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Discussion Starter #1 (Edited)
2001 Thunderbird, about 17,500 miles, Mikuni carbs, all stock. Runs great although it is sometimes difficult to start when cold. My problem is that the carbs run dry even when the petcock is on reserve, the tank is full and the fuel tank cap is off. I removed the petcock and disassembled it, found nothing wrong. Reinstalled petcock, filled tank and turned petcock to 'on' position. Fuel just dribbles thru the clear fuel line until carbs are full and then stops running when the fuel line is about half way full. I checked the petcock standpipe screens when I had it apart, even sprayed carb cleaner thru them so thats not it. As I said, the bike runs fine, pulls hard to redline and idles smooth until it randomly runs out of gas. Once the fuel line fills again all is good. I looked inside the tank as well as I could and found nothing floating around. I looked from the filler and thru the petcock hole in the bottom of the tank. Breather tubes on the carbs are clear as well. I am stumped. Anyone have any ideas? Thanks
Edit to add that the float levels are set at 15mm.
Edit #2, using 5/16 inch ID clear fuel hose, single hose feed to carbs, no fuel filter.
 

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Are you saying that if you take the hose off the carb and let it hang, and open the petcock, fuel just dribbles out? If so, there aren't many places to look. You should see about 8 gallons per hour that way, or, if you get a pint in less than 60 seconds you're doing fine.
 

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We have witnessed some folks (I had a friend locally with this issue as well) having this problem in the past, turns out the majority of the time there is fine particles from the liner sloughing off and clogging the fuel screen. The particles are tough to see and the screen(s) will look fine.

What seems strange is you have the problem on reserve, I’m assuming there is no screen on reserve?

Have you removed the petcock, disassembled and cleaned it?
 

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Discussion Starter #4
WSC I have not tried that but I will. Doesn't seem likely to me that the problem is in the petcock since I have removed cleaned and reassembled it just last week but I suppose anything is possible.

BRG I did remove the petcock. Disassembled and cleaned screens. No change. The issue presents in both positions, not just on reserve.
Thanks
 

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Is the fuel tank full? Or is there quite a bit less than full? Have you measured how much fuel flow it has in a minute like WSC stated? I suppose you could have a vent issue in the fuel cap or the tip over valve located at the left rear of the fuel tank, if there is still one installed.
 

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My problem is that the carbs run dry even when the petcock is on reserve, the tank is full and the fuel tank cap is off.
That would seem to indicate the petcock is not delivering enough fuel to the fuel line.
I removed the petcock and disassembled it, found nothing wrong. Reinstalled petcock, filled tank and turned petcock to 'on' position. Fuel just dribbles thru the clear fuel line until carbs are full and then stops running when the fuel line is about half way full.
Have you checked the "duck bill" filter? If the fuel cannot get into the manifold on the carbs it will not flow down the line. If you do the test and get good flow with the line off the carbs, then there must be a blockage in the carb rack itself.
 

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Discussion Starter #7
I pulled the tank and petcock today. Pryed the filter screens off and rinsed them from the inside with carb cleaner. The standpipes were clear. Reassembled and reinstalled the petcock. Fuel flow checked good. Disassembled the gas cap and gave it a good cleaning just for good measure. When last I had the carbs off I had removed the duckbill filter so it can't be that. I reinstalled the tank and when I turned on the fuel it appeared to flow forcefully but then stopped before completely filling the fuel line. The bubble just will not go away and I am still stumped.
 

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The bubble is not your problem. If you want to get rid of it, take the line off the carbs, lift the end of it up above the petcock, and open the valve to fill the line. Then put your finger over the end and reattach it to the carbs. It may just come back, though.

Did you do the flow test?

If it just randomly runs out of gas, especially with partial tank after riding for a while, Check the tip over valve. Make sure it isn't stuck shut, and make sure it is oriented right end up. It should open right side up and close upside down (then reopen if you flip it over!)
 

