Another petcock question.

Pulled the carb hose off my petcock yesterday. Fuel flows in PRI and ON, stops when set to RES. I assume fuel flows through RES with vacuum because the bike runs well when set to RES.

I've been unable to find a diagram that shows me what is really going on in the petcock.

Before I order a rebuild kit I have a question of those who have torn their petcocks down. Will I need a rebuild kit, or am I likely to find something stuck in the valve and be able to get this to function correctly with a cleaning?

Thanks in advance for your insight.

 

Test and inspect before ordering rebuild kit.

Test:
Remove vacuum line from CARB intake manifold -- leave connected to PETCOCK Vacuum valve.
Set petcock to ON or Fuel (NOT Res or Prime)
Draw vacuum on line like taking a drag off a cigarette. Fuel should flow as vacuum valve opens when drag on line applied -- off when no vacuum present.

Inspect:
Remove unit from tank.
Check that filter screen is in place.
Look for matter on screen / clean as necessary.
Remove two screws on faceplate.
Remove faceplate.
Remove wafer spring washer.
Extract nob by pulling and alternately twisting left and right to allow O-ring to move with extraction pressure.
Examine rubber valve washer for correct placement. Both open ports. Valve washer "seated" on locating dimples.

Re-new O-ring on outside diameter of knob. (Standard metric sized 0-ring usually stocked at major hardware stores.

Install new o-ring.
Lube with synthetic waterproof grease. Very, very lightly. Just a sheen.
Lube inside diameter of petcock housing. Again; just a film (sheen).

Install knob.
Replace remaining parts in order of removal.

The petcock rebuild kit is PRIMARILY a vacuum valve replacement. The vacuum valves fail after time. The other parts -- which normally remain stationary ... are either protected form the ellements or neoprene.

If your vacuum valve is shot ... use all the new parts.

 

Rick,

Thanks for the reply.

This gets to the crux of my question. Vacuum valve works fine on RES, fuel flows without vacuum when petcock set to ON. Does proper functioning on RES rule out a vacuum valve problem, or is there another valve for the ON setting?

 

Smells like the Vacuum Valve is stick OPEN. This can happen if a chunk of tank rust lodges in there.

Do the in-through-the-front exam ... with the petcock wide-open and all the innards on the bench ... take a vacuum on the line ... like mentioned before ... while you are holding the vacuum valve OPEN ... shoot some carb cleaner in the fuel outlet port.

You can also carefully extract the amber petcock screen and shoot some cleaner in those ports; too. Draw a vacuum on the line and tap on the vacuum valve with a rubber mallet.

It is possible to remove and clean the vacuum valve. You do this at your own peril. The diaphragm is usually sealed to the petcock body and it requires a "Plastic surgeon's touch" to lift-off the diaphragm's outer edges.
Should you attempt to do this ... a slip-up will cause the patient to die on the operating table. Order the kit.

If you successfully lift-off the edges of the diaphragm -- note how its placed.
Set the diaphragm aside and clean-out the valve body and give the whole thing a major spraying clean with carb cleaner.

You need to seal the very edges of the diaphragm upon reassembly. Use a whole box of toothpicks to spread-on a light film of sealant. Be sure not to allow the sealant to clog the tiny rectangular vacuum break along the side.

 

MiCarl:

I don't know the proper name for all of the internals, but I assume what you're calling the vacuum valve is the diaphragm on the back side of the petcock that opens and closes with engine vacuum.

The other major part, on the "outside" of the petcock, is a rubber disk with three holes in it. The holes each have raised rubber shoulders, to seal them from the adjacent holes. When you turn the petcock to the various positions, a hole in the metal disk of the petcock lines up with one of the three holes in the rubber disk, allowing the various functions.

I suspect your diaphragm is fine, since it works as designed on Reserve, but the rubber disk probably has torn places in the rubber shoulders around the On and Prime holes, which is allowing fuel to leak from the Prime hole to the On hole, when the petcock is in the On position.

