Throttle Valve Shaft Seals

[Rikugun]

There are several reasons to sync the carbs and one is to have the throttles open the same amount (vacuum readings the same if you prefer) at idle, to ensure they open in unison, and yes even so the engine pulls under load. Here's the key point - they can be off slightly and you'll never notice it under load. If you think you can notice it under load, you still can't. You are noticing some other engine deficiency or you simply want to believe you can notice it.

Where you will notice it is at idle. Call it a conspiracy if you wish. The factory wishes their product to run smoothly at idle - it makes for happy customers. Is this at the expense of performance elsewhere in the rev range? Absolutely not. Not for street riding in the real world anyway.

If your engine is in good health and the carbs are synchronized at idle, the rest of the rev range will take care of itself. How can it not? The throttles are in sync. Yes, vacuum readings will vary between cylinders at different RPMs. This is normal (within reason) and unavoidable. So if you wish to choose an alternate RPM to set them at for your sake I hope just by chance it also means they're balanced at idle. If not you gained nothing and lost a smooth idle.

Now this is important....If you sync the carbs at idle with good equipment and technique and notice any of the following:

-rough idle
-surging at steady cruise
-a miss at high RPM
-a stumble off idle or any other RPM for that matter
-poor fuel economy
-lack of power
-erratic popping under decal
-sputtering, coughing, wheezing, or any other flu-like symptoms  ;)
-or any other performance woe

it is not the fault of the carb sync. There is something else (or several things) wrong with your bike.  :(

Many people tend to over dramatize the significance of the mysterious carb sync procedure and blame it for things it can not possibly effect. At it's core, it's main function is to ensure a smooth idle. That's it. Everything else just falls into place as a result.

[QBS]

Thank you Rik.  Total agreement. Excellent and through explanation.  Wonderful for those of us that began our travels in the world of fuel injection, never having interfaced with comparativley archaic carb technology, and all its hidden details and secret handshakes.

[pinholenz]

I have posted a chapter from the Vision carb tutorial that I have been working on. I'd appreciate comments and suggestions -especially if my terminology becomes confusing or if I am plain wrong!!

This first part deals with replacing the throttle shaft seals.  and is posted over on the general forum.


http://vimeo.com/70672209         (with music)

http://vimeo.com/70668221

[Fuzzlewump]

Now this is important....If you sync the carbs at idle with good equipment and technique and notice any of the following:

-rough idle
-surging at steady cruise
-a miss at high RPM
-a stumble off idle or any other RPM for that matter
-poor fuel economy
-lack of power
-erratic popping under decal
-sputtering, coughing, wheezing, or any other flu-like symptoms  ;)
-or any other performance woe

it is not the fault of the carb sync. There is something else (or several things) wrong with your bike.  :(

This is most disconcerting! When I adjust my carbs so they're synced according to the gauge, I have any of these symptoms that are listed...sometimes all of them! I've gotta get into that top end...

[The Prophet of Doom]

I have posted a chapter from the Vision carb tutorial that I have been working on. I'd appreciate comments and suggestions -especially if my terminology becomes confusing or if I am plain wrong!!

This first part deals with replacing the throttle shaft seals.  and is posted over on the general forum.


http://vimeo.com/70672209         (with music)

http://vimeo.com/70668221

A most excellent video tutorial John. 
I'm sure that will help a lot of people.  More would be very welcome I'm sure

[Rikugun]

I agree, very good tutorial!  :) I'm sure the video length belies the actual time invested in it's creation so thank you. I liked the use of a second set of screws to set and hold the plates prior to application of threadlocker. Not sure if that's your idea or someone else but a good tip.


Roro, you brought up a good point relative to throttle plate position vs. vacuum readings. I commented on this but it may have gotten missed due to the page break. I edited my comments in an effort to be more clear - hopefully it is?

And on the topic of the carb sync, I should apologize for my part in the hijacking of the thread. :-[ It's a worthwhile discussion but should have been made it's own topic.

[The Prophet of Doom]

Thanks for the reply I did miss it due to the page break - I really love how RoV has developed lately.  Less proselytising, a lot more thought, and consideration of ideas.

My carbs are squeaky clean, with new jets and tested for leaks.  Even so, there's a noticeable sync difference between low idle and 3000 RPM.   I assume because the 122.5 / 127.5 jetting was suitable for a new bike with stock air filter, and stock exhaust - none of which apply to my bike.

I've been tuning mixtures to highest vacuum (at lowest possible idle) rather than highest RPM since I got my proper manometer.  It's easier to see the smaller changes, and quicker since you do it on a per carb basis.

[Cdnlouie]

Just for the record....

