Fuel starvation issue.

[sq61708]

Hello everyone.

Quick question here. 

Can a leaking petcock be the cause of fuel starvation when riding at highway speed by letting the carbs suck in air along with the gas ?

[MAN OF BLUES]

we need more to go on with this issue... a lot more.

[sq61708]

1991 Connie (Canadian model) 106 000 km on the odometer (about 66 000 miles)

Just changed the oil, fresh sparkplugs, valve adjustment, clean and re oil the air filter (not too much and I let it dry before re-install). 
When I took the gas tank off to do the valves I noticed that the fuel line was ripped near the petcock so I cut 1 inch off and decided to install an inline fuel filter (Napa 3011) to make up for what was cut.

Re installed the tank, filled it with fresh gas and it started right away, idling was smooth.
Took it out for a short run and everything is fine.
That night I take it to go to work, 50 % back roads 50 % highway.   On the last leg of my way to work it feels like it’s missing sort of like when I have to put it in reserve doesn’t last long because I get to work.  I park it and forget about it.
Next morning I leave to go home and take only the highway to get back.  It stalled on the highway after about 6 miles.  I pulled over and put it in prime for a minute and it started right up.  I take off and ride for about 6 more miles and it stalls again.  Same procedure, prime for a minute and it starts goes another 6 miles and stalls just before I get home.
At this point I thing that either my fuel line is kinked or the filter I just installed is too restrictive but I have seen it used by other in past posts.
When I parked it I notice that my petcock is dripping a drop about every 15 seconds. I put a catch under it and go to bed.  When I get up it has stopped leaking so I decide to take care of it on my next day off.
Today I emptied my gas tank by attaching a clear fuel line to it and draining it on prime to a gas can.  This is when I notice that there is air going down along with the gas as it drains. Must be coming from the small leak letting in air I think.
I then shorten my fuel line by ½ inch to make sure that it is not bent enough to kink, I change the vacuum line from the petcock to the carbs just in case , tighten the 4 screws in back of the petcock by ¼ turn.  I reinstall my gas tank, refill with gas and take it out for a short ride. Everything is fine.
I take the bike to work again tonight, 50 % back roads and 50 % highway.  It stalled after about 3 miles on the highway.  Again I put it in prime for a minute and it restarts.  I get back to work but this time I try to keep my revs below 4000 rpm and just cruise to work.  When I get there I notice that the petcock is leaking again.
So here I am.  It feels like the carbs are not filling fast enough when I go fast (over 4500 rpm for a few miles) but they are ok if I’m going slow on city streets or back roads.
That's why I asked if the leaking petcock could let in air down to the carburetors and make them not fill fast enough to keep up with the demands of the engine for fuel at highway speeds.
Feel free to ask if you need more information.

[Nosmo]

I agree with MOB, more info needed for a real troubleshoot, but pending that: If it runs in PRIME but not in the normal ON position, the difference is that PRIME doesn't require vacuum but ON does.  Check your vacuum hoses to be sure there are no leaks or kinks.  Lack of enough vacuum to fully displace the diaphragm and poppet valve can cause lack of flow/starvation.  The leakage of the petcock is probably a separate issue that should also be dealt with.  Sounds like time to repair/overhaul/replace the petcock.

[sq61708]

I tought that my previous post was more precise.  Let me know what information I should include.

It's a canadian model, I am aware of how the petcock is vacuum actuated.  There is just one vacuum line that goes from a nipple on carb #2 to the petcock and I changed it this mornimg making sure there was no kinks in it. 

The small  lines that go from carb to carb look fine and where never touched.

If I remember correctly I tried running it on prime the second time it stalled last week and it didn't help.  I will make sure to go back home with the valve on prime all the way home after my shift  this morning to see if it makes any difference.

I will report tomorrow night.

As far as the petcock goes I will probably order a OEM one to replace it.

[T Cro ®]

Try removing the inline fuel filter... For the most part they are known and do cause starvation issues... Be sure to fix or replace the leaking petcock... Also just because you've read that they are used does not mean that they are 100 % as some folks have problems and some don't. I installed one years ago and it caused me grief but it was due to a slight kink that cautious routing and trimming worked out; but results vary a lot by brand too...

