Dumb question time - why do we GS'ers need a vacuum petcock?

[Guest]

I have to ask just having read a post about a pingle petcock being left on accidentally and the results of that.
Why do our GS's need a vacuum petcock? Other makes running the same carbs don't need it.....is it because of our carbs, the near 6gal tank size or what? Not ranting, just curious.
I've got the vacuum set up like everyone else and don't plan on removing it for my own reasons...but I just have to ask why the floatbowl needle/seat don't want to seat up fully, with just gravity feed from the tank, like other makes can/do?

[John Park]

Tank volume is irrelevant; all that matters is 'head' - the height above the carbs of the top of the fuel.

IMO the needles are rather primitive. They wear and don't seal all that well. Having a viton rubber tip would help immensely, but having a master rubber topped shutoff made that irrelevant.

Somewhere in the owners manual that always gets lost or disregarded it probably talks about draining the tank and carbs for storage, and if people actually did that there would be far fewer petcocks sold.

[JTGS850GL]

I have to ask just having read a post about a pingle petcock being left on accidentally and the results of that.
Why do our GS's need a vacuum petcock? Other makes running the same carbs don't need it.....is it because of our carbs, the near 6gal tank size or what? Not ranting, just curious.
I've got the vacuum set up like everyone else and don't plan on removing it for my own reasons...but I just have to ask why the floatbowl needle/seat don't want to seat up fully, with just gravity feed from the tank, like other makes can/do?

Because Suzuki thought ahead and realized that the danger of a flooded engine would be easily averted by using a vacuum controlled petcock. You just had the very same problem they made the fix for.

[robertbarr]

The vacuum petcock is an ingenious and simple approach to preventing the incident you read about. I love safety devices that require no effort or diligence on my part.

When you start your bike, your engine generates vacuum which opens the petcock to replenish what was already in the bowls. You could do something similar by determining when the engine is running to switch the fuel supply on; for example, most (all?) modern EFI systems 'know' when the engine's running so it runs the fuel pump. If the engine stops, so does the pump. The vacuum petcock does the same basic function, but with (usually) absolute simplicity and effectiveness.

[gsrick]

I agree with the rubber tipped float valve tips, but I've also dealt with many a ATV with the fuel tanks in the normal position, leak if the petcock is not put in the off position. I haven't run across many street bikes that don't use a vacuum petcock and the ones that don't have an off position in place of a prime, usually feed a fuel pump located under the seat as a lot of cruisers do. The pump itself keeps it from flooding the crancase.

I have a Vulcan 800 at work that I got going with a lot of work. It has a direct tank to carb set up with a vacuum port and just came back in because the gas leaked into the crankcase. Either from it being left on prime by accident or because of the slow drip drip drip the petcock developed. We are waiting on a new one, but it wasn't notice while at the shop, because I drained out the old nasty fuel before I removed the tank. The tank was hooked back up before I put anymore fuel in it and then it was only enough for test rides. I'm hoping he didn't do any damage to the cylinder walls and rings, but his oil was thin enough to leak from the front sprocket seal and you can see where some came out the head gaskets.

[sleepy]

The problem with the design is where the vacuum ports are on the carbs... when the diaphragm fails, fuel flows down the vacuum feed and into the bore of the carb - down into the engine and past the rings... I replaced the vacuum petcock with a manual petcock on one bike.

[JTGS850GL]

The problem with the design is where the vacuum ports are on the carbs... when the diaphragm fails, fuel flows down the vacuum feed and into the bore of the carb - down into the engine and past the rings... I replaced the vacuum petcock with a manual petcock on one bike.

The problem is that a 30 year old petcock fails. Seriously, these petcocks don't fail for many years. Don't look at common problems today as being a problem with the original system or part. After 30+ years I had certain parts fail myself.

[John Park]

The problem with the design is where the vacuum ports are on the carbs... when the diaphragm fails, fuel flows down the vacuum feed and into the bore of the carb - down into the engine and past the rings... I replaced the vacuum petcock with a manual petcock on one bike.

The original design had a double diaphragm and also a one way valve on the vacuum. Mine turns forty next year. I did replace the rubber insert on the three way valve, but the rest is fine. With the one way valve, the diaphragm isn't subject to fluctuating vacuum. There's a lot there to go wrong before it feeds back through the vacuum line.

I avoid the later type which suspiciously got simpler.

[JJ]

The original setup works great as long as the petcock and needle valves function properly. If you are having problems, do the required maintenance.

[mmattockx]

Why do our GS's need a vacuum petcock?

They don't 'need' it, it is just so that riders don't have to remember to shut off a manual petcock after their ride is over. Personally, I prefer the simplicity of a manual petcock but lots of riders don't like having to remember anything so they prefer the no brain required approach of a vacuum petcock.


Mark

[JJ]

They don't 'need' it, it is just so that riders don't have to remember to shut off a manual petcock after their ride is over. Personally, I prefer the simplicity of a manual petcock but lots of riders don't like having to remember anything so they prefer the no brain required approach of a vacuum petcock.


Mark

I have used both. I prefer the simplicity of the vacuum-operated petcock.

[Steve]

The original design had a double diaphragm and also a one way valve on the vacuum. ...

I am going to guess that the "original design" never made it to production, or maybe it was never released on bikes destined for the US market.

I have never seen a double diaphragm or a one way valve on ANY bike, whether it was Suzuki, Yamaha, Honda or Kawasaki.

Yeah, my observations have been limited to those that were sold in the USA, so I don't know what was offered to the rest of the world.

.

[Sandy]

I've lived with both types and really prefer the vacuum ones. Not remembering to off a manual one is not as bad as forgetting to turn it back on again. Carbs hold just enough fuel to get you out into traffic before it dies.

[nejeff]

Vacuum is just simpler and better. Fool proof for foolish, absent minded riders. That said my 79' KZ400 ( 1/2 of my daily rider bikes) had a bad petcock beyond repair. OEM was (is) very expensive, like $130 bucks. Bought a $10 shipped one off Ebay and I just make a point to turn it to off everytime. Also helpful, if the Kawi would overflow, which is hasn't, the carbs have overflow tubes that take the fuel to the ground and not into the crankcase.

[mmattockx]

I have used both. I prefer the simplicity of the vacuum-operated petcock.

Vacuum is just simpler and better.

Vacuum may be simpler for the rider, but is mechanically more complex with an extra failure point with the diaphragm leaking. You won't know that it is leaking until you discover a crankcase full of gas or bend a rod on a hydraulically locked cylinder. I don't mind having to remember to turn a petcock off, but it looks like that is the minority viewpoint here. To each his own.


Mark