I would like to go on record to state that throttle sync is primarily all about balancing the load during acceleration between cylinders exactly as Rick G and a few people that know what they are talking about have said, and it didn't take more than one sentence to say it.  You really want to check to see that that vacuum draw is as even as possible at various rpms.  2500 is a good spot to check it at.  Idle is set with the mixture screw and can be fine tuned after the sync is checked. They do affect each other, so you get your idle decent, (idle screw backed out as much as possible to prevent holding throttle plates open) then sync checking for even vacuum pull at load rpms (holding it steady for check), and then come back to fine tune the idle with mixture screws.

[Rikugun]

Thank you for your opinion Cecil.

[Rick G]

Its NOT just Cecil's opinion, its a fact !!

[The Prophet of Doom]

Its NOT just Cecil's opinion, its a fact !!

That was dogmatic.
This argument is on every bike forum in the world.  Surely there is more than one way to tune a bike.

[Rikugun]

I concede there are several generally accepted methods of syncing carbs. To suggest there is only one way would be arrogant. Having said that, it has been my experience that the Japanese manufacturers' intent is to have carb balancing done at idle. I should also like to limit this observation to street prepped, minimally modified, multi-carb engines with mechanically linked throttles and unified idle speed adjustment screws. Like the Vision for instance.  :D

I have made a real effort to be more open to differing ideas - admittedly the older I get the more difficult this becomes..... :)  However, anyone who has read my contributions to the site knows I have no problem questioning convention when I see a reason to. I also realize "standards" often become standards for good reason. With that in mind, I invite the "2500 camp" to change my mind. Convince me and be persuasive.  :) Certainly there is some good evidence out there you can point to that demonstrates one way to be superior over to another.

BTW, I do not recognize the opinion of forum members (myself included) as expert. I'm going to need documentation from reliable known and peer reviewed/accepted technical sources. Factory service manuals would be acceptable for instance. Which brings me to another interesting point no one has brought up yet. Unless I missed it somewhere I don't see in the Yamaha Vision service manual that it specifies a speed to balance the carbs at. I'm curious to learn forum members' thoughts on why that may be.

[Cdnlouie]

The point that I am making here disagrees with your statement Rikugun as you said,

"At it's core, it's main function is to ensure a smooth idle. That's it. Everything else just falls into place as a result."

My point is that you can easily adjust a smooth idle with the mixture screw (make it run perfectly at idle) and have it run like a bucket of junk without a proper synchronization.  The synchro is used to get the carbs into synchronization for running under load. In order to check synchronization you have to open the throttle plates and that necessarily takes you out of the idle circuit.  That is the phase above idle where it greatly affects engine performance and you can feel the difference, especially in multi-cylinder machines.

A mechanic generally uses carb synchronization to confirm many things about the engine performance.  It is a check of general engine wear, engine faults (air leaks, etc.) that may affect performance dramatically all under "load" conditions.  Generally you start the synchro above the normal idle circuit anyway and after setting the idle  (a slightly fast idle by adjusting the carb stop) to do a proper comparison of what is happening under load.  At this point you are out of the idle circuit and into the pilot jet circuit, then at (? rpm) the main jet kicks in.  Now, if you have a jet blockage you will notice a drop in vacuum pull in the rpm of either the pilot or main jet and this can show up in the synchronization if you do more than just check it at idle which is really not a proper synchro at all.  As I said, a smooth idle is set with the mixture screws not the synchro and that is a scientific fact, and normally common sense because of carburetor design.

So your theory of this only being important to set the idle is just plain wrong.  I wish to be politely clear here, it is not my purpose to convince anyone who knows everything, but to enlighten those who need or want the help, so hopefully this helps someone.  This post is for those who want to listen not those who already have their minds made up, so feel free to ignore it if it doesn't suit your fancy.  BTW if you find a shop manual that tells you everything about tuning an engine let me know, I want a copy.

[pinholenz]

And, (from my limited experience), you need to get the carb sync vacuum right at idle first, simultaneously juggling the mixture screws to get a smooth idle. Adjust one and it affects the other. I guess that is why the mixtures screws were capped off after setting at the factory.

So if I understand Cdnlouie right, unbalanced sync vacuums at  RPMs higher than say, 1300, signal that there is else something adrift in the system - such as restricted airflow, partially blocked jet, valve clearance problems, leaking YICS,  (restricted exhaust?)  and so on. That makes more sense than trying to re-sync the carbs at different RPMs and having to choose one setting in preference to another.

However, surely you HAVE to have the butterfly valves in sync at idle AND the mixture screws nicely adjusted in order to get a smooth idle?

