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General Category >> Rubber Side Down! >> Vacuum Suction Test for Stock Vac Petcock
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Message started by Oldfeller on 08/25/11 at 17:21:47

Title: Vacuum Suction Test for Stock Vac Petcock
Post by Oldfeller on 08/25/11 at 17:21:47

http://www.hunt101.com/data/500/medium/DSCF00168.JPG

REMEMBER -- AIR IS NOT GASOLINE, THIS TEST IS APPROXIMATE

The rubber suction bulb comes from an ear wash kit that you can get at WalMart or any drugstore -- ask your wife, you probably already have one stuck away somewhere from when you raised babies.

(if you don't already know you have one for your own ugly earwax issues)

Is the bulb too strong (stronger than the vac level provided by the bike's intake tract)?? Maybe -- but you get to pick how hard it sucks by how far you squeeze the bulb before you cram the pointed end down into the suction tube.

I guarantee it sucks harder than the bike does if you collapse it all the way, so mebbe you don't want to do that ....

You can suck on the fat tube to judge the flow of gasoline at the various suction levels you can get out of the bulb -- it is very educational to play around with this for a bit just to get a feel for it.

You can put your fingers in the bulb dent and counter-squeeze just a little bit, reducing the vac suction level between "full" and "zero" at will. You can feel the change in the flow through the big tube like playing a little musical instrument .... a little death flute if you will.

You can actually feel and hear the diaphragm and the valve moving and engaging.

Lastly, leave the squeezed bulb engaged and go away for a half hour or so -- you are now doing a leak down test to see if your diaphragm is leaking or not. If it fails, glue the silly thing in place to reassure yourself that it isn't the junction between blue tip and black hose doing the leaking ....

What did I learn?

On my old dead vac petcock I had a slow leak in the diaphragm/drum system and my gas never really ever shut off completely at zero vacuum. I always had "gas on" all the time at some small flow rate. I also learned we have been ignoring leaks from the drum selector system getting old -- these can happen too.

Lastly, sucking and blowing on the fat tube taught me that the supply rate of gas on a vac petcock is a variable, and it never is "freely flowing" from the screened side the way you would think it would be.

Prime position (mechanical passageway) has a much greater max flow rate than the ON or RESERVE positions do even at full input vacuum levels. This may be a function of the relatively smaller diaphragm valve "full open" passage area compared to the simple "hole" area of the Prime passageway.

Now, if you have a used vac petcock laying around I invite you to test it and post below what you see from your tests -- anything we can learn is good information.
I just have the one dead petcock, some other folks need to test theirs so we can refine this test method some more.

REMEMBER -- AIR IS NOT GASOLINE, THIS TEST IS APPROXIMATE

Title: Re: Vacuum Suction Test for Stock Vac Petcock
Post by ralfyguy on 08/26/11 at 04:54:09

That's a cool and interesting test. I just wonder for the fact that there is a difference in the flow between ON and PRIME, with the ON flowing less than PRIME and even varies, how much flow the bike actually needs.
When I got my new petcock last year I tried something similar out of curiosity. With both the old and then the new one installed, I compared the function of the vacuum operation on both. Both just took a little bit of suction to have it pouring out of the nipple and looked about the same flow when turned to PRIME. But even the old petcock was just a few years old then, so I believe that yours is much older and the diaphragm is more stiff now.

Title: Re: Vacuum Suction Test for Stock Vac Petcock
Post by Oldfeller on 08/26/11 at 05:01:39

Yep, we are looking at moldy dead ones "after the fact" trying to understand why they died.

It is interesting that the stiffening of the diaphragm makes so much of a flow difference ("ON or "PRIME") between your new one and an old one.

It is good to hear that a new one at least works well when it is new !!

Title: Re: Vacuum Suction Test for Stock Vac Petcock
Post by bill67 on 08/26/11 at 05:30:28

That looks just like a 1983 Suzuki GR 650. petcock It lasted me 28000 miles 22 years.Never a problem with it.Sold it. last I new 36000 miles.It was stored for 15 years driven 5 miles a year.

Title: Re: Vacuum Suction Test for Stock Vac Petcock
Post by Routy on 08/26/11 at 05:30:39

I believe the main question here is, how much is the minimum vacuum needed to hold the valve fully open, (need gauge in line) or at least to hold it open enough to satisfy the thirst of a WOT condition,.......and compare that w/ the max vacuum produced by the engine at hi sp WOT.

I also believe that a problem in which the engine does not produce enough vaccum to hold the valve open enough to sustain hi speeds, can more time be cause by an engine that is not in the best condition, (rings, valves, cam, timing, etc etc) than onto a defective valve, or a "mis-engineered" valve, as is always portrayed here.

Title: Re: Vacuum Suction Test for Stock Vac Petcock
Post by bill67 on 08/26/11 at 05:30:54

That looks just like a 1983 Suzuki GR 650. petcock It lasted me 28000 miles 22 years.Never a problem with it.Sold it. last I new 36000 miles.It was stored for 15 years driven 5 miles a year.

Bill, I ignored the Tempter the first time you waved it under my nose, and I will pretty much do it again no matter how often you show us that Suzuki had an in-line 4 bike that uses the exact same petcock that we have now -- that "Here's a petcock that we are already buying in quantity" type thinking on Suzuki's part was a good bit of getting us in the mess we are in now.

Title: Re: Vacuum Suction Test for Stock Vac Petcock
Post by Oldfeller on 08/26/11 at 08:09:44

If the valve gets old and stiff and its parameters change to outside what an equivalently aged Savage engine can produce, well, then it is poor applications engineering. Some of the vac petcock failures have been on stock bikes, so yes -- it was mis-engineered for those bikes. They did not take aging of the diaphragm into consideration.

Those of us who have opened up their air filter systems to remove air flow restrictions (and have lowered their intake vacuum levels accordingly) we must take part of the blame.

Running a K&N or other cone filter or a low resistance flat filter puts you outside of what a stock vac petcock can support properly. In my case, I accept it is my own fault now for wanting that extra power from the engine.

Extra power requires extra gas -- no matter how you went after it (big pistons, hot cams, etc) once you get that extra power and try to suck the extra gas to support it you may go outside the vac petcock's design parameters.

So me, I am at least double guilty. I also run a Raptor now.

Title: Re: Vacuum Suction Test for Stock Vac Petcock
Post by verslagen1 on 08/26/11 at 08:29:25

You bad bad man 8-)

Title: Re: Vacuum Suction Test for Stock Vac Petcock
Post by Oldfeller on 08/26/11 at 10:26:18

:) You is bad too, you just ain't recognized it yet ....