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Old 03-06-2009, 02:43 PM   #1
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Default Engine leaning out as the fuel goes down ... help please ...

Hi guys,

I have a friend who bought a Hyper ST Truggy (the pro version) and he got problems with air leaking through the tank o-ring, he changed the tank for a new one and now his engine its leaning out as the tank goes empty so the engine start very rich, by middle tank its fine and on 1/4 or less its veeery lean ...

Any trick to kill this problem ?,

thanks,

Pat
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Old 03-06-2009, 02:51 PM   #2
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thats sounds more like a tune issue, beginning of tank motor is cold, and runs fine while warming up,then starts leanning out after about 5-6 min.. thats a bad tune.. Are you getting a lean bog?? i would put the needled back to stack setting and retune the motor
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Old 03-06-2009, 03:12 PM   #3
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thats sounds more like a tune issue, beginning of tank motor is cold, and runs fine while warming up,then starts leanning out after about 5-6 min.. thats a bad tune.. Are you getting a lean bog?? i would put the needled back to stack setting and retune the motor

No lean bog ... engine act good on track or table, but as the fuel goes out, engine leans out, just that ... no leaks on carb, no leaks on back door of engine, no bad fuel line, maybe no good tank.
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Old 03-06-2009, 03:20 PM   #4
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Try lengthening the pressure line or using a Mugen pressure chamber and retune the engine. The longer line provide more consistent tank pressure, so it should not be too rich when the tank is full.
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Old 03-06-2009, 03:20 PM   #5
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any bubbles in fuel line? Fuel filter almost pluged? obstruction in pressure line or tank? loose fuel inlet on carb?
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Old 03-06-2009, 03:21 PM   #6
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what do you mean it leans out??? usually when a motor leans out, it acts differently,do you mean keeps heating up???? if its a new tank, then its going to be a motor issue, whether its tune, or maybe a air leak.. I had a motor run great good when i fired it up, run better by half tank, then run worse and loose a little power towards bottum of tank, that was due to a lean engine.. once i retuned it, it ran great..
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Old 03-06-2009, 03:45 PM   #7
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Originally Posted by Tabushi View Post
Hi guys,

I have a friend who bought a Hyper ST Truggy (the pro version) and he got problems with air leaking through the tank o-ring, he changed the tank for a new one and now his engine its leaning out as the tank goes empty so the engine start very rich, by middle tank its fine and on 1/4 or less its veeery lean ...

Any trick to kill this problem ?,

thanks,

Pat
You still have a low back pressure issue between the pipe and the tank. Most often it is caused a leak in the tank (lid or crack), a cracked/loose pressure nipple on the pipe or a loose pressure line.

so, check the nipple on the pipe and tank and wire tie the pressure line in place. Make sure the lid seal is good and clean.
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Old 03-06-2009, 04:40 PM   #8
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Try lengthening the pressure line or using a Mugen pressure chamber and retune the engine. The longer line provide more consistent tank pressure, so it should not be too rich when the tank is full.
Assuming you have NO leaks anywhere; this is the answer.

Run longer pressure line; I prefer 21-23 inches for a .21 on a 125cc tank.

I'm sure you can run 18-20" on a larger tank.
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Old 03-06-2009, 04:53 PM   #9
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Try this simple cheap fix. Take aprox. 4 small rubber bands and rap them around the tank and lid. Use the kind your wife or G/F uses in there hair. They are petroleum resistant. This will keep pressure on tank lid creating a better seal. What may be happening as the fuel gets low in the tank the pressure is lost. The tank lid may have just enough of a leak that when the air void in the tank is large from used gas. The small air leak causes just enough pressure loss to make engine lean. 1/2 to full tank less space pressure loss is easily replaced by incoming exhaust. 1/2 to empty the void in the tank is greater than the what can be easily replaced by incoming exhaust.
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Old 03-06-2009, 05:00 PM   #10
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What exactly does a longer pressure line do? Seems like that would add volume and make it even harder to keep pressure up?
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Old 03-06-2009, 06:45 PM   #11
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What exactly does a longer pressure line do? Seems like that would add volume and make it even harder to keep pressure up?
When you fill the tank, there is less air to compress, so the tank gets more direct pressure from the pipe (rich mixture). As the tank empties, more air space is available, so the air in the tank will now compress more, forcing less fuel to the carb (lean mixture). Not every engine and car is the same, but in general, to want to keep the fuel pressure feed constant between full and low tanks. On-road guys run long lines. Offroad guys generally don't know about this.
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Old 03-06-2009, 06:55 PM   #12
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How long is the fuel line? If line is too long, when the tank gets empty that can happen.Some tanks will act like a bladder & exaust will compress in there if its having a hard time pushing through a overly long line to begin with, when you let off throttle it backflows back into the pipe.Your friend did change the line when he changed the tank didnt he? Was it replaced with the same length? Some tanks do get updated & the exit hole diamiter changes causing a change in tune. Are you sure the pickup line is good inside the tank? If its the kind of tank that has a short piece of fuel tube between the exit at the top & stone filter at the bottom I have seen those tubes split & cause an internal air leak when the fuel drops below the split. I have 3 cars that will work with excessivly long fuel lines & 4 that wont work/tune right unless the line length is close to what the manual calls for. I know you said the tune is good but it does sound like & tune problem to me unless the second tank in a row goes back to a rich tune & it does that all day.The fuel line theory is just something I pulled out of my ass since it has happened to me before & sounds just like what you described.
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Old 03-06-2009, 07:30 PM   #13
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This is the tank ... :




I will test out the fuel lines and all, just checked carb engine (my friend left the truggy with me so tomorrow i can work on it as a favor) and all seems ok on the carb side no air leaks (i mounted the engine on my buggy and tested 2 tanks perfect).

So the problem seems to be the lines or the tank, tomorrow will see and post here,

cya !

Pat
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Old 03-06-2009, 07:42 PM   #14
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This is the tank ... :




I will test out the fuel lines and all, just checked carb engine (my friend left the truggy with me so tomorrow i can work on it as a favor) and all seems ok on the carb side no air leaks (i mounted the engine on my buggy and tested 2 tanks perfect).

So the problem seems to be the lines or the tank, tomorrow will see and post here,

cya !

Pat
You mounted the engine with the same exhaust pipe & header? If not, that can be subject to leakage also (the header to pipe gasket).
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Old 03-07-2009, 10:57 AM   #15
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After checking all and change 2 tanks (NIB Hyper ones), i put a CEN Truggy tank and the engine its 100% ok ... lol !!

Someone told me on another forum that the Hyper ST being a very good truggy, sucks a little on tank air filtering ...

cya and thanks guys,
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