half tank lean

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savagecre

savagecre

Well-Known Member
Messages
392
What is this exacly Ive got alittle over 1 gallon through my savage25 ltd ed and Ive noticed that on a full tank and just strarting up its slugish for a little bit but at like half tank its rippin and more responsive I don't have my temp reader here so i cant tell if the temp is rising or not is this the half tank lean that you guys have been talkin about ?
 
Yep, that would be it. Be careful though, the temps can get way over 300 towards the bottom of the tank. I recommend a mid tank, but thats just me.
 
Xjeepguy said:
Yep, that would be it. Be careful though, the temps can get way over 300 towards the bottom of the tank. I recommend a mid tank, but thats just me.
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thanx but what is a mid tank? sorry I'm new. can I just go down to my lhs and get one?
 
No, it is a mod you have to build. Basically it is putting a 1/8 scale buggy tank in the radio box, and mounting the throttle servo next to the engine. If I can find some pics of my old savage, I will post a pic or two in the morning when I get home.
 
from what i know, u can also get a newer stock tank that does not have a primer pump. this seems to help cut down on the mid tank lean issues.

i've also heard of people running longer pressure lines or adding a fuel filter to the pressure line.
 
what is exacly happening is more air being drawn into the motor if i new what was happening I could better fix the problem I'm trying to emagine whats happening but confused????/
 
there are a few design flaws causing the half tank lean problem. some never have it and some its really bad with. I'm not really sure why that is, i know the newer primerless, and rear fuel pickup tanks really helped[ i have heard of some with the old tank not ever having the problem.]. others have changed the length of the pressure lines and had success getting rid of it. i tried all those mods to git rid of it and none worked for me. what i finally figgured out , is the tank when full the fuel level is above the carb inlet. so with the increased tank pressure[tank being full and running wot the engine is putting out a lot of pressure and the tank cant hold it all]at full helps to flood the engine. then it needs to be leaned to compensate for the higher pressure. then when the tank is below the carb , around the seam in the tank[1/2 tank] there is room for the pressure inside the tank and fuel flows normal again, which makes the carb settings be too lean.but as the tank emptys it gets worse as the air volume is increased and pressure isnt . my truck would continue to lean itsself out through the reat of the tank. most trucks do this but nownere as bad. most have the tank mounted so that the full level is just at the bottom of the carb inlet, so its not as noticable,if any. the header tank and the mid tank mods are the only real fix for the problem[ at least out of everything i tried]. i mounted a small tank in the back of my truck and i put it lower than the carb and it really helped, the header tank is full all the time and dosent change flow or pressure going to the carb. it makes the engine run a lot more consistent, through the entire tank of fuel. my first set up of the heaer tank i mounted it on the side of the roll bar. it fixed the problem but was easily destyoyed in a roll over with the body off .it also would flood the engine very easialy while it was running causing me to have to lean the carb more than needed and would make the engine run hot. that way really narrowed my tuning range.[ from the point of being too rich and flooding badly and too lean and running too hot] the header is a bit of a pain to fill on the first run and draining it after the last run.i only run it empty if i am done for the day then i have to pump out the little bit left that the engine wont pickup.its worth the little extra work needed on the first and last runs because it really works.here some pics.DSCF0019.webp
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RIGHT ON thanx jrstorm hey them wheels look good man got to get me a set still tryin to save up the money.... I'll be makin a tank next .
 
So let me get this the pressure coming from the exahust pipe hose to the tank is very high at WOT causing the fuel to less'n it's flow to the motor and getting worse as the level goes down... If that is correct then wouldnt an inline pressure flow control help that without the other tank..at my job we have little flow controls for air pressure up to 120 psi. we use them to control the rate that the air moves through they come with barb fittings that fit the I.D. of the fuel line ........cuz if I'm under standing you correctly then this should work also??
 
