Half tank lean

[Blazer X]

Alright so I got my new lrp 28 dialed in but for some reason this engine has a severe case of half tank lean...I mean so bad it will barely run. Should I just fatten it up on a full tank and let it be? I've tried two different tanks and fuel line is good.

 

[Jeremy]

This is what mega told me and IT WORKS !

fill the tank up all the way, and warm it up, start tuning at half tank.. after that don't fill the tank up, just fill it half way,

 

[Blazer X]

so you never fill your tank up?

 

[Jeremy]

not all the way no

 

[Fx]

Mega told me the same thing at the rc jam. Fill it up to the shelf and retune. My axial used to run 280+ now it's hard to get it over 230.

 

[savagecre]

My Platinum kinda did the same thing but I tuned it at the half tank. Now when I fill her up all the way it runns a little boggish for a bit. But quickly becomes a power house.. I snapped my front dog bone yesterday just about in the middle..not sure if its from the motor or just old...

I dropped the dynamite fuel and started using Odonnels 25% and temps have dropped. The platinum is insane at temps of 220 so I run at 220 & 230 very reliably.

 

[savagecre]

Oh and if I top her off I do just fill it just above the half way point so yeah try that.

 

[polystyrene]

half tank lean is nothing to do with the shelf in the tank. the pressure never changes. heres a better explanation:

"The issue with the savage is different, the carb pick up is below the top line of the tank (when fitted to a Savage) as liquid finds its own level its much richer at the start than at the end, because at the end of the tank the fuel level is below the carb pick up.

On a buggy/truggy the entry to the carb is higher than a full tank of fuel."

 

[Jeremy]

This thread could go on forever ...............................

 

[Fx]

half tank lean is nothing to do with the shelf in the tank. the pressure never changes. heres a better explanation:

"The issue with the savage is different, the carb pick up is below the top line of the tank (when fitted to a Savage) as liquid finds its own level its much richer at the start than at the end, because at the end of the tank the fuel level is below the carb pick up.

On a buggy/truggy the entry to the carb is higher than a full tank of fuel."

Hmm don't think anyone has yet said half lean has to do with the shelf.

 

[DOOBEEWHA]

This thread could go on forever ...............................

Heard that... It's just to bad the the dang engine doesn't get the pressure up enough on the return line to make this issue a thing of the past. A fuel pump is the ultimate answer to this issue if you haven't already gone mid-tank.

 

[awya14]

I almost bought a rd logics 200cc tank yesterday ..thought If I had more fuel I could change the lean factor ..but I guess i was wrong
http://www.rdlogics.com/cgi-bin/category.cgi?item=SVG-075&type=store

 

[awya14]

you'd think someones figured something out by now ..theres gotta be a good mod ..somewhere

 

[DOOBEEWHA]

Hey awa14.
Like I said in the above post, bro. The mid tank can address it.
At least thats what JT told me when I asked him about his conversion some time back... He said he had no issues with it.
And I believe him.
And then, theres this mod.
If you want to go through the trouble to install and maintain a fuel pump, it can be employed to regulate the fuel at a constant pressure. It works good , but requires batteries.

There's also a little trick thats been done where you use an extra long fuel line with the excess tubing coiled up above the tanks level which in theroy, re-establishes the tanks fuel
draw point higher with a volume of fuel stationed above the carbs location. It's a quick way to simulate a secondary tank. I haven't tried it yet, but mega was explainning it in the shout box one time and told me its been done. Good Luck

 

[Blazer X]

Another thing that baffles me is that my hpi engines did it but nothing even close to this bad.

 

[Wooha]

you guys gotta tune ur fferratiszstat in the truck but yeah... how about that rear fuel tank mod? it makes it lever if not a little higher then the engine

 

[gzus11]

someoine please go and read about the basics fluid machanics, I am a qualified engineer, and these theories about half tank lean are not correct, and the mid tank mod does not address the issues of half tank lean.

 

[awya14]

someoine please go and read about the basics fluid machanics, I am a qualified engineer, and these theories about half tank lean are not correct, and the mid tank mod does not address the issues of half tank lean.

well, enlighten us ..full page summary ..on my desk 10 o'clock tomarrow morning!

 

[DOOBEEWHA]

Yea...totally, awya14.

