This is what I heard.... carb stuff

[KYGS500E]

OK... so i'm not a carb idiot or anything... but it does seem that i should know this

I rebuilt my carbs (o-rings, cleaned out, emulsion tube cleaned, new gaskets) and sync'ed them.  I am curious, though, as to how to accurately set the mixture screw.  I have the rejet matrix and I know what the settings should be but here's what I've heard...

I heard that to set the mixture screw one should run the bike on one cylinder and turn the mixture screw down slowly until the bike dies... then turn it up about a half of a turn and it should be about right. 

Does anyone knwo this to be true or know a better/more accurate way to set it?  I searched... no luck!

[ambisinister]

Huh! New one on me

[The Buddha]

Equal ... what ever happened to setting it equal turns out ...
Cool.
Srinath.

[Mandres]

I heard something about screwing it in and then unscrewing it until the rpms plateau.  I don't really know, I just set both of mine to 2.25 turns out (the stock setting for the non-US GS models according to the Haynes manual).

-M

[patrick]

Reading these would be a good start to learning how to tune carburetors. Understand that the second link is for slide carbs, not CVs.

http://factorypro.com/tech/carbtune,CV,high_rpm_engines.html

http://vma.cape.com/~patrick/brm/carbkei.htm

[KYGS500E]

hey guys... thanx for your help... srinath... what do you mean by equal turns out?  Turn each side out equally?  Not sure what you're meaning.... thanx though

[LimaXray]

Makes sense.  When tuning idle you want it to be as lean as possible and still be able to idle well.  Assuming you have the right jetting, this should give you a good starting point for the rest of the power band.

I think what srinath means is to tune both carbs equally

hey patrick what are you doing with my awesome cylinder head?

[KYGS500E]

thats what I figured... thanx for the help though...

[patrick]

Makes sense.  When tuning idle you want it to be as lean as possible and still be able to idle well.

No. You don't want them as lean as possible. You also don't want them as rich as possible. It's like Goldilocks and the three bears. One bowl of porridge is too cold, one is too hot, one is just right. Same thing with tuning carburetors. It's either too lean, or too rich, or just right. There is no "as lean as possible" if you're tuning for best performance. Here read this. This is how you do it:

--
With the carbs synched, the engine warmed up to operating temperature, and the idle speed set, adjust the fuel screws so that when you rev the bike in neutral and release the throttle, the revs quickly drop to exactly idle speed again. If, when you release the throttle, the revs hang up a few hundred rpm above idle speed, then drop to idle, the idle mixture is probably a bit too lean. Turn the fuel screws out. Use a minimum of half turn increments until you know you've just about nailed it. You'll drive yourself up the wall trying to tune the thing in eighth or quarter turn increments if you're a mile off. In extremely lean cases the idle will hunt between the proper speed and something above it. If, when you release the throttle, the revs drop below idle speed then pick up, the idle mixture is probably a little bit too rich. Turn the fuel screws in. In extremely rich cases the engine will die after revving the bike and releasing the throttle, unless you've turned the idle speed screw way in, in which case it may act like the idle mixture is a little bit too lean. In slightly rich cases, the engine will respond well to throttle blips when cold, but will die or dip slightly below idle speed when hot. While having an exhaust gas analyzer is almost mandatory during tuning of main and needle circuits, do not rely totally on them to set idle mixture. Use an accurate tachometer (like your ear) and adjust the fuel screws to find the highest vacuum for each cylinder or highest idle speed, which will often be the same setting.
--

hey patrick what are you doing with my awesome cylinder head?

That's a chamber from an FZ750 head I ported a million years ago. Here's another shot:



Patrick

[LimaXray]

Tuning of idle should be based on intake vacuum, you want it to idle at the greatest vacuum possible, as it says in the article.  I doubt anyone is going to go through that trouble when setting up idle on their GS tho.  When tuning EFI, I base my idle soley on my VAC gauge.  Although, it has been my experience, there is a small range where it will idle well (it's not as well defined and easy to find as the article makes it sound), and you want to be on the lean side of idling well otherwise you risk fouling your plugs.  (this is magnified if the engine has a low CR and cold plugs)  Also notice I said lean and idle well, not just lean and idle.    But hey, my experience is all with EFI on a totally different engine, I know little about carbs so I could be all wrong.

so wait thats a MC head? thats crazy! i didn't know there were 20v MCs out there.  oh snap.  well my avatar isnt really my head , its a 25v head off of an Audi S1 WRC car

[scratch]

Yamaha has been producing the 5-valve Genesis motors since '85, when it was first introduced on the FZ750.  They now have a 6-valve 660cc single cylinder that is being used by MuZ in their bikes, one of them being the Baghihria(sp)/Black Panther.

[LimaXray]

Thats crazy! I knew Toyota and Ferrari had 5-valve engines too, but I never knew there was a MC with one.  6 valves is even more insane     

[scratch]

Would you be even more amazed that Yamaha once produced a 250cc 6-cylinder, that had a 22,500rpm redline, in the 70's?

[LimaXray]

250 cc - no
6 cyl - kinda
22,500 rpm - yes thats nuts
in the 70's - very much so

Mechanically it makes sense, the stroke would be super small so the piston speed would be low enough not to destroy itself at that rpm.

The valve train tho, that must have been tough, not to mention the ignition system to keep up with 3 ignition events every revolution 22,500 times a minute.