the pressure is part of the problem. fluid seaks its own level. so in the savage tank the fuel is above the carb at full tank and then below the carb after half tank. so the tank position and pressure inconsistencys together are causing the problem.i don't know how high the psi is at wot in the tank, but i played with the tank and blowing into the full tank with a old line to see what happens. it really dosent take much pressure to make the fuel flow. the pressure needs to be automaticly addjusted for the flow to be consistent. what the header tank dose it it is a constant supply of fuel. when the tank is filled by the main tank it stays full throughout the entire run time til the main tank is emptyed.so the pressure changing really dosent affect the engine, and it never leans out. adding a pressure fitting might help but it has to be addjusted as the tank emptys. the header tank cost me 3 dollars and it took a few mins to install it and eliminated all running problems caused by the poor tank design. the pressure in the line from the exhause isnt the entire problem it dose aggrevate the problem though. iv heard of people using longer lines to help, iv also heard that using a diffrent pipe also helps some too. iv even heard of adding an inline fuel filter with the filter removed to act as a pressure chamber to help with the problem. i tried all of these and none worked for me.the header tank fixed the problem for me because it consistently feeds the carb a constant fuel supply , and the header tank isnt affected by any changing pressure from the exhaust.,since the pickup is always flooded and the tank is small enough that the pressure provided by the main tank is always keeping the small tank full. the engine never really has a chance to empty it until the main tank is empty.even then it really never leans out til its pretty much empty.what i do when I'm done running for the day is, i let the engine use all the fuel it can from the header tank.then when the tank[header ] gets pretty close to empty then i let the truck sit and idle, then i turn the header tank so it has all the fuel around the pickup in the tank. then i let it run til it sucks up all it can. it leans out a little but not enough to hurt anything.it works pretty good and gets out all but a few drops of fuel. i hope this helps.
 
No offense Jrstorm, but to make a long story short. The header tank, or mid tank, supplies a constant supply of fuel to the carb allowing it to not have to pull fuel, it is just supplied with it no matter what is happening with pressure, and doesn't care if the fuel is sloshing in the main tank. I have seen air bubbles in the line leading to the header tank, but never make it to the carb line. It also supplies it with fuel when it is flipped over, and adds a little runtime. Do it Savageacre, your engine will love you for it. I bash in rough country, so I mounted mine inside the carry handle so it don't break. You have to turn the air filter 90 degrees to the left( if you are looking a the rear of the savage), but you gotta watch it or it will rub on the spur. I hope this helps, and don't make mr storm upset because he helped me through this same ordeal with mine.
 
no prob garagedoorguy. just trying to help.i have noticed the things you mentioned also. there are so many diffrent theorys and versoins, and things that have worked for others and not for all. some have not ever even had the problem.you pointed out and explained what i was trying to, only in simpler terms. the fact is the header gives a cleaner and more constant supply. it works where other ideas don't. i think if hpi adapted this design it would make a lot better truck for everyone.
 
NO KIDDING. The Warhead comes with a header tank. DUUUUHHHHH. You can look at the warhead, and see the similarities to the savage. Maybe HPI leaves a little "hobby" in it, but nobody will make me believe that they never came across this problem while testing.
 
yeah It helps I get how the header tank works especially if it stays full untill the main tank runs out.. it acts like a buffer between the main tank and the carb cool my lhs dosent know anything about it. then when I read on this site which is the best site around I think No sooner then I was reading about it , it started acting weird.
 
Take what the guys at the hobbyshop say with a grain of salt. They will send you out the door chaising your tail in a heartbeat.
 
one of the lhs around me have never been able to get rid of the half tank lean prob. i showed them my truck and they were really impressed and are going to start installing the header tank when they have a truck that comes in and cant be fixed.my truck didnt do it til almost a gallon was ran through mine. i don't know what the real problem is but the header tank really works.
 
Ok let's put it into easy terms. The savage like's fuel like a cold beer. When it's full it's happy. About half way down, it wants to just chug it down, and on to the next tank, or beer. lol. If it has a constant flow, it delays the chugging factor. Hope this helps.

P.S. Do not take this as valuable information. lol.
 
Yeah hey I don't want to make any body upset last thing on my mind. Just have to know why its who I am. I payed 580 out the door for the truck starter kit extra set of rims and tires fuel don't want to have to spend 100 to 200 for a motor that fast I've only had this truck like one month got one gallon one quart through it now and well like I said I HAVE TO KNOW WHY IT IS on enything so THANKS YOU GUYS ROCK ...
 
no problem. iv had my ss 4.6 savage since thanksgiving. i was really dissipointed in the 4.6 engine from the start. the pull cord broke on the first pull. all my other trucks have been on the bumpstart. so i had no real experiance with pullstarts[ enough to hate them] so i was a little dissapointed in the pullstart. well after i got all that worked out i pretty quick found out about the half tank lean problem. I'm like you i want to know why somthing dose what it dose. i tried all kinds of mods to fix it. most info on the problem i could find, all pointed to changing the pickup position of the tank[mid tank mod, and the header tank] all the other mods were just good gueses at the problem. some of the other mods help some people but not all . i tried every mod i could find, and none helped me. i first thought it was a header /pipe combo that was causing it[ tried a diffrent one -no affect] then i was reading about someone doing a air pressure mod and i tried that it didnt work. [ they take a clunk and a pice of antena tube and put the tube on the nipple where it goes into the top of the tank and put the clunk on the other end of the antena tube and let it go to the bottom of the tank . they say this changes the pressure in the tank ,but all it did for me was put air bubbles in my fuel.-like an aquarium air stone] so i really started looking at the fuel tank design and saw where the header tanks solved the problem. so i tried it and it worked. so i have had a crash coruse on fixing savages . glad to help..
 
