Petcock not flowing enough

[Junkie]

It seems my petcock won't flow enough fuel.

If it's set to "on" or "res", it flows enough for sustained freeway speeds if there's a decent amount of gas in the tank, but does not flow enough for 75mph up a 3 mile 7% grade - it'll start sputtering and then die about halfway up.

If it's set to "prime" and the tank is close to full, it seems to flow fine for just about anything. It'll do the 3 mile 7% grade at whatever speed I want to without starving for gas. However, when it starts to get low on gas (meaning around when I'd have to switch to reserve - a bit more than 2.5 gallons down from full, I have a CA bike with a 2.8+1 gallon tank), it'll start starving for fuel at normal freeway speeds. Not by much, but it certainly won't be running quite right.


I haven't yet taken it apart in order to see if there's a bunch of gunk in the petcock or anything, but would like to know if my odds of being able to fix it are decent. I remember reading here that the rebuild kits aren't particularly good...


I'd be happy to have a 100% manual petcock, with on/res/off, if it means that it flows enough. I'd really prefer not spending a bunch of money for one, though.


And yes, I have confirmed that fuel starvation is indeed the issue - I don't have the side panels on and have a clear fuel line, if I look in and it's less gas than air I think that's a pretty good indication that it's not flowing enough. I don't have a filter or anything that would restrict the flow, either.


Any suggestions?

thanks


edit: oh, this is on an '85 GS700. I put a fresh vacuum line on to see if that was the problem, it didn't fix it though. The petcock doesn't flow at all when there is no vacuum, if that means anything - in other words, the off with no vacuum part is working fine, just not the on with vacuum part.

 

[Nessism]

Suggestions?...how about measuring the flow rate on both Prime, where you say it flows well, and in On and Reserve (after pulling a vacuum). Be scientific and determine how much it flows as a start point. One other possibility is the floats are set too low - going to have to pull the carbs to check that out though.

Good luck.

 

[Junkie]

With the tank all the way full, it runs fine for quite a while with the throttle wide open.

As for measuring the flow rates... I gassed up 5 miles from the bottom of the big hill. Filled the tank all the way up to just below the lip inside the filler cap. Got on the freeway, headed up the hill, and it died halfway up. Waited a minute on the side of the freeway with the bike on Prime, and it was fine for the rest of the trip.


At this point, I tend to gas up every other round trip - each direction is 18 miles, so that means about every 75 miles. I see 40ish mpg on average. I gas up in Atascadero, because it's a little bit cheaper here than down in SLO. Atascadero is up the hill. If I've got it on prime and am heading from SLO to Atascadero and plan on gassing up when I get there, it's fine.

If I'm just riding around town or even at normal freeway load (ie from Atascadero to SLO, doing 75ish but not up a huge hill), it's fine set to On or Res.




I also don't happen to have anything to pull a vacuum with, but the fact that it starves for gas up the grade if set to On no matter what else I do really makes me think it isn't flowing enough. The fact that it's fine when on Prime makes me think it isn't the carbs - in fact, it's fine on On with the throttle open most of the way for a minute+, it just takes some time for it to drain the fuel out of the line.

 

[BassCliff]

Just do it!

Just do it!

Mr. Junkie,

I encourage you to do it right the first time. Bite the bullet and get a new vacuum petcock. Then you won't have to worry about it for another 20 years.

Also make sure your gas cap is venting properly.


Thank you for your indulgence,

BassCliff

 

[tkent02]

Mr. Junkie,

I encourage you to do it right the first time. Bite the bullet and get a new vacuum petcock. Then you won't have to worry about it for another 20 years.

Also make sure your gas cap is venting properly.


Thank you for your indulgence,

BassCliff

What he said.

 

[Junkie]

The gas cap is fine, I don't think it seals too well actually... I probably ought to take care of that. It certainly isn't holding pressure though, and I accidentally tested the venting once - it does so fine, and the gas cap is willing to spew gas too. I filled it up, maybe a little bit more than normal but not much. Walked the bike to the other side of the gas station, waited a few minutes when doing stuff and noticed that gas was streaming down the side of the tank from the cap, as well as out of the overflow hose out the back. It was on the sidestand in the same was as it had been before, so I'm not sure quite what was up (especially for it to flow out the top), but it certainly does not hold pressure.

http://www.z1enterprises.com/detail.aspx?ID=1715 is the right petcock, right?

