81 XJ650 - High Idle When Synched

Hello everyone. I noticed some issues that I thought were due to poor synching, but I ran into a problem when trying to synch the bike. Went for a ride, got the bike nice and warm, and sure enough, the #2 carb was out of whack when I connected the Carbtune. I synched #1 and #2, but then when I synched 1-2 with 3-4, the idle went up to around 2000 rpm once the pressures were equal. The idle screw was already very low, and I could only back it out a little bit before it was fully out.

So, in order to get the idle down, I had to intentionally raise the vacuum on #3-4. Something is clearly not right. If it helps to troubleshoot, here’s a list of carb work I did over the winter:

• Replaced #2 carb body
• Replaced #2 throttle shaft seals
• Cleaned enrichment passages
• Set fuel levels

Any chance there’s a vacuum leak or something? I assumed having the idle screw all the way out would close all the butterfly valves, so I’m surprised it can even run like that.

 

By "Colortune" are you meaning the gizmo that replaces the spark plug? That is used to set the idle mixture screw that hides under the pressed in cap on the engine side of the carburetor bodies. Adjusting these will affect the idle speed some. To adjust the idle speed and carburetor balance you adjust the screws down between the carb bodies on the throttle shafts using the vacuum ports on the rubber isolators next to engine head. It sounds like you are on the right track with the right gauges and ideas. Using the screws on the throttle shafts should be the answer to your issues. To be complete in your tuning you will be going back and forth between the 2 adjustments. Tedious but not difficult.

 

thats where a bench synch would have come in, so obviously one or more of the butterflies is slightly open.
not so easy now with the carbs on the bike, but you would need to screw the idle screw in some ,then back the butterflies off till they close together,
i know , i hate taking them off but you would be able to see the butterflies

 

he said Carbtune , not Colortune

 

I did bench synch the carbs when I put the rack back together, but I must not have done a good job. What do you mean by “back the butterflies off till they close together?” I assume you mean to adjust the synch screws until they are properly aligned?

 

Yes he did. Oh well, I need to read more carefully...... continue Gentlemen.

 

yes, screw in the idle screw slightly ,so all the butterflies are open slightly, then use the adjuster screws between the carbs to close the butterflies until they are all closed together the same , as in doing a bench synch, then when you start up and do a running synch, you will have in and out adjustment on the idle screw

 

One of the "trick" methods to setting the butterflies slightly open (as bensalf mentioned) on the bench/off the bike is to adjust them to a very slight opening using the corner of a business card or the straightened tip of a small paper clip as a feeler gauge. This allows a consistent adjustment of gap from carb to carb. That will get you in the ballpark, so that there isn't a lot of adjustment to go once you mount the carbs and final-adjust using the Carbtune (vacuum-sync adjustment gets things really evened out).

 

And just for clarification (and hopefully a New Era with a uniformity of language usage)----there are 3 different screws involved in adjusting/tuning the carbs:

1) the pilot circuit mixture screw......this is the one on the (front) engine side of each carb body, with it's straight-slotted-head facing the sky. This is the one that is originally (from the factory) closed off (hidden) by the small disc "plug" which must be removed in order to gain access to the screw. It's used to adjust the fuel mixture for the pilot fuel circuit ..... also called the "idle mixture screw" ....and is what's adjusted when using a Colortune see-thru spark plug. Typically not messed with while performing the synch process, although subsequent adjustments to this screw will affect synch, and for best results----once a mixture screw does get adjusted, then a re-synch of the engine is a recommended step.

Adjusting these screws inward/downward ("tightening" them, i.e. clockwise) restricts (closes) the fuel passageway for the pilot circuit, thus leaning-out the pilot fuel circuit for that carb/cylinder.

2) the idle speed screw. Located at the bottom center of the carb rack, it's has a rather large, castellated head, which faces the airbox, and is used to adjust the idle speed (rpm's) of the engine. The tip of this long screw presses on the #3 throttle shaft bracket and opens (or closes) ALL of the throttle blades simultaneously. Turning this screw "in" (clockwise) opens all of the throttle valves ("butteflies") an equal amount. thus increasing rpm's, and turning the screw out (counter-clockwise) allows all of the butterflies to close an equal amount (the butterfly valves are always "trying" to close themselves fully, as they are under spring pressure to close --- that's the purpose of those circular springs used on each throttle shaft end bracket).