Anyway, the secondard tank is probably the most popular remmedy for the half tank thing. And.... it adds more fuel capacity
to the trucks overall fuel payload. It does have the issue of being in a wierd place to work properly and maybe also has some
vulnerabillity to road rash.

A good example of this is to check out the one thats on the Warhead's setup.

 

[DOOBEEWHA]

someoine please go and read about the basics fluid machanics, I am a qualified engineer, and these theories about half tank lean are not correct, and the mid tank mod does not address the issues of half tank lean.

Tell it to JT.... But before you do, why don't you enlighten us.
If that mid-tank is below the level of the carb input valve, It sure in the **** does address it.
But since you've already read the ins and out on the concept, please share with us the fundamentals. While we wait on this insight, let me give
you an annalogy of this principal that best suits your statement...

Ok, say just for a momment gus11, you have a baby pascifier in your mouth, connected to a tube, that attatches to a big boob of mothers milk....

Are ya still with me here?....
ok... At this point, we'll have to assume that your thirsty... Maybe you've already read this in your text books, I would imagine. But for the rest of
us understudies, I'll continue.
When the boob is above your mouth, the milk flows fast and freely, that is, while you suck.
Now.... If Mums desides the to bring you to her while shes laying down, with you a top, notice that the flow is not flowing so freely.....That's half
tank lean...Does this sound right, anyone?...

Of course, I may be incorrect....
But this works for me. Here's to looking up at our mothers....burp!!!!

 

[DOOBEEWHA]

Hmm don't think anyone has yet said half lean has to do with the shelf.

I don't know about that one, FX. I think polystyrene's right on this point. Besides,... It goes along with supporting my BOOB theroy....lol

 

[gzus11]

Most the mid tank mods I have seen pictures of, the tank is still located so the carb is approximately mid height of the tank.

Doob that analogy is accurate, but as the level of milk reduces it also reduces the flow as the pressure head decreases. And this is ignoring friction in the pipe which severely reduces fluid flow from theoretical values, so adding more fuel line below the tank has no benefits, except restricting the fuel further.

now look at the fuel tank (as it is what we are talking about) in the same way as your analogy Doob, raise it above the carb, increase the pressure head....
pressure out=pressure in(exhaust) + g*h(height between fluid level and carb input) .....And with this we lean the HSN to restrict the flow.

Place the tank below the carb reduces the pressure head..... Pressure out=pressure in - g*h(height between fluid level and carb input) .....So in this case we richen the HSN to allow fuel to flow more easily.

These are the initial conditions, which we can tune for.
OK

Now as we run the engine the fuel level drops, reducing the pressure head in the tank. Max pressure at tank outlet is.... Pin(exhaust) + g h(depth of tank),
and min pressure.... Pin .... Which will be fixed values for every tank (different depth tank, different value) wherever the tank is located in relation to the carb.

This creates another variable we need to consider while tuning the engine. Now without testing and a good handle of differential equations, it is not easy to determine how the fuel flow varies as the tank empties.

So choose a mid point (half emptied tank) and tune the engine to these conditions. - A little rich at full tank and slightly lean at empty tank. This is the reason people recommend run the engine for 2-5 mins to warm it up before tuning the performance, allow the engine to reach operating temperature and fuel level to drop in the tank.

 

[DOOBEEWHA]

Hey gzus11, Thank you for the prompt reply.

And also for taking my analogy as lite hearted as it was meant.
You obviously have some techical background in the understanding and explainning of these physical interactions. This, in regards to SOME of the principals stated here.

Yes, like SJ said, this thread could go on forever...
I guess, the end goal here is to have a solution to this sinario with the half tank lean bug thats forced upon us by design.

At the risk of not having to explain where every single molocule of fuel is located in the fuel supply, we can try to educate the young and uninformed, or simply expand on
esoteric realities that all lead to a bunch of bull.

Instead of this, lets come up with real soultions, rather than judgements. There's no professors here to consult with about dogmatic therories to the laymen.

 

[gzus11]

I suppose what I was trying to say is that this "half tank lean" is not a bug, or a dark art, it is merely an effect of the physical world, fundamental laws of physics, fluid mechanics etc.

Moving the tank, mid tank mod , tuning etc are not solutions to "half tank lean" and in theory have little to no effect on the symptoms.

Proper tuning techniques are the best way to reduce the effects of leaning as the fuel is used.

