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Discussion Starter · #1 ·
This was moved from another thread:



Yesterday afternoon (March 14) I took this past winter's project bike, a GL650I, out for a shakedown ride. A dozen miles went by with everything working great so I pointed it north on the interstate to head to the mountains.




After about ten miles of high speed riding it ran out of gas. One cylinder stumbled and died, and a short distance later the other one quit. Only one problem - the fuel tank was almost full.




I opened one of the carb drains and a little gas dribbled out, but not much. Several attempts were made to start it with no luck. Then I switched to reserve and it finally started after some more coaxing. The ride home went fine with no more problems.



Once home, the vent hole in the tank cap was checked and found to be open/free. The bike was run for an hour in the garage with no more problems.



The petcock vacuum hose is new and well-secured on both ends. It is fairly stiff and thick-walled, and therefore extremely unlikely to collapse under vacuum.



Tonight I hooked up a vacuum pump to the petcock and checked the fuel flow into a funnel/gas can. There was plenty of volume in both the on and reserve settings, and the vacuum controlled the flow as it should.



The tank was then drained so the petcock and filter could be pulled for inspection. It was drained in the on position and then switched to reserve. Only a little more gas came out when switched to reserve. So reserve is not working - it's good to find out now rather than when it's really needed.



The petcock and the in-tank filter were inspected. The filter was very clean. The tank looks almost new inside with no signs of debris. The petcock looks fine, including the tube used for reserve.



At this point it's a mystery as the problem has disappeared.



The petcock will be replaced so that reserve will be available. It's also high on my list of suspected bad actors.
I'll probably disassemble it to see if anything obvious has failed internally.
 

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Discussion Starter · #2 ·
After several attempts to track down the problem it has finally been solved. Here is the guilty culprit:







This piece is located inside the petcock and functions as the valve to cut off fuel when there is no vacuum. As part of the restoration process a petcock rebuild kit was installed, and this is one of the pieces provided in the kit.



The aftermarket replacement is much stiffer than the original Honda part. The Honda part flexes much easier. When the original Honda part was put back in the problem cleared up.



The fuel starvation was occurring at speeds above 70 mph, generally when going up a long hill. The throttle was mostly open in this condition.



I connected a vacuum gauge to monitor the vacuum during a test ride. To my surprise the vacuum was only around 2 psi under the riding condition that caused the fault. The vacuum was at its lowest when the fuel volume was needed the most!



All is well now. An extended test ride showed no further problems.
 

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Yes that makes sense, glad you tracked it down. Perhaps a weaker spring would work with the aftermarket valve. Vacuum is highest at idle I believe, and drops during acceleration. I found that out as a 16 year old when the vacuum operated windshield wiper in my Model A Ford would work like a charm sitting at a light, and stop completely when I hit the gas!
 

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After several attempts to track down the problem it has finally been solved. Here is the guilty culprit:







This piece is located inside the petcock and functions as the valve to cut off fuel when there is no vacuum. As part of the restoration process a petcock rebuild kit was installed, and this is one of the pieces provided in the kit.



The aftermarket replacement is much stiffer than the original Honda part. The Honda part flexes much easier. When the original Honda part was put back in the problem cleared up.



The fuel starvation was occurring at speeds above 70 mph, generally when going up a long hill. The throttle was mostly open in this condition.



I connected a vacuum gauge to monitor the vacuum during a test ride. To my surprise the vacuum was only around 2 psi under the riding condition that caused the fault. The vacuum was at its lowest when the fuel volume was needed the most!



All is well now. An extended test ride showed no further problems.




Yes that makes sense, glad you tracked it down. Perhaps a weaker spring would work with the aftermarket valve. Vacuum is highest at idle I believe, and drops during acceleration. I found that out as a 16 year old when the vacuum operated windshield wiper in my Model A Ford would work like a charm sitting at a light, and stop completely when I hit the gas!


Maybe I am not reading this right, but the statements seem to conflict. Did you say the problem occurred during high throttle/speed, and as a result low vacuum, which caused the petcock diaphragm to close down the fuel flow? That seems backwards to me. Count me not very knowledgeable in this arena.
 

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Maybe I am not reading this right, but the statements seem to conflict. Did you say the problem occurred during high throttle/speed, and as a result low vacuum, which caused the petcock diaphragm to close down the fuel flow? That seems backwards to me. Count me not very knowledgeable in this arena.


When the engine is idling the throttle plates are closed, the engine has to draw air through the idle circuit and a bit around the throttle plates so it builds a greater amount of vacuum than when the throttle plate is wide open and it can draw air freely.



That is why these carbs will not work right without the correct air filter and air box set up. If there is not enough restriction in the airbox, there is not enough vacuum to raise the sliders.