oh yea, i have been thinking the engines only lasting for 3 gallons or so, might of been because of the lean problem.my first engine was doing it bad and didnt really notice it til i started to lean for power. i had to keep it really rich at the bottom of the tank and lean it out to run at a full tank. even the rich settings at the bottom half of the tank made it still overheat with any hard bashing. i think it helped wear out the engines quicker. especially since i had to tune it all the time. the second engine lasted a gallon longer than the first did. but i had added the header tank at maby 1.5 gallons. the picco was just a problem from the get go. it was having problems in my other trucks too, so i really think i just got a bad one and need to send it back to ofna for replacement. i wouldent worrie about the engine , the .25 engine is an excellent engine and is very reliable. the 4.6 the 2 i had were problems but by the time i got the header tank installed on the second engine i think i had already done damage. so i really cant fairly criticize the 4.6 engine. but the main bearing went on both 4.6 engines i had, and thats nothing that the tank problems would of caused. when i finally took the last 4.6 engine out for good , it wasnt running. it would die very quickly or wouldent run at all. the problem was the main bearings were skipping their races and jambing up the crank and stopping the engine or making it stall and not start easy.what engine do you have?
 
you know its probably the manufactures way of makin money they rely on us buyin another motor from them lol Ive got the 25
 
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yep i agree. my buddy and i were talking about that this weekend. the mfg would rather us having problems. my lhs told me that they make more money in parts than on trucks sold there.so thats probally true of the companys too. if the trucks were indestructable then they would loose money.
 
I’m having a similar problem with my savage X 4.1. After reading this thread I realized that the problem started after I installed the optional exhaust cone. Could this be causing my half tank problems? It would make sense that it is causing more pressure in the pipe as it restricts the exhaust flow resulting in a more erratic fuel flow. I’ll try removing it tomorrow.
 
one thing iv seen thats part of the problem that i haven't figured out yet, is the exhaust. iv heard of people who have changed to all copper lines and the problem being fixed.iv also heard people have put a empty fuel filter in the pressure line and it fixed it . iv tried the filter in the pressure line and it didnt work for me. iv also heard of people changing the exhaust and it fixed it. it didnt work for me either. my truck didnt do it til i was into the second gallon of fuel and it was getting worse the more i ran it. i had to do somthing and nothing i tried worked for me.try changing the cone out of the pipe and see what happens. the exhaust pressure is part of the problem with it so the more flow to the tank the better. good luck...
 
Removing the cone helped, it made the truck drivable but there is still a problem. I have a spare fuel filter so I'm going to try that. I hope it works, if not I’ll be building a header tank.
 
i still fail to see how the header tank helps. surely the pressure from the exaust to the fuel tank still fluctuates and the amount of pressure needed to keep a steady flow of fuel from the fuel tank to the header tank and on to the carb would be the same as needed without a header tank in the system?

but if it works i'll try it, the only problem i have is that to drive/race or bash on UK offroad tracks you are not allowed modified fuel tanks so i'd have to take the header tank out and retune before i could go on a purpose built track 🙁

oh and just to clarify - the best position for the header tank is below the carb needle or above?

Thanks
 
you are right . the exhaust pressure is still going to flucuate. but having the header tank installed makes the engine get a constant supply of fuel to the engine nomatter how whe pressure changes. the pressure is what delivers the fuel. the stock tank shape is more of what causes the problem, because the carb has to work harder to get the fuel to flow to it correctly. the header tank stays full all the time so the carb has a constant resivore to draw from. when the main tank goes below the carb height it causes the engine to get leaned out, but with the header tank the engine never sees this so it dosent lean out. also when the stock tank is full it is above the carb which makes you tune it leaner to get it to run correctly ,so when the fuel is nolonger being forced to the carb[because of gravaty] it gets leaned out. the best place iv found for the header is behind the shock tower. it is close to even with the carb, you still don't want it to be above the carb too much to keep it from flooding the carb. also this isnt a mod for more speed or power ,or really even longer run times, its simply for making a bad designed tank work correctly. there are a few trucks out that come with it stock
 
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