SUZUKI GSXR750 Carbs Mikuni

[xschop]

gas issue

Yes, I would like even better gas mileage. That is my new goal,performance and gas sipping....chop

[mro]

Vac hole

Believe hole is .025 inch

When capped off does not seem to have an effect under "hard" acceleration.


mro

[pggg]

My XS seems to run REASONABLY with the vac hose disconnected and acceleration is smooth enough, but the timing is all astray at higher revs, also it can be too 'fluttery' down low and basically the motor just doesn't perform 100% without it.

[xschop]

yeah

yeah,diddo what pggg said, hey pggg, the new 4th gear roll on is killer....

[TopCatGr58]

Re: Vac hole

Originally posted by mro
When capped off does not seem to have an effect under "hard" acceleration.
mro

Hey Mike, and other interested persons!

You're right in that full throttle is where the vacuum "Retard" unit shines, cause it actually lessens it's pull under hard accel, allowing the timing to retard to the more optimum less advanced level for POWER. Under low throttle input is where it creates more advance which is what is needed with lean small throttle input!

See this Cycle Mag Jan. '78 article and below excerpt for the full technical explanation!

Although the Yamaha Eleven taken as a whole is a "first," if only because it's both fast and civilized, the bike has in its makeup only one genuinely innovative feature: its ignition system. Yamaha's advertising probably will stress that the Eleven's ignition system is transistorized, with no mechanical breaker-points to replace periodically. There are plenty of motorcycles for which that may be said. The difference, favoring the Eleven, is twofold: first, that it has an inductive transistorized system; second, that it's the first to be fitted with an automotive-type manifold vacuum controlled timing advance mechanism. The first of these is a triumph of engineering judgment over what is trendy; the second will be misunderstood by most people, and dismissed as unimportant by the rest. In combination, they'll do more to make the Yamaha Eleven owner's life pleasant than all the overhead camshafts and zoomy exhaust systems ever invented.

The Eleven ignition system's second important feature, and one that's unique to motorcycles, is the vacuum-actuated advance/retard mechanism. Actually, there hasn't been an automobile produced in at least 30 years without some form of inter-connection between manifold vacuum and ignition timing, simply because spark advance otherwise has to be compromised between the conflicting demands of power and economy. Engines running under slight throttle openings have a thin, low-pressure mixture in their cylinders, and they don't run efficiently unless the fire is started long before the piston reaches top dead center. Much less ignition advance gives optimum results at large throttle openings. Yet, all motorcycles but the Eleven effectively have a fixed spark advance, with a centrifugal device to retard the timing for easy starting. And their off-idle timing has to be compromised, with enough advance to give reasonably good efficiency under cruising conditions but not so much as to cause detonation when the rider uses full throttle. The Eleven we tested had 10 degrees of static ignition advance, and another 26 degrees from the centrifugal mechanism, which adds up to the 36 degrees other engines of similar configuration have needed for maximum power. But then there's also 16 degrees built into the vacuum-advance device, and that pulls the timing around to 52 degrees BTC when the bike is cruising along at freeway speeds. Wind the throttles open, and it drops back to the power-setting; roll off throttle, and it auto-advances for economy; use some in-between throttle position, and the ignition adjusts itself to suit conditions. It's a big step out of the technological Dark Ages, and one that's long, long overdue.

T.C.

[mro]

Thanks TC

I knew that.......sorta...........I guess........



mro

[underdog57]

Good stuff on this !!


So we could place an orifice in the line with a .025" hole .
This could be drilled in a brass round 5MM or similar and inserted
in the rubber vacuum line . I doubt it will move , but it wont suck in the tube on #2 carb any way ...

I smell a project !!!

Bob

[TopCatGr58]

Hey Dog,

Instead of using brass, how about taking a vacuum nipple or adapter/joiner and put some JB-weld into the end of it, let it set, and then drill it to the 0.025" diameter, that way the vacuum adapter will easily fit into the vacuum line?
T.C.

[underdog57]

Hi Tc ,
Can give it a try !

Bob

[MAXIMAN]

Originally posted by 79XS11F
Yes I would guess that the actual hole size on carb #2 is somewhat smaller then what is there to accept the hose. I do know that if you hook up to the carb boot the advance will just bounce back and forth from full advance to no advance because that's the way it was improperly hooked up when I got my bike.
Rob

What? Why would it matter if the vacuum advance line were attached to the carb or the intake boot? Both are sensing engine vaccum. Bounce back and forth?

I just installed 36mm Mikunis from an FJ1200 on my Maxim. There is no vacuum tap on the carbs so we hooked it to a intake boot. The advance seems to work fine...but then?

[mro]

Originally posted by MAXIMAN
What? Why would it matter if the vacuum advance line were attached to the carb or the intake boot? Both are sensing engine vaccum. Bounce back and forth?

I just installed 36mm Mikunis from an FJ1200 on my Maxim. There is no vacuum tap on the carbs so we hooked it to a intake boot. The advance seems to work fine...but then??

Don't see that there should be any problem installing either place.

Having proper amount of restriction is what moderates the pulsing.

May need to do same to project bike that have the flat sides on it, although have not run it long enough to fix other issues.


mro

[pggg]

Originally posted by MAXIMAN
What? Why would it matter if the vacuum advance line were attached to the carb or the intake boot? Both are sensing engine vaccum. Bounce back and forth?

I just installed 36mm Mikunis from an FJ1200 on my Maxim. There is no vacuum tap on the carbs so we hooked it to a intake boot. The advance seems to work fine...but then??

Max, you could remove the brass connecter from the old XS carb, plug the end with solder or epoxy like TC suggested, drill the half mm hole thru, slide the tube up inside the hose, then connect to No1 carb boot. You'd get smoother operation for sure.

[MAXIMAN]

Why? I'm not really grasping what the reason is. Doesn't the vacuum advance simply sense the amount of vacuum and adjust accordingly? Why would it be helpful to restrict the pressure differential?

[pggg]

It's real noticible when balancing carbs with vacuum gauges, you need a tiny hole to meter the vacuum precisely. eg.. if you run a unrestricted hose line, the vac gauges will twitch like crazy, same with the vacuum 'bottle' if the hose is direct to the carb boot. Dampen the pulses with a small orifice and you'll get smoother operation.

[mro]

originally posted by MAXIMAN
Why? I'm not really grasping what the reason is. Doesn't the vacuum advance simply sense the amount of vacuum and adjust accordingly? Why would it be helpful to restrict the pressure differential?

Pressure/Vacuum stays the same, regardless of hole (orifice) size.

Making it small just dampens pulses.


mro