 4, it doesn't actually number the screws. In the text, it references the "No. 1 synchronizing screw" and "adjusting No. 2 screw", but doesn't actually refer to the last screw by number. It only states to "synchronize carburetor No. 4 to carburetor No. 3." So, my quandary is, what do we call it? No. 3 since there are only 3 synching screws? Or No. 4 since it is clearly attached to the throttle shaft of No. 4.

I'm going to say that, on further review of the text, I've erred. A diagram in the back of the book shows the carbs with circles around the synching screws. Arrows from these clearly point to the carb numbers. I'll fix my mistake.

 

Take any prize off the Second shelf.

 

I'm not so sure... I think we need a government sponsored study to determine if that screw should be designated as number 3 or 4.

Perhaps we should call it 3.5 so that we do not alienate either screw or be seen as to be showing a preference for odd or even screws. :roll:

Nice job Gamuru this should be a sticky.

 

kontiki, if I didn't know any better, I'd say you had been reading my blog (WARNING: political content).

 

Ahem; since my education is in Computer Science and Mathematics I feel compelled to inform you that 0 is the first non-negative integer. Hence, screws should be labled 0, 1, 2.

It's time to end the underrepresentation of the #0. It is, after all, 10% of the digits.

Oh, in case you wondered, 0 is an even number.

 

Actually, zero is not "really" a number, it's like the "color" black, it represents the absence of a quantitative value. Zero is part of a "counting system", though, and serves as a place-holder or a reference point for "real" numbers.

Western number systems didn't even recognize the number "zero" until sometime around the 12th century....invented by those pesky Persians.

Calender systems in the western world don't recognize the year zero (it went from 1BC to 1AD), and thus each "century" (i.e. a hundred-year interval of time) actually starts with the first year of the millenium, e.g. the FIRST year of the 21st CENTURY is really 2001, not 2000.......

Heavy stuff, this is.

Number are hard animals to tame.

 

COULD YOU CALL EM A,B,C?

 

That'd be too easy.........

 

The Factory Manual lists them as 1, 2, and 4.

 

MiCarl:

Code:

Option Base 1

Does that fix it for ya? :wink:

For the rest of you scratching your head (WARNING: Geek Content), Option Base 1 is a computer command which is defined as being: "Used at module level to declare the default lower bound for array subscripts." The numeral 1 indicates that arrays start at 1 instead of their default base of zero.

 

I'm not sure that is available in FORTRAN :wink:

 

MiCarl: No, but arrays in Fortran start at base 1 by default unless otherwise defined.

Who knew: carb talk and a little computer science all in the same discussion. :grin:

 

Those kind of things tend to happen here.

 

Does this suggest that the predominance of our forum tend to be geeks?

 

Get out and ride ya nerds.

 

I just wanted to say Awesome Walkthru Gamuru. I followed these directions this weekend on my XJ650 and It runs so much better.
Just gotta get my hands on a yics stick now.

Thank You

 

Hey Loki, here they are!:


vv7) Reproduction YICS Port Blanking "Block-Off" Tool---this is the tool you'll need to perform a proper carb synchronization on any YICS motor......without it, you're just wasting time. This block-off tool is used to isolate each cylinder from the others while perfoming a carb synch. Updated version of the factory #90890-04068-00 tool. Some people claim that this tool is NOT needed to perform a proper synch; we disagree, and so do the Yamaha engineers who designed the YICS system. Quality aluminum tool features a snap-open and shut trunnion handle design like the original. This is NOT a cheap home-made tool and has all the proper seals, dimensions, and high-temp insulators as an original factory tool. IN STOCK!

HCP93 reproduction YICS port blanking tool, each:
$ 37.00

 

Hey Chacal

When is your website going up? When it does are you going to have pics? Man, sometimes when you get going on techinal stuff my head starts to spin. I don't know what everything is called so pics might not be a bad idea. Of course that is if you have space.

Wm

 

Eventually, I promise!!!

 

If I bench sync that means I don't have to syncronize my carbs with a syncronyzer once they are installed it?

 

Bench sync puts them in the ball park to get the bike running, then you must sync with vac' gauge/s.

 

I already did the sync with vac gauge/s but I wonder if I should bech Sync and then Vac again, but my grandpa' would say "if it works don't fix it".

 

You're good with the vac sync. Bench sync is to get it at a good running position so you CAN vac sync

 

SLKid- Got it. Thanks for your reply!!

 

Thanks for the explaination on bench sync, I finally get it... I bought a 85 Xj700 Maxim X a couple monthes ago. I've got what I think to be carb problems, Just trying to understand them a little better before I start asking questions. The knowledge you guys have about these bikes is amazing...

