Petcock Diaphram Question

[Old Geezer Richard]

will a wore out petcock diaphram cause my bike to act like I'm running out of gas at higher RPM'S or a pin hole in one of the diaphrams thats under the Velocity Stacks could cause this problem ?  ....  my Fat Lady idles fine and runs great at slow speeds , but when I run it thru the gears and start getting up towards 3500 to 4,000 RPM's in first gear and then run thru the gears getting up to 5000 RPM's the old girl does not have anymore " GO "  it feels like its stalling or running out of gas .... now if I back off the throttle alittle and continue  thru the  gears and RPM's , it pulls good but stops going any faster as I try to get past 4800 RPM's .... no popping , new vacume hoses , no kinks in the vent line ..... so what could it be .... how about a clogged-up high speed jet ? I can ride up to 80 - 85 mph on the hwy  easy but nothing faster ..... now I did not ride my Fat Lady this past summer because of my neck surgery and it had a full tank , could that of clogged up the high speed jet Thanks the Geezer

[3W-lonerider]

sounds just like the petcock diaphram to me..mine done the same identical thing..found the diaphram had a tear in it.

[Daniel Meyer]

Yep. Diaphragm. Easy to check...pull a vacuum on the petcock. Should not leak down.

[Old Geezer Richard]

  As usuall , thank you all for the help .... thats what I thought , it was starving for gas Thanks the Geezer

[John U.]

It is also possible that the screen is clogged. Will it act the same if the petcock is set to reserve? Do the symtoms get worse with less gas in the tank?

[Old Geezer Richard]

Hi John , yes sir it does the same whether its " on " or  " Res. " ....  matter of fact I'm running some 109 octaine fuel thru the gas and it seems to be clearing up the high RPM stall ... went for a ride last fri. after work and ran the Old Girl thru the gears and she is wanting to go past 5,000 RPM's now with just a little stalling  .... but I am going to change out the old diaphram because it is the original and my bike has about 68,000 miles and still ticking .... Thanks the Geezer

[Ricky-D]

One thing that can be done to improve the life of the diaphram is to incorporate a plenum in the vacuum line.

An easy to use plenum is a gasoline filter. The kind that has a hose barb on each end. They are air tight and easy to incorporate.  Get one that is easy to fit within the area you want to install the plenum. I think one about 1/4 cup volume would suffice.  The larger the better actually, tho within reason.

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[Old Geezer Richard]

 Ricky D , can you give me an approx. dimension  ( length & width) of what I would be looking for ... I have seen different shapes and sizes .... it can't be too long or too large in diameter because of the lack of room under there .... Thanks the Geezer

[Ricky-D]

Let me preface this with a statement that I have a Pingel gas stopcock.

It's the constant flexing of the stock petcock diaphram that's causing the problems.

So attenuating the flexing should lead to a much longer lasting diaphram.

A plenum in the circuit can have that effect. I just used a gas filter as an example. All that is needed is a can that can store the vacuum so as a result the pulses are smoother and less abrupt.

Another way to accomplish the same effect would be to incorporate a choke in the line. For instance a short piece of copper tubing filled and drilled with a small drill bit down in the .015" size.

Another way would be to utilize a one way air valve (in the vacuum line) that's been been adulterated to allow a slow leak down.

The possibilities are unlimited and only restricted by your own imagination.

I'm not trying to be difficult, but rather just floating an idea.

Like I said, I'm using a Pingel!

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[John U.]

Another way to dampen the vacume pulses is with a snubber. This was reccemmended some time ago by CA Exhaust Coatings. I installed one in each of my Valks; though not enough time has gone by to assess how they work, in theory they should extend the life of the diaphragm.
McMaster-Carr has what you need, it is small, takes up minimal room. I used 2 brass barbed vacume fittings and 1 snubber, part number 4034K11. The snubber cost 8.69 in May of 2009. I didn't give you the number of the barbed fittings because as I recall I ordered the wrong ones and had to get the right ones locally. Be careful fo the male/female fit, I think I remember that's where I screwd up.
Thanks again to CA for the suggestion.
http://www.mcmaster.com/#pressure-gauge-snubbers/=ajvprv

[Old Geezer Richard]

Hey RickyD/ JohnU , I think I know what your describing to me .... on my Camino I installed a one way type plastic valve for the vacume line going to the power brakes so the air/vacume would not go back and forth .... I guess that one-way valve is called snubber ... RickyD , I guess I can use a small fuel filter on the vacume line to lessen the pulsations from going backwards like the snubber idea .... I'll give it a try when I replace the petcock diaphram .... Thank you both for the tip ... Thanks the Geezer

[Westsider]

hey og ..a pretty sharp dude told me once if i were to run the stock petcock to tee off vac. line to at least one more intake so petcock has little to none pulsating ,,diapham should last long time that way...but i switched to a pingle anyway....