Turning the petcock rubs the rubber shoulders around the holes, tearing them over time. (A good reason to leave the petcock in the On position, and refill BEFORE you need Reserve.)

My petcock was free-flowing in the On position, and the rubber disk was the problem. Which was a good thing, since the rebuild kit (from the dealer), included the rubber disk and o-ring, but not the vacuum diaphragm!

Hope you get it sorted out-
Herb

 

Yep ... You have to do an inspection of that "Rubber Metering Wafer" ... which appears on the parts diagram as one part of the VALVE ASSY.

If yours is shot ... I can rummage through my endless supply of spare parts and look for a replacement for you.

I've NO idea how long it would take for the Mail to get it to Spain!

Rick

 

Thanks guys.

That is the info I was looking for. Since I won't be street legal until late Sept., and the thing is fine on reserve I may just make sure I keep it topped up and use reserve until the season is over.

I'll let you know what I find when I get into it.

Carl

 

Since I'm slow:

ON & RES both should not flow unless under vacuum.
PRIME should pour out.

I'm rebuilding this weekend - which is fine since it's raining...

 

I think RESERVE is free-flow, too. No vacuum needed. I'm not ready to play that hand "All-in" ... but, I'm sure it's that way on some ... if not all.

 

I with you on this one Rick. Reserve should flow regardless of vaccum signal.

 

I'll have to agree with the earlier post, the only setting that should flow with vaccum is ON, Pri and Res should flow free without any vaccum being needed.

 

This is why I ask these things!

PRI & RES: Flow Always
ON: Flow only under vacuum

You folks are the best!

Thanks again

 

HMMMMM Maybe my valve is assembled wrong.

With no vacuum

Flows on PRI,
Trickles when ON
No flow on RES

Bike runs just fine on RES so apparently flows with vacuum when set on RES

 

If any of your friends have an owners manual ... doesn't need to be a Yamaha owners manual - (they all have the same pictures and text; just about) - flip through and get familiar with "Asian" ... can't hurt! You'll get familiar with how things look and work. There are plenty of similarities between Manufacturers. When Yamaha makes something that works real well; the others guys make cosmetic changes. Lighting and some other electrical's are pretty-much all the same; everybody using out-sourced common brands ... like we do for GE, DelCo, and AC. Yamaha's relays look like Bosch. All the new Fuel Injected bikes have a very familiar look to them, too!!!

 

Sorry guys but RES and ON are both vacuum actuated. Only PRI is free flow. Was that way on my 82 Seca , my 83 Maxim and currently on my 83 Maxim's petcock borrowed from a Yamaha ATV.

 

Rick and Robert:

It is with great temerity that I disagree with XJ gurus of your stature, but my petcock, and every reference about automatic petcocks I see elsewhere, says that "On" and "Reserve" are both vacuum operated.

My operators manual indicates this as well...

_____________________________________________________________

"NOTE: In the "ON" and "RES" the cock works on pressure from the engine turning over. If the line connecting the cock to the carburetor intake manifold is not connected or has a leak the cock will not function properly.

PRI: This indicates "PRIME". With the fuel cock in this position, fuel flows whether the engine is running or not."

Also, from the safety warnings...

"When transporting the motorcycle in another vehicle, be sure it is kept upright and that the fuel cock is turned to the "ON" or "RES" position."

_____________________________________________________________

The only difference in "ON" and "RES" is the height of the stand pipe inside the tank where the fuel is drawn. Logically, you wouldn't want free flow on "RES" any more than when it is "ON", or it could still flood your carburetors if parked on "RES".

Have a great weekend and stay safe-
Herb

 

Noted Herb, and thank you for your contribution to the fray. We (your post included) collectively have the right answer and I hope for all concerned that the correct answer bubbles to the top. Gad I hate it when I screw up. Must mean my petcock is dorked up, time to face the facts of an overhaul. I was hoping to avoid this. I've been sitting back here with a smile on my face thinking I'm in the clear with this gremlin, darn it!

