Primerless tank does lean out when the fuel level drops

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rossb

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Last week I said that the optional HPI primerless tank I used with my SS kit did not lean out as the fuel level dropped. Well, I was wrong. Today I ran about 5 tanks thorough the truck and each time I waited until the tank was almost empty before refilling.

Today the outside temps were high 80s, low 90s and the humidity was about 90%. When the tank was full I tuned the HSN so that is was about 1/8th of a turn clockwise (lean) from the rich bogging point. The motor made pretty good power and the temps after a few wide-open blasts were about 245. The LSN was tuned so that it took off smoothly after a 1-10 second idle and a pinch test caused the motor to lean out and die after about 3 seconds. The motor felt like it had a bit more in it and it may have been bogging very slightly at high RPM, especially after letting it sit and idle for a while.

I changed nothing on the truck and when the fuel level dropped to about 50% the truck felt like it got faster. It was lifting very small wheelies on pavement (first time it did that) and it was lifting and riding out perfect 50 degree power wheelies in the grass when it hit an uneven part in the grass. I took the temps after a few blasts and they went up to the high 270s. I refilled the tank and the truck slowed a bit again and the temps dropped.

So basically when my tank was full the truck was very close to the rich bogging point. If I let it idle and cool for a while with a full tank it would bog for the first few passes until it heated up. When the tank got near empty it leaned out quite a bit (almost too much).

Does this sound right?
 
Tune the engine for optimum power at half a tank so when it's full, it's rich, then by then time it's down to nothing, it'll only be slightly lean. It sounds like you're pretty close tunine wise
 
Mine's like that too, but I haven't tried to re-fuel it when it starts to lean, that things tank is huge and can't afford to waste fuel.
 
you can try doing the dual tank mod. Fairly simple to do and not that expensive.
 
Have you looked into the UniFlow mod?

Basically you drill out the hole on the underside of the fuel tank cap, then thread a fuel pressure-fitting into the hole. Then run a piece of fuel line from the pressure fitting to the bottom of the tank (an airplane-type fuel "clunk" works well to keep the fuel line at the bottom of the tank). Cut the piece of fuel line so that with the cap closed, the clunk lays horizontal on the bottom of the tank without flopping around too much.

I just did this before firing up a Mach .26 for break-in, and I get constant temps for the whole tank (it leans out right at the last bit of fuel in the tank, but I really dont think it happens long enough to create a problem). I was able to tune at the top, middle, and towards the bottom of the tank, all the while getting pretty much the same temps (180-210).

I think its a little easier than the header tank mod. One "gotcha" is that you need to run a longer pressure line from the exhaust to the tank (15" is recommended) because fuel will come thru the line in the tank if you try to prime it. A longer pressure line gives you a lot more time before priming causes fuel to come back out.

-Chris
 
I have read on one of the forums, about putting a one-way valve between the exaust and the fuel tank. Has anyone tried this?

Purchase a $2 valve for an aquarium. Make sure the arrow is going towards the tank. The engine is only making pressure on the "exaust" stroke. The one-way valve is keeping the pressure in the tank while on the intake stroke.

Here is a link that explains it better. http://globalservices.globalhobby.com/globalforum/viewtopic.php?t=1964&start=0
This is from the X-term forum, but would work on all cars. I haven't tried it yet. Bought the valve, but everything I have is broke. If you are thinking it is a fix for the 24/7 engine only... Don't. Haven't had any problems with mine, yet.

Would like to here what everyone thinks. Does make sense. :cheers:
 
Couple Savage guys have tried this, and they noticed that at idle, it causes fuel to fill up and flood the carb after shutting down. The pressure built into the tank has to go somewhere!!! The check valve doesn't allow it to go that way, so instead it forces fuel into the carb after shutdown (unless you can quickly open the cap!!!)

Seems like UniFlow might be the best fix, but the check valve is easy, as long as you remember to pop the cap.

-Chris
 
chrisexv6 said:
Have you looked into the UniFlow mod?

Basically you drill out the hole on the underside of the fuel tank cap, then thread a fuel pressure-fitting into the hole. Then run a piece of fuel line from the pressure fitting to the bottom of the tank (an airplane-type fuel "clunk" works well to keep the fuel line at the bottom of the tank). Cut the piece of fuel line so that with the cap closed, the clunk lays horizontal on the bottom of the tank without flopping around too much.
Click to expand...

How is running a fuel line through the cap to the bottom of the tank any different than the HPI primerless tank setup? The primerless tank also uses a pickup at the bottom of the tank with the exit near the top of the tank.
 
The reason it leans out is because as the fuel level drops, you have more air in the tank, but the same amount of pressure coming from the exhaust. Since we can't adjust the amount of pressure coming into the tank, the only other option is to regulate the empty space/fuel in tank ratio.

By adding the pressure fitting, line, and clunk, you are in effect extending the pressure line right to the bottom of the tank. What that means is that you have the same amount of fuel flowing because the clunk will always be submerged in some amount of fuel.

Its used commonly in RC planes (in fact, I was able to get most of the parts for this mod by purchasing a small inexpensive airplane fuel tank), and works well in land vehicles as well.

I dont know if linking to other forums is taboo, but try reading here: http://www.savage-central.com/ftopic22106.html

-Chris
 
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Oh I see, I mistakenly thought that you were running a new pickup to the bottom of the tank. You are running the pressure feed line to the bottom. Does that mod create air bubbles in the fuel, like putting a straw into a full glass and blowing into the straw?

I'm not really convinced that this mod will actually work. Even though the pressure line is submerged there is still the same amount of air in the tank as there was before the mod. I don't think that putting the pressure line under the fuel would change the pressure in the tank.
 
Last edited:
Like I said, during break-in, I was able to get consistent temps from full to empty tank. I'm using a Savage '25 (primerless) tank, mounted in the radio box position (moved the throttle servo back next to the engine).

Normally (with my Tmaxx) I could tell when I was low on fuel by bringing the truck in and noticing higher temps. Now I bring the Savage in and actually have to check the fuel level because the temps are so much more consistent.

This also explains it better than I can: http://www.fraserker.com/heli/uniflow/how_uniflow_works.htm

Its worth a shot at least!!! I'm absolutely convinced it works on mine. And I havent seen any air bubbles in the fuel the few times I've checked.

-Chris
 
Thanks for the link, it is very helpful. Do you have pictures of this mod on your ride?
 
You could try a different tank as well.

072704-SavageTankSide.JPG


I've been using this tank for about 1.5 gallons. Very minimal leaning effect as it empties and it does really well against foaming. I just made little metel L brackets and drilled a couple holes in them to mount them to the truck and the tank to the mounts.

The only downside is that it's a 125cc tank. But on the upside, minimal (if any) leaning effect, durable and relatively cheap. I think it's $10 at tower. It also allows easier access to the side of the engine.
 
Same tank but cleaner than do it yourself hardware. You can do it yourself for about $12.

And this one mounts in the stock tank place:Ofna Stock
This one mounts behind the rear shock tower:Ofna Rear

This is the tank by itself: Ofna 125cc Tank

IMHO, the new era one that mounts in the stock place puts it too close to the rear shocks.
 

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