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Old 04-07-2010, 06:01 PM   #1
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Questions?? Tune over a tank of fuel

I think I've got tuning under control, I can get the engine to tune just how I need it. BUT... the tune changes during the tank of gas. i.e. it's richer with a full tank and it gradually leans out as the tank burns through. If I tune it just right with 1/2 tank, then I'm a little rich with a full tank and a little lean at the end. Is this expected or typical? If not, what could I look for to try to alleviate the issue?
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Old 04-07-2010, 06:15 PM   #2
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Default Best question ever......

You just asked the million dollar question. I've had buggies that didn't change that much,and then some that do. My 808 changes more than I'd like, but I've found that tuning at half tank is the best way but only when it's warmed up really good. It's normal to run a little fat at the beginning of a race because your running more than two tanks during a race and it is better to err on the rich side for the longer mains. Unless it's late at night and gets humid or other changes. Afte the engine gets race hot the next fill up will be not so fat. I like to warm my engine up as the race before is going on, then shut it off so it will warm up quicker or if the race has a couple of laps left just leave it running unless there is a big break between races, but yes, it's normal to have this change in tuning as the tank gets empty, but some tanks are worse than others in my experience....
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Old 04-07-2010, 06:26 PM   #3
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try a longer pressure line, helps me keep a more consistent tune through the whole tank.
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Old 04-07-2010, 06:29 PM   #4
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this is a question with kind of a yes and no answer. Yes you are going to have a little change as the tank runs dry but it should not be to drastic. after you start your engine, it has a vacum effect on the fuel inlet. also when the tank is full, you also have a slight feed from gravity. as you burn through the tank it has less of that gavity effect and you engine is forced to suck a little harder ( not in that way ). that is why we run pressure from the exhaust. it pressurizes the tank to keep the fuel flowing to the carb. if you take the pressure of the tank, all you have is the vacum from the engine. it is very hard for it to keep the fuel flowing at the correct amount. I would look very hard at your tank. if there is a crack in the pick up line it will be sucking air as well as fuel when the tank gets to a low amount. also check your pressure line for cracks and try to keep it as short as possible without kinking it. a long pressure line will lower the pressure in your tank and cause it to tune a little eratic like you said. hope this helps.
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Old 04-07-2010, 07:25 PM   #5
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Quote:
Originally Posted by j_blaze14 View Post
try a longer pressure line, helps me keep a more consistent tune through the whole tank.
Quote:
Originally Posted by Jerm13 View Post
this is a question with kind of a yes and no answer. Yes you are going to have a little change as the tank runs dry but it should not be to drastic. after you start your engine, it has a vacum effect on the fuel inlet. also when the tank is full, you also have a slight feed from gravity. as you burn through the tank it has less of that gavity effect and you engine is forced to suck a little harder ( not in that way ). that is why we run pressure from the exhaust. it pressurizes the tank to keep the fuel flowing to the carb. if you take the pressure of the tank, all you have is the vacum from the engine. it is very hard for it to keep the fuel flowing at the correct amount. I would look very hard at your tank. if there is a crack in the pick up line it will be sucking air as well as fuel when the tank gets to a low amount. also check your pressure line for cracks and try to keep it as short as possible without kinking it. a long pressure line will lower the pressure in your tank and cause it to tune a little eratic like you said. hope this helps.
Well, this isn't confusing at all.... Anyone care to clear this up?

I've seen recommendations that said to run a loop of tubing in the pressure line, but that's to correct the full tank lean bog cause by fuel sloshing into the pressure line on RH turns with a full tank. I don't know how tuing length would affect tune consistency between full and empty.
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Old 04-07-2010, 07:34 PM   #6
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I'm guessing you are trying to tune your engine cold. To get a good tune your engine needs to be up to temp before you try to tune it. Like mentioned before, your tune should not drastically change as your fuel drops in your tank. Your engine is probably just starting to reach temp half way thru the tank. Pressure is not even needed for these engines to run. I use to run my gas truck with a .12 without a pressure line. It had a much smoother throttle response. You had to tune it different, but it ran great without it.
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Old 04-07-2010, 08:44 PM   #7
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Quote:
Originally Posted by Jerm13 View Post
this is a question with kind of a yes and no answer. Yes you are going to have a little change as the tank runs dry but it should not be to drastic. after you start your engine, it has a vacum effect on the fuel inlet. also when the tank is full, you also have a slight feed from gravity. as you burn through the tank it has less of that gavity effect and you engine is forced to suck a little harder ( not in that way ). that is why we run pressure from the exhaust. it pressurizes the tank to keep the fuel flowing to the carb. if you take the pressure of the tank, all you have is the vacum from the engine. it is very hard for it to keep the fuel flowing at the correct amount. I would look very hard at your tank. if there is a crack in the pick up line it will be sucking air as well as fuel when the tank gets to a low amount. also check your pressure line for cracks and try to keep it as short as possible without kinking it. a long pressure line will lower the pressure in your tank and cause it to tune a little eratic like you said. hope this helps.



so 8 inches of fuel tubing to the exhaust is to much?
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Old 04-07-2010, 10:23 PM   #8
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I've always used the same amount of pressure line as I do fuel line.. about 8" both ways.. Seems to work well..
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Old 04-08-2010, 12:36 AM   #9
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yes 8" BOTH WAYS
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Old 04-08-2010, 03:37 AM   #10
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Next time I'm at the hobby shop I'll pick up some more line and give that a shot.

The motor is definitely hot when I tune it, it just burned 1/2 tank before I started tuning! The change over the tank is probably equivalent to 1/8 or 3/16 turn on both HSN and LSN.
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