TOPIC:

why would someone use a vacuum petcock as opposed to a 'gravity fed'? petcock?

Aside from a desire to keep a given machine in OEM configuration, there's an ergonomic factor; if your needles and seats weep a bit, it's kind of nice to have fuel flow shut off along with the motor, instead of having to remember to manually shut the petcock.

Avoids those embarassing little puddles all over the floor and those gas stains on your countershaft cover. Saves on gas that way too.

Obviously, they're a bit more complex mechanically, but they ain't rocket science - and in proper working order they'll last quite awhile.

Also arguably the vacuum ones are much more reliable. I "had" like 10 manual petcocks for 1977 KZ650s at one time. I thought having 10 would be a lifetime supply, but no, between leaky ones, ones that the stupid lever froze on and broke off ect, I finally abandoned them all in favor of the vacuum setup after mixing and matching parts until I was blue in the face. Even with regular lubing of the lever and replacing the lever oring and bowl gaskets, I could never get the damn things to work.
Conversely I've got close to 300,000 relatively trouble free miles out of vacuum ones, just set them and forget them, unless the diaphram goes bad, then $11.00 and back on the road again.

Despite what other may say about vacuum petcocks, I like mine. I think they were a great idea. Like Ron says "just set em and forget em"

I have a 78 KZ1000 A2 will mine have one? And if not how do you hook one up? Mine is off and I dont know the diffrents from one to another.

they look different and have different markings. on a manual, it has on off res on a vac it has on off pri (i think)
check www.z1enterprises.com

The biggest advantage to a vacuum petcock (in my experience anyway), is that you don't have to remember to turn it back on before you take off on the bike.

When you are moving along in first gear on gravel and the engine shuts off because you forgot to turn the petcock back on, while riding a 750 twin, I can assure you that the back tire will stop turning instantly and life suddenly gets interesting

It caught me by surprise and I almost dropped it

RonKZ650 wrote:

Also arguably the vacuum ones are much more reliable. I "had" like 10 manual petcocks for 1977 KZ650s at one time. I thought having 10 would be a lifetime supply, but no, between leaky ones, ones that the stupid lever froze on and broke off ect, I finally abandoned them all in favor of the vacuum setup after mixing and matching parts until I was blue in the face. Even with regular lubing of the lever and replacing the lever oring and bowl gaskets, I could never get the damn things to work.
Conversely I've got close to 300,000 relatively trouble free miles out of vacuum ones, just set them and forget them, unless the diaphram goes bad, then $11.00 and back on the road again.

I agree. Also, if the bike falls over while parked (like getting hit by cars driven by idiots that don't know how to parallel park) , the vacuum petcock is off whne the engine is off. A manual one might be left on and then your bike is a weenie roast waiting to happen.

another quick question, whats the prime for if you have a choke? when in prime does it act like a gravity fed petcock to fill the bowls? also i was reading the mikuni vm carb manual and it said something about having a 'starter system' as opposed to a 'choke' system, what are they talking about?

I rode an '81 KZ440 for ten years. It had a vacuum petcock. When I recently switched to a '78 KZ1000, I have had the hardest time getting used to the manual petcock. I keep forgetting to turn the darn thing on and off. I was so used to just leaving the petcock alone, and letting the engine decide when it needs fuel.

I picked up another (larger) tank that I am prep'ing for paint, and it has a vacuum petcock. I'm loooking foward to putting that on.

fische wrote:

another quick question, whats the prime for if you have a choke? when in prime does it act like a gravity fed petcock to fill the bowls? also i was reading the mikuni vm carb manual and it said something about having a 'starter system' as opposed to a 'choke' system, what are they talking about?

All prime does is allow the fuel to flow into the bowls, then you switch to on or reserve and no additional fuel will flow until the engine is running and vacuum pulls the diaphram in the petcock and lets fuel flow. Once the engine is shut off vacuum releases the diaphram and fuel is shut off so if your floats stick you don't get gas all over the floor.
What they are talking about on the "starter system" vs a "choke system" is the way they richen the mixture for cold starts. On a "choke system" like on an automobile carburetor, when you "choke" the carb, you are doing exactly that, you are turning a plate to restrict air which results in a richer mixture.
On our Mikuni carbs with a "starter system" the exact opposite happens. When you pull up the choke lever, the starter plungers pull up and allow additional gas to be drawn in resulting in a richer mixture.
Basically 2 ways to richen a mixture, either by restricting air or by introducing more gas.

I've been riding oldies so long, I've never had a bike with a vacuum petcock. But I've always made a habit of turning off my ignition with my right hand and simultaneously turning my petcock with my left.
It's rare when I forget. So far anyway.