If you have a vacuum cleaner with a 3 inch hose, it will have less vacuum/suction than if you reduce the end of the hose down to 1 inch.



This is also why I believe that once the throttle plates open, the idle jet/circuit stops providing fuel.

Because of the rubber plug in the idle jet tower, the idle circuit draws fuel through a hole in the side of the primary jet tower.

Once the throttle plate is open, the fuel is drawn through the primary emulsion tube by the airflow across the top of the it, and it bypasses the idle circuit. Dave says he measured 2 psi of vacuum, that would easily be canceled out by the fuel flowing past the hole in the side of the primary stack that leads to the idle circuit.
 

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Me and vacuum petcocks dont get along.



I bypassed mine.


When you say you bypassed it do you mean put a non-vacuum type in? If so what did you use?

If you bypassed it and used the stock petcock can I assume it is not hard to figure out how?

I have never taken apart a petcock. Just have not gotten there yet...
 

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When you say you bypassed it do you mean put a non-vacuum type in? If so what did you use?

If you bypassed it and used the stock petcock can I assume it is not hard to figure out how?

I have never taken apart a petcock. Just have not gotten there yet...
Just moved the spring inside the petcock to the other side. Be careful when taking it apart, and if possible do it on a clean garage floor, or driveway. When i first did mine I did it over a bunch of pavers with big gaps in between them, and I dropped parts everywhere.



Oh, and a piece of vacuum line with a golf tee stuck in it works well to block the vacuum. I've also used the back of a cheap pen before on another bike.
 

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When the engine is idling the throttle plates are closed, the engine has to draw air through the idle circuit and a bit around the throttle plates so it builds a greater amount of vacuum than when the throttle plate is wide open and it can draw air freely.



That is why these carbs will not work right without the correct air filter and air box set up. If there is not enough restriction in the airbox, there is not enough vacuum to raise the sliders.



If you have a vacuum cleaner with a 3 inch hose, it will have less vacuum/suction than if you reduce the end of the hose down to 1 inch.



This is also why I believe that once the throttle plates open, the idle jet/circuit stops providing fuel.

Because of the rubber plug in the idle jet tower, the idle circuit draws fuel through a hole in the side of the primary jet tower.

Once the throttle plate is open, the fuel is drawn through the primary emulsion tube by the airflow across the top of the it, and it bypasses the idle circuit. Dave says he measured 2 psi of vacuum, that would easily be canceled out by the fuel flowing past the hole in the side of the primary stack that leads to the idle circuit.


Allan... that is a great and accurate description of why vacuum at the petcock decreases at speed... Mine is getting a "makeover" this evening as I had this exact issue on a long shakedown cruise yesterday. 20mins at 75+ mph and it killed as if going to reserve level, switched to R and nothing.... filter looked almost mt. Let it rest a while, blew into the vac line, checked everything, noticed the fuel filter was full, started right up as if nothing happened, and rode it glitch free for a couple more hours around town (slower)
 

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Discussion Starter · #12 ·
I think the vacuum petcock is a great design and a worthwhile convenience. But after three decades it is not unreasonable to expect the rubber parts to deteriorate.



I'll be contacting the aftermarket manufacturer sometime this week to pass along these findings so hopefully the problem can be corrected in future production lots. Manufacturers need this sort of feedback; they cannot take corrective action if they don't know a problem exists.



The aftermarket rebuild kit was chosen solely because of the cost savings. If I had to do it again, certainly a Honda kit would be used, at least until the aftermarket issue is resolved.
 

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if a petcock is bad would it effect the idle/slow rpm, i had a problem the other day with my carbs leaking from the drains on the bowls like crazy only when i would rev it up would it not leak, i picked up another carb at the local bike junkyard rebuilt it. and now i have the exact same problem with my new ones as i did with the old ones. i have to tease the choke for 10 mins and now it wont start. my suspect is that it is getting flooded. also i do have pod filters, i know you guys dont like thouse
is it that there is not enough suction to lift the needles? the whole time ive had the bike i have never been able to figure out this problem so

would an old petcock be directly related to my problem as well?
 

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if a petcock is bad would it effect the idle/slow rpm, i had a problem the other day with my carbs leaking from the drains on the bowls like crazy only when i would rev it up would it not leak, i picked up another carb at the local bike junkyard rebuilt it. and now i have the exact same problem with my new ones as i did with the old ones. i have to tease the choke for 10 mins and now it wont start. my suspect is that it is getting flooded. also i do have pod filters, i know you guys dont like thouse
is it that there is not enough suction to lift the needles? the whole time ive had the bike i have never been able to figure out this problem so

would an old petcock be directly related to my problem as well?


Your problem is more likely with the Carb floats and Float Needle Valves(FNVs)



http://globalcxglvtwins.hostingdelivered.com/viewtopic.php?f=19&t=110
 
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