[Fuzzlewump]

I love this forum.  :)

[Cdnlouie]

Absolutely Pinholenz, the idle circuit is a component of balancing that starts with the mixture screws and the throttle plates closed.  The next aspect includes the rest of the throttle range where (depending on the linkage used with the type of motorcycle) you synchronize the lifts of each carburetor so they all pull evenly throughout the rpms.  Things like vacuum flutter will perhaps indicate a leaking valve or some other engine condition that you may want to look into if they don't pull evenly.  That's why this action must only be performed after checking valves and other tune-up items so that you know that all those aspects are covered and you can do a general performance check to see your motorcycle is in good condition. In the eighties when the Vision first came out Yamaha also introduced the use of an exhaust gas analyzer that was to be used to get the mixtures set properly first and then the synchronization would check the throttle opening as the plates began to lift. That first off-idle zone, say 1000-2500 is where you are watching to see that you get even pull and that is more important (or perhaps indicative) than the idle zone which be a bit off because of more fluctuation in vacuum at this point.   

[Rick G]

Good luck with the factory manuals , there full of errors and don't alway agree with each other.
My "opinion " come from working with people who  were in the trade when the Japanese invasion started and the accumulated knowledge   was  better than any manual.

[Rikugun]

Just for the record....I would like to go on record to state that throttle sync is primarily all about balancing the load during acceleration between cylinders exactly as Rick G and a few people that know what they are talking about have said, and it didn't take more than one sentence to say it. 

So Cecil, Rick and a few others know what they are taking about and the rest of us are just a bunch of schlubs. Good to know and believe me, we appreciate hearing it! Also, apparently one sentence wasn't sufficient after all! Good for you!  ;)

I wish to be politely clear here, it is not my purpose to convince anyone who knows everything, but to enlighten those who need or want the help, so hopefully this helps someone.  This post is for those who want to listen not those who already have their minds made up. 

Having been at the receiving end of your snarky comments in the past I'm guessing this is aimed at me? The irony is you could also be describing yourself! Much like me you are a bit of a know-it-all and don't like being wrong.  :P Interesting, huh?  And BTW, starting a sentence with "I wish to be polite"... doesn't absolve you of whatever comes out of your mouth next.   ::)  ;)  I honestly won't take offense to your comments so just say it and own it. We're all big boys here. Fair enough? Good.

BTW if you find a shop manual that tells you everything about tuning an engine let me know, I want a copy.

Gosh, I sure haven't but will let you know if I do.

I guess this is in reference to my query regarding Japanese factory shop manuals and the 2500 RPM sync? If so, my question remains unanswered. It is a serious question and I'd like to know.  Certainly someone must have a service manual to a Honda Kawasaki Suzuki or Yamaha multi cylinder bike laying around? If anyone has a reference to this they can share I sincerely hope you will.  I've looked in the few manuals I have and they say idle - except for the Vision manual which doesn't indicate a specific RPM. On that note, I'd still like to learn others opinions of why engine speed during synchronization is not mentioned in the Vision manual. This is not a test or trick question but rather a genuine interest in learning others ideas on the matter. 

I sense that Rick and Cecil don't regard factory service manuals as an authoritative and reliable source of technical info. I do (with certain stipulations) so humor me. We can argue how accurate a factory manual is but if we see references to 2500 RPM sync over and over from several makes and models, I'd think some amount of credence can be given to the claim. If this is the preferred method I certainly want to know. And if only for certain models, why just them.

[The Prophet of Doom]

This post has got me thinking about all this again - timely as I'm in the middle of rebuild v3.0.

Despite how I've done it in the past, when I think about it, syncing at idle makes no sense.  The butterflies are closed.  They are supposed to be closed.  If you need to open one of them a crack (which is what you are doing if you deploy the sync mechanism) then you really need to take another look at your mixture settings.  Since most people seem to use the fairly crude "midpoint between stumbles" technique there's a good chance that their mixture settings are sub-optimal.

Jetting is a coarse adjustment - the implication being that you can't fine tune the jets to be balanced between cylinders unless you also employ the sync mechanism. 

Idle mixtures on the other hand are an infinitely fine adjustment, if one cylinder is slightly stronger at a given sync setting, you can easily back that one off to achieve balance.  I think you can get away with a less than optimised burn (maximum power) at idle - it's not like you are going anywhere but it will need to be balanced for smooth transition.  Again - this can be done with the mixture screws. 

My plan then is to adjust mixtures using max vacuum and EGA, sync at a higher RPM, and then go back and readjust idle mixtures to have balanced vacuum.  Repeat until perfect.  The result should be balance across the entire RPM range, and optimal power while the bike is going, rather than when it's idling.  Can anyone see a flaw in this plan?

PS I also wonder if High RPM syncing not being recommended is not so much that it's undesirable, but more due to the dangers of potentially sucking mercury through the engine.  Not an issue with my metal stick manometer :-)

[Rikugun]

Oh not you too!!   :D  :D You're over thinking this my friend...