Petcock Issue: Starving For Fuel (Occasionally?)

So I am having some fuel starvation issues with the Petcock on my FJ600. The Valve is vacuum operated on "On" and "Reserve" with a Prime setting that is full flow when turned in that position.

Sitting in the garage, fuel flows on both on & reserve when a vacuum is applied and gas flows without vacuum in the prime position. So far everything is good. After ten minutes of riding (On position) the bike dies due to lack of fuel. When I turn it to Prime, fuel flows and the bike starts. Tonight while riding I switched it to prime after it starved for fuel, but ten minutes later it died again due to lack of fuel, even on Prime. The bike sat for a minute, it started and I scooted home.

I have a clear fuel filter so I can see it is not getting fuel when it dies on the road. It is the original petcock, never been opened. What is inside this petcock that would cause it to not get fuel on Prime?

Any ideas?
Thanks
TR

 

Just a thought. Maybe your fuel tank cap is not venting properly. Once the fuel flows for awhile, a vacuum is created if the tank cap vent is clogged. Next time it happens, and dies on you. Open the gas cap and close it again. You might hear air getting sucked into the tank. See if it starts immediately. If so, worth cleaning the cap out.

 

I am not familiar how the cap vent works but it seems strange that enough venting couldn't be accomplished through the key lock mechanism. I will dismantle the cap tonight, from looking at it the starting point would be the two small screws on the under side of the cap.

 

the key mechanism is sealed, and has an o-ring around it. It won't let enough air in. If it did, it would also let ridiculous amounts of rain in, too.

The cap is a a rebuildable cap on that, and inside it there is also a very tiny air valve.

I did a write-up a couple years ago on rebuilding this cap. This cap is shared by a number of bikes...... Here's a link:

http://www.xj4ever.com/turbo%20gas%20cap%20rebuild.pdf

 

Perfect, Thanks for the reply. I wouldn't have thought a stinking fuel cap was so complicated.
Thanks again
Tom

 

tmrastatter, did you confirm that it is the cap that is causing the issue? Would hate for you to go through the extra trouble of trying to fix the cap and still end up with the issue persisting. Testing the cap vent first is very easy.

 

No, I did not confirm/test the cap. How would I go about doing this? But after reading the rebuild instruction/link, If I were a betting man, I would say that is my problem (plugged vent).

 

Just go for a ride until the bike starts to stall and dies on you. Then once stopped, open the cap. You should hear the vacuum releasing if that the issue you are having. And the bike should start immediately once you relieve the vacuum by opening the cap. There are some that try testing it by leaving the cap not closed all the way and going for a ride to see if the symptoms show up. Don't like this method much. If you should go over good sized bump you could end up having fuel all over the tank and on your lap.

 

Honesty it takes all of 15 min to R&R the fuel cap. Just pull it, clean it out, and put it back on.

 

k-moe is right, I did it a month ago, a piece of cake!

 

Another possibility is a cracked/split vacuum line; OR somebody replaced the vacuum line with regular "hose" that is collapsing in on itself under vacuum once warm.

 

No time tonight to clean out the cap. Maybe tomorrow, I replaced the vacuum line myself, plus it was starving on Prime too. So I am pretty sure it is the cap.

Thanks to all
TR

 

Problem fixed, I took the cap apart and cleaned out the check valve and the little filter. Neither looked dirty but I cleaned them anyway. Went for a 30 minute scoot, bike ran fine.
Thanks to all
TR

 

So my fuel starvation problem never really went away. I cleaned out the Gas Cap passages even though they weren’t plugged. Checked my fuel filter for clogging and flow, no problem here. Petcock flows gas on Prime, With vacuum applied, it flows on “ON” & Reserve. When the bike sits for any length of time, the fuel filter fills up all the way to the top.

So I ride the bike and the fuel level in the filter drops continuously until the bike eventually stalls due to no fuel. This occurs even when the petcock is on Prime (remember, the fuel flows ok on prime). The last time this happened on the side of the road I immediately pulled the fuel line off the filter and gas flowed like a river. I thought about it for a while and I tried to blow into one of the two carb vent hoses. I couldn’t get any air to blow into the carb (I don’t know if this is normal). Eventually I get enough fuel trickle into the filter to ride home. If it sits overnight the filter fills up all the way (on “ON”).

I wet set the float height after the carb rebuild, new float valves etc... I am at a loss to understand why the carbs are starving for fuel. Is it related to the vent hoses? From what I remember when the carbs were apart, the vent hoses basically vented the top part of the bowl to atmosphere, there weren’t any valves to block air/fuel flow.

Would the length of the fuel line make a difference? Right now the fuel line is a little too long and it loops down towards the bottom of the bowls. I did this so I could see if I had fuel flow. I would not think it would matter as gravity should push the fuel to level required.

Any help would be appreciated.

 

Mine died on me a couple times becuase the fuel line was so long it actually kinked and starved fuel. Just had to jiggle the fuel line and she started right up.

You're saying that your fuel line fills up overnight with the selector to ON? Then it's probably the petcock that needs a cleaning/rebuilding. Perhaps the in-tank filter has a bunch of gunk in it that occasionally dislodges and blocks fuel flow. Might as wellmreplace that sucker when you pull the petcock.

 

What bike are we discussing here?

 

Sorry, the FJ600

 

According to the fiche, the '85 FJ600 has the #2 and #3 carb manifold vacuum ports connected via a tube; and the #1 and #4 "teed" together with the tail of the tee running to the petcock. http://www.yamahapartshouse.com/oempart ... ir-cleaner Thus drawing vacuum from two manifolds to operate the petcock.

Are you set up that way, or are you running a single vac line to the petcock?

And if you ARE set up as in the fiche, there are a whole lot of possibilities for a minor vac leak that could affect the petcock's willingness to flow fuel.

 

I am running a single line from #3 to the petcock. The other 3 are capped off individually. The original carbs for the bike we're butchered, so I bought a spare used set, the spare set was plumbed that way, I chose to plumb it the way I have it now. Do you recommend a switch to original configuration?
Thanks
Tom

 

I do.

Yamaha must have plumbed them the way they did for a reason, I'd put it back to the configuration shown in the fiche.

 

Yep, I will try that. But in terms of the vent Tubes... If a person were to blow into one of them, shouldn't they vent out to the other Tube? When I blew into one of my it was like they were plugged up. I amjust trying to figure out why the fuel isn't flowing into the bowls.

TR

 

I just blew through the vent tubes on my 700 and, yes, air should go somewhere. Air isn't going to come out the other tube because they aren't interconnected between carbs 2 & 3. Could be some little creature built a nest in the tube. My tach quit working a couple years ago because a spider spun a nest inside it.

 

Is it possible that your have the filter or fuel line positioned so that you are getting something like a vapor lock caused by air in the line or filter? I have noticed that when riding my filter gets air in it, but since it is positioned horizontal with the tank end slightly elevated fuel still flows.

 

Yeah It might be, I currently have too long a section of fuel line. It loops down almost below the bowls then loops back up. This weekend I am going to shorten it as well as re-do my vacuum lines to factory spec.
Thanks
TR