rmossesq
Posts: 12
Joined: 5/29/2003
From: Anaheim, DE, USA
Status: offline
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I have had quite a bit of sucess with both the 3000, the 3250 and the 2300 from super tigre over the past 15 years. I have two 4500s but have never mounted them on an airframe (gonna do's you know). The problem that every one is having is quite simple. When you have a tank that is over 16-20oz the fuel level change between full and empty is dramatic to the carb do to gravity. That having been said, I bet some of you are using even larger tanks which makes the problem worse.
Start the engine on the ground at a perfect setting and it will go lean in 5-7 mins in the air because of the fuel level drop. This is why back pressure is so critical to these engines as it helps pushing fuel to the carb against gravitya s the level drops and the carb to the fuel level gets steeper and steeper. Gravity is to blame, not the plug or the muffler. In most cases the engine will stop after about 2/3 of the fuel is gone cause these engines are so thristy they won't spend any time running lean and burning up in the air like a smaller engine would - they just stop all of a sudden.
Unfortunately, must of us fly arfs now and the engine and tank location(s) and postion(s) are decided for us by the manufacturer. In almost all cases, the tanks are too low, particularly when you get to tanks over about 16oz.
I know what some of you are saying already, "But I have another brand of engine and never had a problem". That is because not all engines are the same. Some engines are designed by the factory to have much better fuel draw then others. OS engines are a great example, they normally trade a little peak power for good fuel draw in all of there engines as well as easier overall handling. Super Tigre over the years has tried to make their engines have better fuel draw by modifying the carb venturi size repeatedly, but then when they get the venturi to the size that draws well, people complain that the peak power is down, so they tend to go back to the larger throat carbs. A small venturi 3000 will make the same power as a 2300 and is significantly larger and heavier.
I have always mounted my ST's on the side so that the carb is in line with the top 1/3 of the tank at the least, and in line with the center of the tank if possible. Yes, it will siphin when the tank is full and the engine is stopped, but you just have to use a clamp in the fuel line to the carb to stop that. Another big help is a perry pump with backplate pressure. This will solve all of your problems, but you will have to fiddle with the needles once the pump is installed (Don't adjust the nut on the pump - ever!). I have run a 2300 on back plate pressure alone very sucessfully (There is a huge difference in back plate pressure vs muffler pressure if you have never tried it - huge!). Others run the 7D OS carb on the St's. This is unnecessary in my experence and the 7D is touchy to needle settings when used this way to me, but some like it. They say that the transition is much better and the idle is more steady (Which is true, they really idle nice and pick up instantly with the 7D). If you are swapping the 7D for a narrow throat St carb, there will be a significant increase in top end power also.
If you are mounting any of the big tigres rightside up, you are asking for problems. The tank will be too low 99% of the time. Incidentally, the big MDS engines have the same problem. To get an MDS to run really well, use a perry Pump off of back plate pressure and get rid of the machined carb and run the cast (Later version) carb. Run it rich for at least 30 mins on the ground before trying to fly it. It will swing an 18x10 almost 9,000 rpm on all that cheap old fuel that has been sitting in your garage for years and never quit or wear out.
Another trick is to use a hopper tank, a small one oz tank that is fed by the main tank, but that is mounted on the firewall close to the carb. This fools the carb into thinking that the fuel tank is in line with the carb. Ducted fan pilots used this trick for years as they had the same problem and it worked just fine.
Hope that this helps.
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RE: ST2300 Testing and Experimenting - 