Oh, I looked around and couldn't find a guide for adjusting the valves on a 16v. I'm doing so tomorrow afternoon, so I think I'll take some pictures and write one up. I haven't done it on this bike before, but I've done it on other vehicles a few times so it shouldn't be too hard.

 

[Steve]

Have you tried to put it on PRIME before going up the hill at your 'normal' 75 mph?

Is your thottle near wide open while going up the hill?

Have you tried going up the hill a little slower?

I am thinking that at 75 mph, the throttle will be near wide-open and vacuum might be low enough that the petcock might be shutting off.
As much as I like the idea of a new petcock, too, I don't think a new petcock would help in this situation.

We have gone up some long steep hills and not had problems, but they were at slower speeds on winding roads, so the throttle was never wide open, let alone for three miles.


.

 

[Junkie]

Yes, I have tried putting it on prime before going up the hill. Unless the tank is almost empty, it does it fine in that case.

Near wide open... yes, it is.

It's a freeway, so traffic moves pretty quickly. Maybe I'm not quite doing 75 up it (I don't have a working speedometer right now), but even if I'm going with the slower cars (but not the semis, going VERY slowly) the carbs manages to run out of fuel by the end of it. I tend to move with the faster part of traffic, it isn't like I'm passing everyone going up it or anything - the speed limit is 65, and to me it's more important to be safe (in other words, to be paying attention and to control my encounters with most vehicles whose drivers aren't really paying attention) than to be legal. A ticket hurts my wallet, but a lot less than going down at freeway speeds would hurt me (and I haven't gotten one in about 16 months, 2 more and I can take traffic school again - never even been pulled over on a bike, although only about 10k on bikes so far).

 

[Dogma]

This sounds very similar to what's going on with my '80 850 GL. Except I can do it on the flat (just going with traffic). This issue has been on hold for me through the cold months. I've disassembled and cleaned my petcock. There was nothing significant in there, just a few tiny chunks of black who-knows-what (overspray from a recent tank painting?). The next thing for me to check is the cap. I've been told to ride it 'till it dies then listen for it to break seal when I open the tank. I' don't think I'll be able to hear anything on the side of the road, so I plan to try to open it up. A new one on BikeBandit is about $50

 

[Junkie]

I'm quite sure it's not the gas cap, as I said before it doesn't seal all that well. The vent on the back works fine too.


While it might not be the best idea, you could try riding along with the gas cap off and seeing if your problem still happens - I mean, you wouldn't want to do it for too long, but I can't imagine several miles would hurt (presuming it'll happen that quickly).

 

[Dogma]

I'm quite sure it's not the gas cap, as I said before it doesn't seal all that well. The vent on the back works fine too.


While it might not be the best idea, you could try riding along with the gas cap off and seeing if your problem still happens - I mean, you wouldn't want to do it for too long, but I can't imagine several miles would hurt (presuming it'll happen that quickly).

The thought has crossed my mind. I'm mainly concerned with making sure nothing splashes out during normal maneuvering.

My cap has also had opportunity to leak fuel, demonstrating that it can't hold liquid in. But I've been wondering what the thing's internals are like. Is it possible to leak fuel out but not allow air in? In my case, a half tank of fuel (bike laying down, broken tie-downs, very scary) probably created a lot of hydrostatic pressure on whatever seal is there. But pulling vacuum against the other side of the seal may be a whole different story. Now that I think about it... leaking fuel when the bike is laying down might be grounds for replacing the cap anyway.

Mostly I responded to this thread because I wanted point out that I've had very similar symptoms to you but found nothing significant when I took apart my petcock. At any rate, disassembling the petcock to check it is a piece of cake (on my 850 anyway). If the diaphragm and seals are so delicate that you can't put it back together, you need a rebuild or replacement anyway. So, to get back to your original petcock question:I say continue on with your methodical troubleshooting. Take it apart and see if there's anything wrong in there. There is a little filter screen that could be gunked up. The rest of the passages should be wide open.