The mid tank mod has many other benefits which just mask the leaning. Bigger tank, longer duration until severe leaning occurs, position protects the tank from damage, cracks and punctures, and it looks neat when done well, though most servo repositioning and control rod arrangements need much more refinement.

The leaning will continue to occur unless as you said doob a fuel pump is used and the carb develops to be more like full scale carburettors, with a float chamber and better venturi.

To the original post, it is best to try to tune the engine correctly, or get advise/help before severe modification of the truck as this can cause more troubles.

 

[DOOBEEWHA]

Fair enough, gzus11, You certainly made your point clear and consise.

Well done.
There's not too many of us out here that can put it the way your last post expressed it.
Your very correct with taking into account, many the other variables here.

Tunning is key to living with the condition. This, I've always understood.
I must say that the standard Savage setup lends to this issue in the worst way with it's tank location.

Thanks for reminding us of the big picture, gzus11.

 

[awya14]

very impressive!! good show ..everything makes sense now..no mods needed ..just a good varible tune ..deal with the fuel gobbling richness/bogg ..getter where ya want it ..should equal out ,makes sense
A+ gzus..A+

 

[Blazer X]

I will try retuning this week but I think it's still gonna be too fat at the top of the tank to even get it to finish the whole tank. It just dumbfounds me that a new engine that looks like it is the same height as the stock one is causing my problems to this degree...but I'll figure it out it's still a very new engine too me I'm still tweeking it out!

 

[DOOBEEWHA]

I would like to express these observations...
Although, I can truely agree to the principals that gzus11 has clearly stated and feel he's right on target with knowing the propper explanations to this condition, I feel
that some of these theroies metioned don't really effect the savages general condition of use.

By this, I mean to say,... Due to a smaller scale of the tank, this may make the symptoms of head pressure, more infantesimal perhaps.

This compared to a larger mid tank scale of the same mock up.
I'm not sure if there's any actual phisical data around with charts and graphs to confirm this.. lol. It's just a feeling base on my own grasp of this in combination with
the experiences of others included.

In other words,.... I feel that getting the fuel tank of the savage to ride below the carb's fuel inlet is very helpful in reducing the effects of the half tank lean issue.
When you consider the solution that the Warheads secondary tank provides for the Warhead, this setup produces a situation that makes the tanks shelf have a higher
draw point with increased fuel volume assisiting in the siphonning effect that's ALWAYS ABOVE the carb's inlet.
Thus, allowing for a constant siphonning effect of fuel to draw from. A gravity flow, if you will....

If this is incorrect in theroy, whatever the process is,.. IT WORKS enough to mark impovements to the condition...

This same assumption also applies to another approach to this.
It just requires a change in tuning.

Note: (The deminishing head pressure issue will always be a factor in both of these examples. Although, the head pressure's effect in both cases, will be a noticably less.
This is due to the idea that the flow of fuel to the carb never has to make a transition from the siphonning gravity effects from a higher shelf position.
This compared to a tank level thats goes below the carb inlet. This transition has a noticably negitive, pressure drop.)

Hence, Mega's explanation:
Just fill the tank and tune at the half tank point. Once tuned, never fill the tank past the half full point...Or to that effect.

By getting a constant flow of fuel to be either ABOVE or BELOW the carbs inlet WITHOUT COMBINING THE TWO, this provides a more stable platform to tune from.
By doing this, you will have taken away one of the most disruptive elements that de-stablizes a passive tuning attempt. Thus, REDUCING the effects of half tank lean.

- I hope I expressed this clearly enough. I can barely read it myself...lol

 

[gzus11]

just another point i forgot to mention.

pressure head is only affected by the difference in height. the fuel tank could be any shape, wavy tube, have a shelf, spherical, long wide short, has no effect on the change in pressure head.

all i can say about adding a second tank is my experience would lean the flow after 1min wether both tanks were full empty or any combination.
adding another tank to the system only complicates things (analytically doubles the size of the equations due to the added variables)
yes i am sure it works for some people but i think it is more luck than desighn.

i still think a propper tune will run the engine sweet for the entire tank. i never had leaning issues nor has any members of my club.

good luck in the quest to eradicate the problem

PS only trying highlight the principles and thorey, so people don't just go around modding things without trying to understand the situation.

 

[Crash]

congrats doobeewha longest post of day. lol
love your attention to detail in your posts.
keep it my internet browsing friend.........................