[BobCT]

Did you put the napa 3006 so that the 90 degree outlet is right at the point where the line goes down to the carbs. You can't have much slack in the line either or it will kink when it gets warm. How old is the fuel line? If you have a spare key try opening the gas cap when it starts to stall to see if your vent is not working. Get a new petcock from Murph.

[connie_rider]

He may have told us the problem.
Said petcock is dripping.
If the Petcock leak is coming from the vent hole, the leak is an indication that the diaphragm may have a hole in it.
If the diaphragm has a hole, it may not be allowing the valve to open completely.

NOTE: It can not be a problem of air being included with the gas flow. Because the gas fills the carb bowls, which vent off any air that might be mixed with fuel.

Your most probable problems are; bend in the hose (that closes when gas line gets hot)
                                                  old gas hose (collapsing when hot)
                                                  Gas tank vent plugged (not operating correctly)
                                                   improper vacuum line hook up to petcock (must be 1 carb (#2) to petcock)
                                                   too small or improper installed as filter
                                                   petcock malfunction (torn diaphragm)
                                                   I may have missed some? (others can add if I did)?

Ride safe, Ted
                                                   

[Steve in Sunny Fla]

tank vent not.  steve

[Nosmo]

He may have told us the problem.
Said petcock is dripping.
If the Petcock leak is coming from the vent hole, the leak is an indication that the diaphragm may have a hole in it.
If the diaphragm has a hole, it may not be allowing the valve to open completely.

It can not be a problem of air being included with the gas flow. Because the gas fills the carb bowls, which vent off any air that might be mixed with fuel.

Your most probable problems are; bend in the hose (that closes when gas line gets hot)
                                                  old gas hose (collapsing when hot)
                                                  Gas tank vent plugged (not operating correctly)
                                                   improper vacuum line hook up to petcock (must be 1 carb (#2) to petcock)
                                                   too small or improper installed as filter
                                                   petcock malfunction (torn diaphragm)
                                                   I may have missed some? (others can add if I did)?

Ride safe, Ted
                                                 

I was thinking along these lines today while hiking.  I think Ted my be on the right track.  The diaphragm has a hole in it allowing leakage through to the external weep hole.  What if the diaphragm is also leaking fuel INTERNALLY, so that vacuum is pulling it into the carb throat through the vacuum nipple, making it way too rich, rather than too lean, and not being metered properly.  When PRIME is selected, fuel flows through the petcock without going through the vacuum circuit, so the engine runs normally.  I am not at home so I don't have my old petcock or photos to refer to.  Just some thoughts. 

I hate to say it has been so long since I got rid of my OEM petcock and vacuum tubes, I can't remember which carbs put vacuum to the petcock.

[sq61708]

Thanks for all your input.

Riding back home from work yesterday morning it ran ok for about 6 miles again then stated to give me trouble again.  I pulled over and put it in prime to get back home, seemed to work but it started to run weird again running rough but at least it did not starve. 
When I got home the petcock was weeping again and I noticed that one of my spark plug boot had popped up. That would explain the sudden rough running.

I checked my gas tank vent lines (2) and they are clear.  I blew air in them and could hear it exiting near the gas cap.
Gas line were  not kinked.
The theory of the torn diaphragm seems very probable.  I put a clear vacuum line to carb #2 so I'll check if there is any gas in it next time I run it.  I'll take the tank off and then get a new petcock.  Don't know if I'll stay with OEM or put a manual one instead.

[Bikenagain]

A lot have trouble as soon as they put a filter into the system. Try removing the filter and see how you go. If nothing else it will eliminate that as the problem.

[Steve in Sunny Fla]

Put a spare key in the gas cap and go ride until it starts to fuel starve. Then pop the tank open while you're still riding. If it clears up in a couple seconds it's the tank not venting, which seems to be an increasing problem. I've written about the nexus between fuel filters and tank venting before, and I am pretty sure that's exactly what's going on here. Steve

[T Cro ®]

A lot have trouble as soon as they put a filter into the system. Try removing the filter and see how you go. If nothing else it will eliminate that as the problem.