 

Hmm...

So since my valve gushes when in PRI (as expected) and dribbles in both ON and RES my guess is that I need the patent pending RickOMatic fix.

My valve isn't seating fully or the spring has lost some 'sprung'...

To top this whole adventure off I touched up the paint, then gave her a shot of rattle-can clear. Which quickly clouded into a crummy mess.

So now Betsy is dripping AND looks crummy.

And I'm moving so I have no time to play - looks like rattle-can primer black for a few months... <sigh>

 

Buff the Primer out.
Use a dab of 3M Buffing compound and keep the buffer moving so you don't burn through.
............

The RickCoMatic Fix might not be a fix ... just a further complication. The results aren't in yet. I'm waiting for the Test Results on Wolfmajor's bike. The alleged fixed petcock is on her bike.

Working with the available data:

I strongly suspect, that pressing the rod on the aluminum disk, to slightly concave the disc, and allow the opposite O-ringed shaft to be moved closer to the valve seat -- restricts the flow of fuel to the carbs at all time --and particularly -- when the petcock diaphragm is not drawn fully open under heavy vacuum pressure.

A low-level of fuel is supplied to the carbs. Fuel starvation symptoms occur when idling causes the valve to either shut or allow only a trickle of fuel to the rack.

If a suitable replacement can be found ... I'll swap the two and resume testing on the one rebuilt with a Generic Rebuild Kit.

Suspicions:

Valve-end O-ring too small to fully seat in the petcock valve seat.
............

 

Rick.

If you push the ally plate into a concave wouldn't you need to fit a longer spring?

Mick Faighaigh.

 

The degree of pushing on the disc (.025 mm) wouldn't require a new spring. The spring pushes on the disc. The disc moves until it comes into contact with the nylon spacer between the two membranes. It's halted there, still under the spring load, but the end of it isn't seating in the petcock valve seat. The O-ring does not seat and lets fuel bleed by. I doubt doing a "spring-job" would help it. But, adding some load, by stretching the spring a scootch-'n-a-half -- might be a solution, too.
I think the problem is where the part was made without quality control

 

While I'm taking up your time... ;')

There should be a small half circle shaped hole in the valve spring housing on
the plate (not body) side of the assembly..?

I'm going to try and get some pics later today - I'm lunching at work right now.

I have to search the board and web about this thing - the drip is starting to haunt me in my dreams...

Thanks for the thoughts Rick about buffing, but now it's personal.
I'm taking the tank off, dumping the gas, rebuilding the valve and stripping the paint. Then I am laying down the sharpest rattle-can finish that has ever graced an XJ! Pictures will come and Betsy will be vindicated...

 

People get freaked-out by hearing somebody say "Rattle Can." But, the secret is: Lay-on the color with a spray that starts OFF the part ... comes across the part laying-down the color ... and continuing the pass beyond the work.

Never point the spray at the work and push the button. Start the spray off the work ... make the pass and cut the spray off after you are off the work.

Touch-up the same way. Start the spray ... move it across the work ... and get off ... NEVER starting or ending the spray on the work.

With no runs in the paint ... it will look nice. But, only nice.
To make it look better ...
Let it dry and Wet-Sand-it-out with 400 & 600. Don't panic when you see the finish getting dull from the wet-sanding. Dull is good. For an even better looking job ... use a bucket of water with a little bit of detergent added. The detergent should be light ... we're sanding and just need the water slipprey to help sand out light imperfections and orange peel effect.

Rinse off the work really well. Let dry.
Fasten seat belt.
Hit the work with CLEAR! Bingo ... Professional results.

After the Clear-coat dry's ... completely dry - 24-hours.

Buff-out the clear coat and put-on the sunglasses!!!

Nice work!