 

[tone]

My thoughts ...... standard ish bike with stock engine & stock fuel tap which was designed to flow enough fuel for this engine under any conditions right ?

conclusions ..... fuel tap is faulty, vent is blocked or fuel line is too long or kinked, vacum pipe damaged or blocked

solutions ..... replace tap, run with cap off to test vent & clean if needed, shorten fuel line so it wont kink, replace vac pipe

easy :-D:-D:-D

 

[BassCliff]

Another suggestion

Another suggestion

Mr. tone has a point. You might consider replacing the vacuum line and fuel line. The old ones may be blocked or cracked from age and leaking. If the vacuum line is leaking it would cause flow issues with the petcock. (Boy, was that a profound glimpse into the obvious!) I'll shut up now.

Thank you for your indulgence,

BassCliff

 

[Junkie]

I replaced the vacuum line about a week ago, it isn't kinked or anything (and is one of those lines with steel braid in it, so next to impossible to kink anyway). The fuel line is clear, and I currently have the side panels off and can see that it's not kinked or blocked in any way.

The reason I haven't pulled the petcock apart yet is because the GS is currently my only working vehicle and while it might not currently be perfect, the workarounds to make it work right are very simple (ie using Prime). I suppose I'll try pulling it apart next weekend, this weekend is going to be about 500-600 miles so I'll get to see how well it works...

 

[Dogma]

The reason I haven't pulled the petcock apart yet is because the GS is currently my only working vehicle and while it might not currently be perfect, the workarounds to make it work right are very simple (ie using Prime). I suppose I'll try pulling it apart next weekend, this weekend is going to be about 500-600 miles so I'll get to see how well it works...

In that case, play it safe and have new seals on hand first. The seal between my petcock and the tank was hard and flattened. It wouldn't seal again when I put it back together, and was leaking fuel way to fast to be ridable. Since it wasn't a simple o-ring sort of thing, I had to order a new one and wait. If your petcock has never been separated from the tank, the seal could very well be in the same condition. Maybe some Permatex product could seal it...I don't know. Good luck.

 

[Junkie]

I'm pretty sure it's been off reasonably recently, as the guy I bought it from has another tank missing the petcock and gas cap which I'm pretty sure was the original one - it was fairly rusty, and he put this one on when he got the bike about 2 years ago.

What is the seal made of?

 

[Dave8338]

Keep in mind...if there is even a tiny pinhole in the diaphragm, air flows easier than a fluid and the diaphragm is tiny. With a small volume of air, a small pinhole (s) and a drop in vacuum (as you have the thing nearly pinned), my bet is on the petcock being faulty. Even if it isn't, replace the roach...it is a safety part and in addition, will rule that out as being a possibility. :?

 

[Dogma]

What is the seal made of?

I dunno. The old one was some black formerly rubbery stuff. It was so hard it almost felt like ABS plastic. There's no chance somebody put the wrong thing in there, since it was molded to fit the grooves. The new one was also black, but the color doesn't say much. Lemme see here.... According to my Parker O-Ring Handbook (no, really) compound compatibility chart... several compounds are "satisfactory" for use with gasoline: Nitrile and Hydrogenated Nitrile, various flourocarbons (FKM, like Viton), and flourosilicone. Regular silicone is "unsatisfactory". If I had to guess, I'd say Nitrile. What happened to mine is classic for Nitrile. Viton might last longer, but another 27 years of Nitrile ought to be good enough.

I looked at the petcock you linked to earlier, and it is different from mine. Maybe it does use regular o-rings, maybe it doesn't.

 

[walt]

Does your bike have an inline fuel filter? If yes it could be dirty and need replacing.

 

[Nessism]

I still say measure the fuel flow if you have not ordered a new petcock already. Get a glass jar and put the fuel line in it. Turn the petcock to Prime and time how long it takes to fill. This is your baseline since you state the bike runs fine on Prime. Next, do the same test with the petcock on ON and RES, you will need to draw a vacuum on the vacuum line input to the petcock. If the petcock flows the same amount of fuel in both situations, it's fine.

A couple of things worth looking at are the screen filters in each carb at the fuel inlet, and make sure the fuel line does not loop upward as it leaves the petcock and flows into the carbs. Both will create restrictions in the line which will reduce fuel flow.

Good luck.