3) the synch screws.......these are the panhead, phillips-drive screws about midway (top-to-bottom) of the carb bodies, on the forward (facing the engine) side, with their heads and drive slots pointing towards the sky. These screws and their associated plunger rods operate on adjacent "sets" of carbs, adjusting the relative openings of the throttle shafts / butterfly valves of each adjacent set of carbs.......thus there are only 3 of these screws per rack: 1 screw for the #1/2 carbs "set", another screw for the #3/4 carbs "set", and the central synch screw (between carbs #2/3) adjusting the openings between the #1/2 "set" of carbs and the #3/4 "set" of carbs.

As their name implies, these are the screws used during the synchronization process to manage the relative (relative to each other) butterfly valve openings. And the synch process calls for first adjusting the #1/2 synch screw (to achieve equal power output ("vacuum") between those 2 carbs), then adjust the #3/4 synch screw (to achieve equal power output ("vacuum") between those 2 carbs), and then adjusting the center synch screw to achieve equal power output between the already-synched #1/2 carbs "set" and the already-synched #3/4 carbs "set"....and you're done ---- now each cylinder is producing the same power output (as measured by the vacuum strength/pull of each cylinder).

AND LEST YOU FORGET: note that when doing a "bench synch" you are attempting to "synchronize" the carbs.....you are setting the butterfly openings to be as equal as possible across all 4 carbs. But when doing the running-engine "vacuum synch", you are attempting to synchronize the the entire ENGINE --- to equalize the power output of each cylinder --- which is why referring to it as a "carb synch" is really an incorrect way of describing what you're doing........the vacuum-synch process actually UN-synchronizes the carb throttle openings from each other, in order to make sure that each cylinder of the engine produces the same power output.

The proper, non-confusing terminology should thus be: an "engine synch via adjustment of the carbs" (a real mouthful, but technical subjects can be like that....) rather than a "carb synch"........"carb synch" is the correct terminology when referring to the initial "bench synch" process, but is absolutely incorrect for the running-engine synchronization procedure.

BTW: if you've had the carbs broken apart from each other during the carb rebuild process, then during the re-assembly process it is noted than when attaching the upper and lower rack bars to the carb bodies, you must have a hard, flat surface (a piece of glass or flat metal plate) under the carb throats (or airbox throats) during the assembly of the rack bars to the carb bodies to ensure that the carb bodies are properly aligned with each other........otherwise the individual bodies will start trying to walk or wiggle their positions in relation to each other during the rack-bar installation process. If this "proper alignment" is not achieved, then the throttle shafts will not be in proper alignment with each other --- they will end up slightly "cocked" in relation to each other, instead of presenting a true and perfect horizontal centerline --- and that makes the proper adjustment of the synch screws impossible, as you''ll get less-than-perfect movement of adjacent butterfly valves while attempting to "synch" each carb set (1/2, 3/4, and then 1/2 to 3/4) as each carb body in a related "set" will get a differing amount of butterfly valve "adjustment".


The devil, as usual, is in them thar' details........

 

This is an extremely thorough write-up. Awesome stuff. So, I do have an update here. I took the carb rack off the bike yesterday and did another bench synch. Straightened paper clips are my tools of choice for setting the throttle opening, since I know they won’t have a ton of give once “grabbed” by the butterflies. Reinstalled the carbs, went for a ride to warm the bike up, then attempted a running synch.

This time, when setting up the carbtune, I zip-tied the vacuum gauge assembly near the back of the bike, so that the vacuum hoses would run as straight as possible, and they would be nearly parallel. That… seemed to make all the difference. Vacuum readings were stable, and the engine smoothed out nicely once I got the gauges to align. The idle screw still has play in both directions, so the idle can be adjusted up or down from where it is.

I suspect that, when I did the synch last time, one or more of the vacuum hoses were kinked or otherwise deformed. So, by “synchronizing” the readings, I was actually throwing them completely out of whack. Lesson learned… check those hose routings carefully.

For what it’s worth, I actually purchased the carb rack alignment plate from you, Len. It made the re-racking process super easy, and I’m confident in my carburetor alignment. I still need to fine-tune the idle mixture screw on carb #2. Waiting for the colortune’s copper washer to arrive in the mail so that I can do that. When I replaced the carb body, I could only estimate the mixture screw setting by copying the “turns out” from the previous carb body.