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Discussion Starter #10
The tipover valve is missing so it can't be that. Before I replaced the tank yesterday I placed the tank on the bike and directed the hose into a pan. I then opened the petcock to both positions and it flowed well, no problems there.
The vent lines are clear so that isn't it.
I guess I'll just have to live with it :(
 

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I guess I'll just have to live with it :(
Good God! With an attitude like that, you should buy a Honda. ;) How long did you run the fuel for? Did you blow air through the vent tube? Just because the tipover valvfe is missing doesn't mean the passage is open. They say the cap is vented, but I'm convinced that is not sufficient. Lastly, you don't have an in-line filter between the tank and the carbs, do you? If the problem isn't the feed it has to be the carbs. (Are we certain it's running out of fuel?)
 

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Discussion Starter #12 (Edited)
Good God! With an attitude like that, you should buy a Honda. ;) How long did you run the fuel for? Did you blow air through the vent tube? Just because the tipover valvfe is missing doesn't mean the passage is open. They say the cap is vented, but I'm convinced that is not sufficient. Lastly, you don't have an in-line filter between the tank and the carbs, do you? If the problem isn't the feed it has to be the carbs. (Are we certain it's running out of fuel?)
Never owned a Honda, probably never will!
I only ran about 1/4 cup of fuel thru the petcock but when I had it out I took the filter screens off and visually inspected. There were no blockages. There was enough fuel in the tank when I had it off last time that I was able to tilt the tank forward and get the fuel level above the filler neck and over the vent hole. When I lowered the tank back to level fuel ran out of the vent tube. I was also able to blow air thru this tube. I dissassembled the gas cap, cleaned everything and reassembled it. I need a new oring as the existing one has cracked but other than that the cap appears functional. There is no in line fuel filter fitted. I am 100% certain it has been running out of gas (clear fuel line is empty), I just don't understand why. When it does this all I do is give it a few seconds and it will restart. Annoying but it hasn't left me stranded...yet.
BTW, the gas cap has a basic one way pressure relief valve. Pressure can only go out so it is IMO not truly vented.
 

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BTW, the gas cap has a basic one way pressure relief valve. Pressure can only go out so it is IMO not truly vented.
That would explain why the tip-over vent is so critical...

I suppose you also checked the hose connected to the tank vent.
 

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Discussion Starter #14
The tank vent (metal tube exiting tank at lower left rear corner) has no hose or tipover valve attached. The paint on the tube looks new, like it has never had anything on it.
WSC thank you for staying engaged and trying to help me solve this!
 

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Try running the bike with the fuel cap slightly loose or next time it runs out of fuel open the fuel cap and see if you hear air rush into the fuel tank.
 

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I’m in agreement with WSC that the air bubble in his fuel line is not the problem. There is a perpetual bubble in my fuel line and it doesn’t affect the running at all.
 

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Thank m in agreement with WSC that the air bubble in his fuel line is not the problem. There is a perpetual bubble in my fuel line and it doesn’t affect the running at all.
As I have one in my filter, but he said the bike runs out of gas so I'm thinkin pulling the carb drain and checking flow from there might show if there is a restriction.
 

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Discussion Starter #19
As I have one in my filter, but he said the bike runs out of gas so I'm thinkin pulling the carb drain and checking flow from there might show if there is a restriction.
Great idea Landlocked! I'll have to see if I have some tubing I can use to direct the gas off into a catch can rather than just pouring onto the engine case. Thanks!
 

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The tank vent (metal tube exiting tank at lower left rear corner) has no hose or tipover valve attached. The paint on the tube looks new, like it has never had anything on it.
There should be a short hose, the tipover valve than a long hose to direct any spills tot he ground instead of the engine or rear tire, If you don't want the valve (no reason not to, as long as it's correctly positioned) then at least put the long hose on it.

Your problem sounds exactly like a venting problem with the tank. Is there any chance the cap is somehow blocking the vent? Can you blow compressed air through the vent line in the tank?

If it is a venting proble you have to let the fuel run for a while before the rate will slow.

Photo of tip-over valve:

721393
 
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