Yes please....

Remove the fuel filter and tell us how it runs without it until you do your wasting your time chasing problems that I'm betting that you installed....

Petcocks do indeed get old and can start to leak not only via the diaphragm but simply past the o-ring that seals the handle to the body.

[MAN OF BLUES]

I'm onboard with the filter being a great probabilty of issue, but only because I've seen soo many people with them having problems...

now, in reality, I'm putting my vote in that the petcock itself is not functioning....

if the poppet o ring fails, you get fuel in contact with the diaphragm.... and it comes out the weep hole to verify its compromised.... you note it is dripping....
if fuel is in the diaphragm area, which it should not be, then the diaphragm is now swollen and warped, and stretched due to softening, this in turn  results in it not opening the poppet fully, as the rubber is now a baloon, not able to pull the poppet open, with the applied vacume it normally sees....it should be stretched tight like a drum to function... pull it apart and look at it, I bet its not tight at all from fuel contamination....
when you switch to prime, you get fuel.... that has to tell you something.

hey, they wear out....just a normal thing.

[Summit670]

If the tank is full and the bike staves for fuel after hot, switch the petcock to prime and see if will stay running while on prime.   Make sure the little arrow on the lever is pointed toward prime.

on my 87, the vacuum line would get hot and collapse, causing starvation.  When i switched to prime it would run.  Plenty of fuel was in tank.   I had installed a new line and it wasnt the oem type line. 

[connie_rider]

I agree with T Cro, Steve, and MOB...

The only way to trouble shoot this, is to start with the obvious.
1) Fix the leaking petcock. Fire is BAD!!!!

2) Remove the fuel filter, (and install new fuel and vacuum line).

If the problem continues, it's definitely the vent...

(If I understand correctly) Steve says that the vent may be marginal., and doing the others may solve the starvation, but may also hide a potential vent problem. Do pay attention to that possibility.

Ride safe, Ted

[Steve in Sunny Fla]

Ok, let me add some info. IMO, the problem isn't a fuel filter, if everything else is fine. I've changed my position on that because like y'all, I've seen adding a fuel filter cause starvation issues. then I was privy to a few different bikes and some experimentation; the result is that the tank venting is really the problem, but the starvation really shows up when the filter is installed. It adds just enough flow resistance that the barely functioning vent doesn't need to functon at all, and then tank starts vacuum locking. That's why I've been saying "tank venting"... because really that's where the problem is starting. JMO, Steve

[T Cro ®]

Ok, let me add some info. IMO, the problem isn't a fuel filter, if everything else is fine. I've changed my position on that because like y'all, I've seen adding a fuel filter cause starvation issues. then I was privy to a few different bikes and some experimentation; the result is that the tank venting is really the problem, but the starvation really shows up when the filter is installed. It adds just enough flow resistance that the barely functioning vent doesn't need to functon at all, and then tank starts vacuum locking. That's why I've been saying "tank venting"... because really that's where the problem is starting. JMO, Steve

Vent + Filter + Kink = Problem
Vent + Filter = No Problem (Maybe)
Vent + Kink = No Problem (Maybe)
Wash + Rinse + Repeat

While not everyone follows the thought this is why I removed the little rubber checky inside the gas cap many years ago to this day I've only once had an issue with fuel flow and it was caused by adding the kink...

[jirod]

1991 Connie (Canadian model) 106 000 km on the odometer (about 66 000 miles)

<snip>
It feels like the carbs are not filling fast enough when I go fast (over 4500 rpm for a few miles) but they are ok if I’m going slow on city streets or back roads.
<snip>

Can't argue with any of the informed suggestions made to this point.  But if none works, check here:

http://forum.cog-online.org/index.php/topic,48929.0.html

I was riding behind the OP of that thread when this happened to his bike. The symptoms you describe are similar to what he was experiencing. The fix is in the thread. Good luck and Ride Safe!