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Discussion Starter #1
I've recently swapped the 2 gallon Custom fuel tank for a nice big Deluxe tank for additional range. The used tank I got did not include a cap or petcock... so I sourced a new Taiwanese-Joe-special knockoff petcock, and a grungy used Deluxe cap.

Everything works fine puttering around and driving up to around 60mph. As soon as I get on the highway and travel over that speed for a mile or so, the bike starts to loose power. If I pull the choke it gives momentary relief, which might seem to indicate fuel starvation by enriching the mixture. (The bike did not do this with the original fuel tank configuration.)

If I pull over and shut the engine off, it seems to return to normal when restarted. I tried putting the fuel cap to my mouth and making sure it vented. It seems to breathe just fine. I pulled the fuel hose off the manual petcock, and the fuel seems to flow readily when the petcock is opened. Now I'm stumped. I read a few threads that point to problematic vacuum petcocks, but mine is manual.

The new Chinese knockoff petcock only has a 3/16" fuel line coming out of it. That's my first culprit for possible trouble on low fuel flow. Any thoughts on that? Am I just a cheapskate and should have bought the proper petrol tap from a reputable supplier?

Is there any way to validate that my fuel cap is breathing properly, other than putting it to my mouth and blowing and sucking through it? Can it / Should it be cleaned out in any way?

Another observation I have had (I don't know if its related or not), is at a cold startup. Starting with the choke pulled, instead of warming up gradually, the bike will stutter once after a few moments like it misfires. After that it will suddenly increase in revs. Its like an instantaneous switch, not a gradual change. I push the choke in, and the revs drop back down. (The bike has always done this prior to the fuel tank swap.)

Any troubleshooting wisdom would be welcome.... :icon_scratch:
 

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Discussion Starter #3
do you have a fuel filter?
Thanks for the thought, MurrayF. I was running a fuel filter initially. I only took it out when I started to have this problem thinking that it was a restriction to flow. Removing it didn't seem to make any difference. I did flush the tank initially with some fuel.

It probably is worth it to drain the tank again, check the petcock screen, drop the carbs and poke around in there.

(If I work on it tonight, I might still get to ride to the Oley swap meet.)
 

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Discussion Starter #4
I performed repairs one at a time to make sure I knew what the root cause was.

Draining the fuel proved that the tank was clean. I removed the petcock and the screen was also clean.
I decided to disassemble the petcock since this Chinese made version was screwed together instead of riveted. That's when I noticed that the passageways weren't drilled clear after the casting process. I drilled all the passageways out to the largest allowable diameter and put it back together.

I made it to and from the AMCA Oley, PA meet and didn't have any problems at highway speeds. :D

So lesson learned.... when possible buy OEM parts.
 
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For those that suspect fuel flow issues consider the following.

Generally, these bikes get about 45 mpg. Assume a highway speed of 65 mph.

The bike will consume about 5.1 oz/mile. Apply the speed to get a flow rate of 5.5 oz/minute. This knowledge now opens the door to some simple testing to check fuel flow.

close petcock
remove fuel line from petcock
install a 1 foot chunk of fuel line onto petcock.
borrow a measuring cup from the kitchen. Don't worry you won't have to return it. When she finds out how you are using it, you'll be getting her a new measuring cup.
get a watch with a second hand
put the open end of the fuel line into cup
open petcock, start timer.
At the end of 3 minutes, close petcock.
You now should have at least 16.5 oz in the cup. If not, then you have a flow problem.
 

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Another consideration is to check your valve clearances. I had similar problems and stalling issues. I checked and adjusted my valves to find that the exhaust valve needed fixing. After that I had MPG improvement and no stalling or loss of power.
 

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check that the air admission hole in the gas cap is clear - if that blocks you will get a vacuum in the tank and the fuel flow will stop.....
 

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I was going to ask if it was a vacuum petcock and if so, had you reversed the vacuum line and the drain/vent line? Don't ask how I know that will cause exactly the symptoms you described :embarrassed:
 

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Discussion Starter #9
All great troubleshooting suggestions. I actually had used Knute's suggestion for determining the flow rate required. That is what told me that there was a restriction in the manual fuel tap.

Thanks everyone. All is well in the world.
 

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First post, not first time here..
My 82 GL500 is doing something similar. Runs fine below 40mph or 4k rpm, get it higher and about a mile down the road the bike will bog down and cough out. Starts up fine after a few seconds and will simply repeat the aforementioned behavior. I've pulled the fuel filter out of the tank, its clean and in good shape, checked the vacuum hoses to the petcock, and they all go to the correct locations. Fuel is drained and clean, and 14 amps when running. Has anyone heard of this before? I'd love to have this running faster than the average scooter...
 

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checked the breather hole in the gas tank cap is clear? Even a slight vacuum with stop the gravity flow of fuel
 

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Good point. And double check the hoses too. The vacuum hose to the carb goes to the barb farthest from the petcock body and the drain/vent hose goes to the middle barb. I mixed them up once and it behaved exactly as you described.
 

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All, here's an update.

First, thanks for the advice. When removing the petcock, I noticed it was loose. I'm not sure if that would have created a vacuum in the tank however. But after putting everything back together, I tried it out and it ran perfectly. No bogging. I had inspected the cap for the vent hole but I could not locate it. I had the key flap open in case that made a difference. I've driven in 240 miles since the last post, and one morning it started bogging down again. I had just enough momentum to roll into a gas station. Since I was there I filled the tank and it took 4 gallons. Back on the road and again no problems. I suspect the cap does have a breathing problem, where do I find that little bugger?
 

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I didn't know the Deluxe tank would hold 4 gallons (even miniaturized US gallons)(if you are in the US - you really should add your location to your profile and your bike's year & model to your signature). But if it did take that much gas it was probably bogging because it was running out of gas that time.
 
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