The Real Dogman
Posts: 2018
Joined: 5/5/2006
From: Tucson,
AZ, USA
Status: offline
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Hey boys, I noticed this problem when I first tuned my Genesis @ full tank . Once I tuned it with half tank it was alot better, but here in the desert were it gets up to 105 degrees in the summer, the problem would re-appear were no tuning would help as well as the hot ambient air was a fight to tune without overheating anyway. So I got to thinking "why". I had a thought,, so after checking a few things on the trucks fuel delivery system, this is what I came up with:
First thing we need be sure I understand how the fuel delivery system works. The Exhaust provides pressure to the fuel tank via a pressure nipple and fuel tubing. The pressurized fuel tank pushes fuel thru the [restrictive]filter in to the waiting carb which via the venturi effect and High Speed Needle actually draws a measured amount of fuel to help delivery, Thus the more pressure in the tank the easier it is to get a good volumn of fuel in the carb. And the most important thing to remember is, Air Pressure=Compressed Air=Increased Air Volumn In A Given Space. Ok, thats it in a nut shell.
Now the problem with this simple system is the pressurized fuel tank cannot hold a constant pressure to feed the fuel line and carb due to the changing volumn of air inside the tank . To be more clear, when the tank is full there is a small amount of pressurized air volumn in the tank , quicker to build pressure. As the fuel gets used up, there is more air volumn in the tank to be pressurized, thus more volumn that can be compressed , takes longer to be compressed and build pressure.Theoretically you will need more Pressurized volumn to keep a constant fuel flow to the carb, however the exhuast pressure nipple will provide more pressurized volumn as the engine is revved higher but when the engine suddenly slows the lower pressurized volumn at exhaust nipple cannot compensate the gain of air volumn in a half full tank quickly enough to maintain the same pressure thru the filter and tube, as well as possible release of pressure exiting back out the exhuast nipple from the tank , thus a flame out due to the sudden loss of pressure at the carb.
When you tune the carb you are actually controlling how much fuel(volumn) is allowed into the venturi, so when you tune with a full tank it is easier to build pressure in a small volumn of space and you dont need to let as much fuel into the venturi. When you tune at lower level of fuel in the tank (preferrably half full) you will need to allow more fuel in to compensate for the lower pressure or lower volumn at the venturi, to minimize flame-outs When there is a sudden drop in RPM's(exhuast pressure). But if you allow too much fuel in you will be running very rich, so a fine line must be maintained to run at optimal performance. This all is highly affected by atmospheric condintions as well, but that is whole new topic.
This is all made more difficult by the somewhat restrictive brass filter in the tank , which I will be removing to see if it helps.
holy$hit!!!I hope that was clear.
Now, if we were to solve this problem what would we need to do?? HHMM...Maybe a check valve between the Exhaust Pressure Nipple and Fuel Tank might help with sudden back feed from the tank to exhaust nipple..but what about maintaining pressure in a higher air volumn condition??have to think about that one some more.....
< Message edited by dogman_1@hotmail.com -- 7/5/2006 2:52:32 AM >
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All Forums >> RC Cars, Buggies, Trucks, Tanks and more >> CEN Racing >> Fuel tank/motor issues.... Again... 
Fuel tank/motor issues.... Again... - 
