Throttle Valve Shaft Seals

[munkyfistfight]

I bought a "damaged screw extractor" set today. It pulled that baby out with no problems. So now that I have 2 sets of 82 carburetors apart, I'll want to replace the screws and the seals. I noticed on the AKVision carbs that one of the carbs had extra washers and a rubber o-ring outside of the seal. I'm guessing this could attribute to my hunting idle problems while tuning.

[Rikugun]

Very possibly  ;) Sounds like they may have been leaking and an attempt was made to seal it with O-rings. Unless they are a snug fit in the carb body and on the shaft OD I can't see them doing much.  :'(

[munkyfistfight]

I got my seals in from Boats.net yesterday. They look good. I have to finish cleaning the carbs and get the gaskets back and we'll be good to go. Let's hope these last another 30 years.  ;D

[Fuzzlewump]

Hey maybe I missed something here guys, but are the throttle shaft seals listed above the correct part for both the '82 carbs and the '83's?   (Yamaha part # 256-14997-00-00) I'm gonna take on this challenge with my late model '82.

[Rikugun]

Yes, my understanding is both years use the same seal.

[Rick G]

Those seals fit quite a few old Yamahas!

[pullshocks]

Definitely glad I have a "practice" carb.  One screw came out easily.  The other is going to require some Dremel work and a screw extractor.  I couldn't believe how little pressure it took to bugger the phillips slots.

[Jimustanguitar]

One important facet of seal replacement is getting the  butterfly back in, so that it seats properly .

I've always left the butterfly slightly loose, and then closed the throttle several times to "seat" it before tightening them down. Is this how you do it, Rick?

On setups with multiple carbs, I also compare the crack of light around the edge (if any) between the different carbs. Be careful not to be fooled by sync/balance screw settings, if the carb is assembled enough for that to matter at this point.

[Rick G]

Yes , basically that's it. But it takes several  tries  to get them seated correctly and it needs to be done quickly  befor the lock tite starts to set up.

[pullshocks]

Well I had the carbs out to adjust the float levels, so I bit the bullet and swapped out the throttle shaft seals too.   The posts above described the process very well.  Thank to  all who posted, and a special thank you to Vintage Bikeworks for supplying the replacement screws.  I would say it took about 2 hours, including trimming the screws to length.

The seals were very loose on the shafts.  I was never able to find leakage, but there is no way they were sealing.

After the switch I had to re-synch about 1/4 to 1/2 turn on the synch adjuster.

The carbs still go out of synch as RPM increases

[Rikugun]

Excellent! How is is running now? I noticed mine too were very loose in the carb and on the shaft but could only get the idle to fluctuate with carb spray sometimes and more often just after a ride when everything was hot. I can't imagine they seal well when they are so loose and have to assume that under load at higher RPM's the effect may be more apparent than at idle when you are looking for leaks.

The carbs still go out of synch as RPM increases

That's normal so long as it's within reason. Just make sure they are equal at idle speed with the engine at operating temp and the mixtures adjusted properly.  :)

[Rick G]

Actually sync is NOT done at idle , but at 2500 rpm. More accurate that way.

[Rikugun]

Rick I'm sure you have your reasons for doing so but to anyone else I'd recommend sync to be set at idle speed.

Old school twins and triples with split cable systems may benefit from Ricks procedure to ensure all the slack has been removed from multiple cables. Anything with unified mechanical linkage that lifts slides or opens butterflies should be synced at idle.  :)

[Rick G]

All bikes should be synqed  at iaround 2500,you don't ride the bike at idle . 2500 is closer to where you do ride it.. As a long time mechanic  , rather than a modern parts exchanger . i learned this from another old timer . who is still active in his own shop at 76 years old.

