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Discussion Starter #1
Hi All,

I have a 2007 VTX 1300 S which I bought a few months back and have been doing some tinkering. The bike is almost 'done' (you know that's a lie...), but the last remaining issue seems to be a starting problem.

Specs first (as far as I know, not much info from the PO): Cobra slip-ons, stock airbox with K&N filter, I did the PAIR mod, new plugs and fluid (Mobil One 4T 10W-40 oil), around 30K kms (18,5k miles) on the clock, new Yuasa battery, A/F screw out 2,5 turns, no SCAR mod (as far as I know).

It goes very well in general, but it can be a real pain to start, sometimes it doesn't start at all. I live in Australia, so a cold day here is ~10C (50F for our US brethren) with a warm day around 105F+. I finally tracked the problem down to fuel supply, I think, and I suspect it's the petcock vacuum fuel valve, although it seems to work when I checked it. Vacuum lines seem OK.

My question is this: the vacuum necessary to actuate the petcock, does it have to be a mere pressure reduction, or an ongoing suction (if that makes sense, barring the TWSS jokes). In other words, when I apply suction with a tube to activate the valve, it takes continuous suction, i.e. breathing in continuously, for fuel to flow. I ask, because a mechanic on YouTube mentions you can apply a vacuum and clamp the line to keep the petcock open, which is not the case with mine.

I've already ordered a new diaphragm for the vacuum-operated valve just in case, but I guess I'm looking for validation that that's the problem. Can you 'normally' apply vacuum to the line and clamp the line to stay open, or does it need constant suction to stay open? With the 'T' in the line between manifold and carb, and the design of the valve shown in the service manual, I'm assuming it only takes a pressure reduction to work, so my guess is the diaphragm is not 100%, but I'd really appreciate feedback from anyone that knows what I mean.

Happy to provide more info. Thanks in advance!
 

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YES, continuous vacuum........
like if you drop the bike.. engine stalls.. but fuel continues to flow to carb and out the overflow and onto the HOT engine.. ( fire )

seeing the amount of work.. WHY not replace the OLD vacuum hoses.....
one very small crack can cause issues in just a few days back on the road
most auto parts stores should stock the size you need.. you just cut to reach.
 

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It takes a constant vacuum to open the petcock fuel valve. However, your understanding of what "constant vacuum" means seems incorrect. If you draw a vacuum on a line that goes to the petcock and then clamp this line with a vacuum still in the line, then this vacuum is constant. Of course, this assumes there are no leaks in the vacuum line, petcock, clamping device, etc. Since at least tiny leaks are always present, you need a continual supply of "new" vacuum (this is not really the correct physics description but it will suffice for the purposes of this discussion) to keep the petcock open. Fortunately, this "new" vacuum is constantly provided by the bike.

One thing to note is that the carb float bowl holds enough fuel to ride a mile or two. The bike should start even with the petcock turned off or malfunctioning unless a long time has passed since the bike was last run thus allowing the fuel in the float bowl to evaporate.
 
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rubber is NOT air tight.. it retains air and or vacuum...
but it does allow both to escape.. over time...
same with aluminum...

a sample from google

First is that nitrogen is less likely to migrate through tire rubber than is oxygen, which means that your tire pressures will remain more stable over the long term. Racers figured out pretty quickly that tires filled with nitrogen rather than air also exhibit less pressure change with temperature swings.
 

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I wonder why nitrogen permeates through rubber slower than does oxygen. The oxygen molecule would seem larger at a MW of 16 compared to nitrogen at 14. Perhaps there is an interaction between the oxygen molecule and the rubber that slows its diffusion through the rubber.
 

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google search

When a tire gets hot the air inside it expands. ... Promoters of nitrogen tires point out they don't lose tire pressure as fast as air-filled tires. Since nitrogen molecules are bigger than normal air molecules, it is harder for them to leak out. This means a tire filled with nitrogen will maintain air pressure longer.

air is not oxygen,,,, its a joined element.

The air you breathe is made up of lots of other things besides oxygen! Oxygen only makes up about 21% of air. About 78% of the air you breathe is made up of another gas called nitrogen. There are also tiny amounts of other gases like argon, carbon dioxide and methane.
 

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Yep, I looked into it further and the nitrogen molecule (N2) is slightly larger than the oxygen molecule (O2) despite the oxygen having a larger molecular weight.
 
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Discussion Starter #9
Thanks guys - appreciate the time!

My initial explanation was maybe not clear. I understand that a continuous vacuum is needed, but I guess my question relates to whether or not air is supposed to flow rather freely when sucked through the vacuum connection of the petcock.

For example, if you have a sealed Coke can and suck against the opening, a pressure reduction occurs inside the can. Stop sucking while holding this vacuum and it stays, i.e. will keep the fuel valve open.

Now, if there's a pinhole in the Coke can, you can also create a similar pressure reduction while sucking, but you have to keep on sucking for the vacuum to stay.

My fuel valve is like the pinhole can - if I apply vacuum, I have to keep on sucking air (quite a lot) for the fuel to keep flowing. Is that the way it should be, or not?

And what's this about the '07 fuel valve recall? Image doesn't work.

Thanks again everyone.
 

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Discussion Starter #10
Whoops, neglected to mention I've also taken the sage advice around vacuum line replacement to heart and will pick some up from my local auto parts shop and replace the old lines.
 

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No, you should not be getting a constant flow of air into your vacuum line necessitating you to continue sucking. Of course, these are not high vacuum lines and connections so there is always some leakage but the description you have given suggests there is a vacuum leak somewhere in the petcock or in the vacuum lines leading to the petcock.
 

