carb synching fun.

[Bminder]

so I finally got motivated to sync my carbs. Engine warm do it at idle correct? do carbs 3&4 together and then do 1 & 2 together and then do the middle screw to balance the two sides correct?
 And I can get all for carbs at the same level but if I blip the throttle a couple times then they're off again. I'll dink around and get them level again then rev some rpms for a while and then they're off again.
It is running smoother and I had to back the idle down a couple hundred rpms but this little procedure seems like an obsessive compulsive person's nightmare because it's never perfect and changes after every throttle blip.
is it normal to do that?

[roverfj1200]

As long as you get them close it will be OK. I like to check the balance at 3 or 4 thou as this is where the bike will be running and where you want it the smoothest. Do very small adjustments and keep blipping as you go. I find I do a dozen or so rounds of this to get things right.


Cheers

[Bminder]

I went for a ride came back and checked them and they were about as close as could be so I'm leaving well enough alone.

[FJmonkey]

I noticed that as well, I thought I was doing the sync wrong. After about 3 tries I realized that it is one of those things you can only get close at. I have not attempted the balance at 3K or 4K RPM, I might try that next just for the experience.   

[movenon]

They will never stay perfectly dead on.  But you can get it close.  I adjust them at idle 1150 RPM + -, adjust and let it settle for about 3 seconds then readjust. Then I do the same with the next set and then the two banks together. Then I recheck them.  IMO bliping isn't going to help anything.  I do raise the RPM up to a constant rate around 2000 RPM to see how they react.  When you get close with a sensitive set of tubes you will fine out that a very slight adjustment makes that last bit of difference.  I find it important to let the rpm settle after each adjustment.

Before I sync the carbs I adjust the fuel air screws.  When doing that I have found it better to lower the RPM to about as low as you can get it, 500 RPM or so then adjust the fuel air screws.  Then set it back up to normal idle RPM and blip test at that point.  That's just how I do it. 
George

[FJ_Hooligan]

At idle, the throttle plates are barely open and thus just a slight difference in them can affect the sync reading.  Blipping the throttle may cause the plate to not settle to the same position.  To avoid this, raise the idle speed for the sync.  That should reduce some of the variability you're seeing.

[JoBrCo]

Yes they'll never be perfect as there are too many variables between 4 different carbs, with many parts of slightly variable tolerance and wear, including throttle shaft seals, which are usually left alone.

Personally I'd sync them at Idle as per the genuine Yamaha service manual.  The reason for this is that at an increased throttle valve opening the difference between them is reduced, while at a smaller opening the difference is more pronounced.

Also don't neglect to do all the adjustments together, running through them several times, playing them off one another, so as to further refine the tune.  In other words lets say you start with the fuel air at the factory setting of from 2.5 - 3 turns, (mine were at approximately 3 before I started the carb thorough clean/overhaul); then adjust idle; then sync; then blip readjusting fuel air if required; then adjust idle again; then sync again, etc. Until no more adjusting is required.  The carb tune requires the most patience, and is the most artful adjustment on any ICE.  The more you do it, the more you'll sense the interactions of adjustments, and the quicker you'll become, and the more precision you'll enjoy.

Don't forget to first ensure there are no vacuum leaks, that can vary, as tuning generates vibrations, especially during the blips.

Sequence verbatim, (though paraphrased), from the genuine Yamaha service manual, Inspection & Adjust section):

Valve Clearance Adjustment
Spark Plug Cleaning & Gap
Canister (CA only) - hoses/canister no cracks
Fuel Lines - no cracks
Vacuum lines - no cracks
Intake Manifold - clamps/bolts tight; no cracks (same with airbox/pods {my interjection})
Exhaust System - no exhaust leaks
Carb Sync -

Note: valve clearance must be adjusted before sync.

1) remove seat, side covers & fuel tank
2) disconnect vacuum plugs & hoses
3) install vacuum gauge & suitable test fuel tank
4) start engine & warm up
5) inspect idle speed - out of spec, turn throttle stop screw (85 fj1100) 1000 +- 50 RPM
6) adjust carb sync steps:
...adjust carb 1 to 2 using screw A (between carbs 1&2) until both gauges the same
...rev the engine (blip) 2 or 3 times & check sync again - VACUUM PRESSURE AT IDLE: 33.25 kPa (250mm Hg, 9.84in Hg) - VACUUM SYNC DIFFERENCE: 1.33 kPa (10mm Hg, 0.39in Hg)
...repeat the above steps to sync carb 4 to 3 using screw C (between carbs 3&4) until both gauges the same
...repeat the above steps to sync carb 3 to 1 using screw B (between carbs 2&3) until both gauges the same

Readjust Idle speed.

========END GENUINE YAMAHA TUNE PROCEDURES=========

My recommendation:  repeat as necessary, (as the idle speed varies throughout the synching process), thus finer tuning shall eventually be achieved.

Bip test for air/fuel is HERE by "racerman_27410," 4th post down (February 12, 2009, 07:12:42 PM)

I hope this helps!

FJ Forever! 

JoBrCo