Did the uniflow tank mod

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rossb

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The S25 RTR primerless tank I put on my SS was causing the motor to lean as the fuel level dropped. I spoke about that here:
https://www.rcnitrotalk.com/forum/showthread.php?t=20420

I went to my LHS (a real big one) to get some fuel line to do the uniflow mod and I spoke to the main tech guy there. He insisted that a nitro vehicle like a Savage will never lean out when going from a full tank to 1/4 tank. I explained that the laws of physics and the pressure differences in the tank with different levels of fuel is what causes the leaning and he said that I was wrong. He said that he works on these trucks every single day and he never saw one lean out from a full tank down to a 1/4 tank. He went on to say that if mine is leaning when the fuel level drops then something is wrong with my setup. He also said that you "cannot believe what you read on the Internet". We disagreed on a few more things and I stopped talking with him when he insisted that stopping the motor by plugging the exhaust outlet will ruin my motor. Even though he insisted that the uniflow mod was a bad idea (he never heard of it before I mentioned it) I left with 3 feet of fuel line and an Ofna high flow screen disc filter. Here are pics of the install.

Put a small section of a Bic ink tube into the cap.

Ran fuel line from cap down to the bottom of the tank.

View inside the tank.

23 inch pressure line with the Ofna filter on the front shock tower.

I did not drive the truck yet.
 
Well bro, most of the info I get is on the intenet!!! Got a hobby shop that is a 5 min. drive. I drive about an hour or so if I need something. Or I order it off the net. This guy want's $34 for a gallon of fuel. I tried to talk him down once. He told me he could get cheaper fuel, but it was old and he didn't want to sell somebody fuel that would mess up their engine. I started to ask him if his $12 glow plugs were fresh too! (O.S. #8) I've ran about 2 gallons of $22 fuel through my X-term. It's still got so much compression I'm scared I'm going to break the string on the pullstart. Had this marked for $45. http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXEMG5&P=ML

And another thing. If they can't make money on fixing your "junk", they have never heard of that problem!!! Doesn't matter what ya have, whatever they have to sell is better! Sorry for getting wound up. I have a bad taste in my mouth from my "LHS". Find a hobby shop that ya like. No matter how small they are, you are their customer. If they don't have any of those, they won't be there long.

Let us know how the mod goes. I haven't ran mine that much. Not noticed it leaning out. Putting the body on raised it by 20 degrees or so. Still not got it tuned just right. :cheers:
 
The Mod does work

I did this style of mod on a T-Maxx tank, of which doesn't have the pressure fitting like a Savage has, but it does work. Had to use the fittings off of a can filler kit and cut where the inlet pressure nipple is on the T-Maxx tank, and screw this in its place, and used a klunk to keep it to the bottom. I run two tanks on this, one being stock, and the other a Ofna tank in the front and used a standard antenna mast to put in the hole in the cap and ran the tube and a klunk that sat on the bottom. Before I did the mods I was getting a temp variation of over 80-100 degrees from a full tank to a empty tank. They are gravity fed and really bad at the bottom above 300 many times. Now peak temps with both tanks are in the 240-250 range.
 
I ran a full quart through the truck yesterday. It started easily and idled easily. After I retuned the motor it ran a consistent tune from a full tank right down to the bottom of the tank. I did not notice any significant bubbling or foaming in the fuel tank.

The only problem I saw was when I was priming the carb fuel started to flow up the pressure line towards the muffler. It never actually got to the muffler but it was fairly close. After the motor started the fuel was forced back into the tank and it stayed there. I stopped the motor once (with the tank still pressurized) and I got a slight amount of fuel flowing towards the muffler, but it never got past the filter in the pressure line.
 
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looks good. sounds like the guy at the lhs has no idea what hes talking about but just tries to act like he knows everything. i hate people like that. haha. but hope it works out for you and the leaning out quits.
savager
 
When you shut down your engine, how did you do so? I noticed that if I pinched it, like I'm used to, the engine is hard to restart warm. Not sure if the UniFlow is causing it, or just the fact that the metal carb may have been vaporizing the fuel before it go to the engine, but figured Id ask how your warm-restarts are after shutting it down (I've also heard it suggested that I plug the exhaust to shut it down, that way there is still some fuel in the engine for a restart).

Thanks!!!
-Chris
 
I plugged the exhaust with a Dubro Kwik-Stop. I let the motor sit (at bottom dead center) for 5 minutes just to make sure the fuel was not going to run out the exhaust. It took about 3 pulls to restart it.

Another thing I forgot to mention, right after opening and closing the tank cap for a re-fuel (with the motor running) the idle dropped a bit. It sounded like it was going to stall. The idle came right back after I goosed the motor a bit and then it stayed at the proper level for the rest of the tank. I tried opening up the idle speed setting on the carb but that just increased the "normal" idle speed and it did not solve the temporary dip I got after refueling. I do realize that the tank pressure is always lost when opening the tank cap. It sounded like my truck was running very rich at idle after opening and closing the tank cap. That sounds backwards because a loss of tank pressure should cause a lean condition.

I am not sure if this was related to the Uniflow mod or if it was related to something else like maybe the weather conditions. I've run 1.5 gallons through this motor and that was the first time the idle dropped right after re-fueling.
 
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I found if you use the antenna wire tube not as much fuel goes up into the exaust and you dont have to use such a long exhaust hose.
 
well I just did this uniflow mod to see how it reacts with my new Wasp .26
So far during break in of the new mill, it has been leaning out a very slight bit with full or half tanks and then drops down to normal. Not sure if just a tuning issue since it's still breaking in but I shall try the uniflow. If it don't work that great then it's a easy fix to take off. I used a threaded inlet on the tank cap instead of a ink pen tube or antenna tube.
 
With the stock tank setup you should be rich when full and leaner when empty.

BTW - What airfilter are you using with the Picco? I read that the Savage Motorsaver elbow will not fit over the Picco's bigger carb. Did you have to shim the clutch bell to get it to align with the spur?
 
Rossb, don't worry. The airfilter fits nicely and so does the clutchbell. I've got a picco and that type of filter and it fits well.
:)
 
dear rossp,
i will put a fuel line to my fuel tank cap!
does it good?i mean does it have good performance?
have you done this before?
 
Wow a bit of dust on this tread....this is from 2004...a relic! Um, I have heard of the uniflow mod before and in theory it should work...but I have never really seen it in action before. Have though about doing it just never have.
 
I just recently ran my savage with this mod. Be careful as it floods the engine really really easy. Seemed to run well though with a very stable tune from full to empty.
 
Yeah Olds seems I do remember hearing you could flood it really easy. I have never tried it so...I have never had much trouble with the half tank lean really so I am hesitant to do it.
 

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