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.