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Discussion Starter #12
No, you should not be getting a constant flow of air into your vacuum line necessitating you to continue sucking. Of course, these are not high vacuum lines and connections so there is always some leakage but the description you have given suggests there is a vacuum leak somewhere in the petcock or in the vacuum lines leading to the petcock.
Thanks Vinish - that's what I suspected, thanks for confirming. Fuel does flow, even when you crank it, but it definitely requires a continuous and sustained suction, which lead me to believe something is amiss.

I'm waiting for the petcock rebuild kit (should take a week or so) and will replace the lines this weekend.

Could something like this cause the starting issue (when the bike stands for a couple of days) and perhaps a sightly lean condition?
 

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Thanks Vinish - that's what I suspected, thanks for confirming. Fuel does flow, even when you crank it, but it definitely requires a continuous and sustained suction, which lead me to believe something is amiss.

I'm waiting for the petcock rebuild kit (should take a week or so) and will replace the lines this weekend.

Could something like this cause the starting issue (when the bike stands for a couple of days) and perhaps a sightly lean condition?
I do not know for certain. I suppose it could be a cause of such a problem is the flow of fuel to the carb is so low that the float bowl is not kept full. Fix the petcock problem and then see if the other problems continue. Have you cleaned and rejetted the carb? That is usually the best way to solve starting/lean problem.
 
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1300 fuel system


tank vent
petcock
float...

a blocked vent will stop fuel flow...

air must replace fuel as it exits... its the law.. and not human.
 

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Discussion Starter #16
Thanks guys - you guys rock. I've today replaced all the vacuum lines and stripped and cleaned the carb. Still waiting for the petcock rebuild kit, but bike's like new!

I strongly suspect there's a leak in the petcock diaphragm, due to the continued suction necessary to get fuel to flow, but it does flow. Slow jet was full of crud, but it's all shiny now. Bike purrs.

I'll see if she starts as easily in the morning like today, but I'm very hopeful.

Thanks again to all who took the time!

Carb was pretty dirty and I eased-up the A/F screw to around 2 out (from 2,5). Zl
 

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Discussion Starter #17
Get a load of this fellas!

Sauntered into the garage this morning with my cup of joe with high hopes the bike's cured. Thumbed the starter and she fired-up, so off to a good start. Need some feathering to keep idling (the enrichener being the devil, of course), and then she promptly dies. Refuses to fire-up again. Like a bureaucrat on a public holiday - ain't doing a thing for love nor money.

So I grab the ol' fishing chair and plonk it next to the bike and take the airbox cover off. Twist the throttle - no fuel pump squirt. So I'm sitting there with my head in my hands fuming over why couldn't I be someone who loved stamp collecting over biking, when I had a lucid moment. I strongly suspect the fuel valve has a vacuum leak, due to the earlier-mentioned continued suction needed to get fuel out. But then I recall the layout of the float bowl, where the fuel pump's inlet is towards the right side of the bike and the bike rests towards the left on the side-stand.

What if, with the crappy fuel supply from the petcock due to the vacuum leak, the bowl is mostly empty, hence why it doesn't want to stay running? It works fine when I ride it, just the starting issue. What if it gets just enough fuel during normal riding, i.e. the vacuum is sustained enough to keep the float bowl full, but there's not enough vacuum at first start (and I try not to redline any motor that's cold-started...) to cause all my starting woes? Once it uses-up the little fuel in the bowl, it's empty and not enough vacuum to refill the bowl.

So I thought to myself, TollingRhunder - an easy way to test this crazy theory without having to strip the damn seats, tank, airbox, etc., would be to lean the bike to the right a little and see if anything changes. So, with only more hope to lose, I mount her and tilt her to the right. Tweak the throttle while precariously leaning to peak into the air inlet, twist the throttle, and on the second twist, the familiar squirt of fuel! Twist, twist, squirt, squirt. Lean her back onto the side-stand, twist again, and it's dry - no squirt. Eureka! The bowl's not getting enough fuel.

When I cleaned the carb yesterday, the float seemed nice and loose and it was working fine when I rode her yesterday. So my current diagnosis is that the fuel valve leak is just enough so that, at general idle or low RPM, no fuel flows, but the vacuum at higher revs causes the bowl to be fed nicely. When I park her after a ride, I reverse park her down our inclined driveway, so she has to idle for a minute or so while I perform this maneuver. I don't generally rev her before parking, as it fills my garage with fumes (and I'm not that rock & roll anymore...).

So I think this combination of phenomena is what's causing my starting problems. I'm anxiously waiting for the petcock valve rebuild kit, so I can replace the diaphragm and see if it works. Although I get the reasoning behind the vacuum fuel valve, what a PITA!

Once I've installed it, I'll report back. I really hope my suspicions are correct, otherwise I might look into that stamp collecting thing.

Can you replace the fuel valve on the back of the petcock with the petcock still on the tank, i.e. loosen nut and rotate the petcock - just keep the petcock closed when replacing the valve, or do you need to drain the tank? I'm not too keen to drain the tank simply for logistical reasons, but I can if I really have to.
 

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Discussion Starter #18
And I should have done some work today, so of course I rather spent time tinkering on the bike...

I'm happy (?) to report that I think I tracked the problem down, in-line with my earlier diagnosis. Don't have the petcock valve rebuild kit yet, but took the valve out (turns out no tank draining required) and after careful inspection, it's perished - photo attached.
 

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that's "Rubber" and fuel... they are not friends...

If you guy's EVER had to use vacuum controlled windshield wipers.. you know what I am talking about..

just saying.

or repairing/servicing vacuum controlled door locks... on a 4 door car...
Headlight doors..... Camaro, Corvette.. etc..
Heater/AC controls... behind the dashboard..

one little crack or pin hole...
 
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