[Rikugun]

The old timer that taught you came from an old time with split cables that aren't used on this bike.  :( All multi cylinder engine manifold pressures will differ slightly at different RPM's. How did your mentor come to accept the arbitrary 2500 number? Using your theory, why settle for 2500? Who rides at 2500 all the time - why not sync them at 5000, 3875, 6104 or redline for that matter?  ???  :o  ;D

The purpose of syncing the carbs is to ensure the throttles open at the same time. The point of opening is from the resting position which is idle. This is the critical point that has multi carb engines idle smooth and accept throttle with the least amount of fuss. And to be honest that smooth idle is really what it's all about - especially when talking about your particular nemesis the inline four.  :) This is what I learned at school where the instructors were old timers who prided themselves on not being parts replacers and who kept up with technology.  ;)

There may be applications where setting them at 2500 works great but I don't think this is one of them. So.... I guess we can agree to disagree then!?  :)

[Jimustanguitar]

Sorry G, but I'm with Rick on this one. I've heard this as well, and it makes sense in my head.

The bike would definitely be easier to start when sunk at idle, but at operating RPM's when the engine is under load and when you're actually riding it would be more critical. I bet the difference between sync at idle and sync at speed are closer to each other than most decently running bikes are to either of these settings before being checked.


Any old school tricks for adjusting sync by ear, or is it truly a gauge only adjustment? (I own a real set of the gauges, I'm just curious because I remember fighting with it a lot before then)

[The Prophet of Doom]

OK this is just my own idle (pun intended) speculation...

The carb sync is designed not so much to ensure the throttles open at the same time, as it is to equalize the AFR between the cylinders.  If it was just for opening, then it wouldn't need adjusting over time, Right?

Since the carbs use different circuits between low idle and 2500 AND since you can't change the AFR at higher RPMs (without changing jetting / float levels) it makes sense to me to sync at 2500 using the sync rod, and then use idle mixture adjustment to sync at idle.

Is that sensible or am I missing the point?

[supervision]

  OK heres an idea, maybe if you put up at 2500 and  sync. it, then drop it to 1500 and see what you have.  The amount of the throtle plate would be open at 2500 is still very small.

[Rick G]

Sorry but the purpose of syncing carbs is NOT to insure they open at the same time, but to insure that they pull evenly under load.  You may rest assured that  the skills acquired over a life time of working on motor cycle engine Will not become obsolete!  In fact new skills are  built on the basis of older acquired skills.

[Rikugun]

Sorry G, but I'm with Rick on this one. I've heard this as well, and it makes sense in my head.

No problem Jim, you owe me no loyalties and I'm not afraid to stand alone in the right  :P But seriously, what is it specifically that makes sense to you? No answer from Rick so I'll ask you: why 2500 rather than any other RPM?

The carb sync is designed not so much to ensure the throttles open at the same time, as it is to equalize the AFR between the cylinders.  If it was just for opening, then it wouldn't need adjusting over time, Right?

Sort of.  :) You are right - it's more than just the opening as that would not change substantially over time other than through wear of the linkage. "Throttles open the same amount" and "vacuum readings the same" are used interchangeably even in technical articles and manuals although they are not necessarily the same thing. In a perfect world they would be. Since it's not a perfect world, we measure and adjust manifold pressure to be equal by moving the two plates relative to each other. Over time this changes and periodically needs to be checked.

The reason sync (vacuum) changes over time is manufacturing variability and wear of the sealing surfaces i.e. piston, rings, cylinder wall, valve seat and face. These things are not the same (between cylinders) when the engine is new and vary more as the engine wears.  Within the realm of the engines serviceable life, adjust for equal vacuum and the plates will be reasonably close. If you were racing you might ignore all this and just ensure the plates were fully open when the linkage is at WFO. Some early forms of race specific carbs didn't even have much of an idle circuit. I however ride on the street in the real world and the Vision is no race bike!  ;D 

A few things to consider....  Equal vacuum readings doesn't guarantee an equal AFR or a smooth idle as in the case of obstructed jets on one carb. You can adjust the sync to achieve equal readings between the 2 cylinders but it still idles rough. Carb synchronization won't clean a dirty carb.  :( You can have one cylinder with a leaking rubber manifold and still adjust the linkage to get the vacuum readings more or less equal. The throttle plates will be way out and the engine will idle rough because a carb sync wont plug vacuum leaks.  :(  There are other scenarios but you get the point. Everything else has to be to spec before you can sync the carbs. A good carb sync won't cure other non related problems.  :)

and finally....

You may rest assured that  the skills acquired over a life time of working on motor cycle engine Will not become obsolete!  In fact new skills are  built on the basis of older acquired skills.

I agree.  :)