 

Bringing this back from the dead.... If this isn't a sticky it really needs to be.

How can I get the carbs in the right ballpark to vac synch them if they are already on the bike? I'm feeling too lazy to take them back off right now. If I can't find a way to get them good enough to get the bike to idle, so I can synch them I GUESS I'll pull them tomorrow morning

 

You dont have to pull the Carbs.

You can SKIP Bench Syncing and just do VACUUM Syncing.

 

What if I can't get it to idle though? That's the dilema. I HAD it idling yesterday, but now can't seem to get it to. Can't mess with it today, because the wife is home, but I will pull it out of the shed first thing in the morning.

 

crank the idle knob till it runs then as it comes into sync keep backing off the idle knob till you get it down to 1K
keep a fan on it so it doesn't over heat

 

thanks a lot.... Also, the mixture screws still have the caps on all of them except for #3. Do I need to drill them out to adjust them or could I be alright without adjusting? I figure they've been as is since 1983 and I'm not changing the exhaust or going with pod air filters.....sooooooo....... ????

 

you wont be able to tune the motor unless you can get to the pilot mixture screws & I would be at least fitting new 'O' rings.

 

New O rings where? Sorry if that's a stupid question.

 

On the pilot mixture screws, idealy change the screws, springs, washsers & '0' rings, complete.

 

Does all of that come in a rebuild kit? I need to just suck it up and order one.

 

Yes.
Funny you should ask!!!

The supply the New O-ring ... AND, And, and ... 4 New Blind Plugs.

You are going to HAVE to Drill-out the Original ones.
You start a small hole in the middle of the Plug
Drill-out just enough material to get a self-tapping screw some bite.

Start the screw into the drilled hole
Grab the screw with Vice Grips
Knock the Vice-grips pulling the Plug out of its hole.

CAUTION
-and-
WARNING

You MUST exercise caution to make the drilled hole deep enough for a screw to take hold ... OR ... Allow the Drill to go completely through the Plug ... WITHOUT ALLOWING the Drill to come into contact with the HEAD of the PILOT MIXTURE SCREW which is located VERY CLOSE to the Plug.

The Drill will DAMAGE the Top of the Screw and make Extraction and Tuning very difficult.

 

Awesome Rick. Thanks for the info. I wanted to call you today, but didn't get around to the bike. I got caught up with yard work and my afternoon nap took the place of working on the bike Now I work 12 hour days the next two days and will be doing family stuff all weekend, so might not touch the bike until next Wednesday. I guess that's perfect, because then I could get all the stuff mailed that I'm going to need. I emailed Chacal, but he just gave me a list a mile long of parts. There were just too many options. I just want to say..... please give me a rebuild kit and someone that knows better than I do what I'll need will put it together in a nice pretty package and send it my way

 

Carb Rebuilding FAQ:


Question #2: What Parts Will I Need?:

The parts you'll need depends on what is there, what is missing, what is worn out, what gets broken or damaged during the dis-assembly process, etc., but the "basic items" needed for the rebuild process (besides any special tools and tuning equipment) is as follows:

- throttle shaft seals (you'll have to split the carbs from the rack to replace these).

- fuel supply tube o-rings (you'll have to split the carbs from the rack to replace these) on the models that use them (some Mikuni carbs do not use these).

- float valve needle and seat rebuild kit (seat, needle, clip, seat washer. The float valve seat filter screens are also available separately). Mikuni owners will also want to replace the float valve seat o-rings.

- float bowl gaskets.

- idle mixture washers & o-rings for sure, and the idle mixture screw springs and screws if they are damaged.

- jets, if damaged or incorrectly sized.

- carb bowl fuel drain screws, if damaged.

- new replacement carb hardware (screws, etc.)---now is the time to upgrade to stainless steel and/or allen-head fasteners if so desired!

 

does anyone have a pic of the idle mixture washers and o-rings torn down? I mean....to show where they go and skool me on what the hell I need to do if I want to replace them.

I got a rebuild kit, but it only contained the float valve seat, clip, washer, and float bowl gaskets. I didn't even bother replacing them, because my stuff looked good.

I don't want to split the carbs from the rack, so I don't want to replace the other stuff above unless I absolutely have to.

I might pick up a jet kit to replace all the jets though.

For now I'm leaving them as is and I just bench synghed them and I'll see how it goes tomorrow when I reinstall them and try to get the bastard running. I'll most likely be back on here asking for help and parts from chacal

 

Good pix of the idle mixture/pilot circuit screws on this thread:

http://www.xjbikes.com/Forums/viewtopic/t=3690.html