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backpressure

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J

JaReD116

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On my losi 3xnt i am having a problem with the tank presurizing. I think that is what is is anyway. When i first start the truck at the begining of the day, i will cover the pipe and pull the starter to prime it. It will do it fine and it starts of the first pull. But if the truck dies for any reason, usually out of fuel, it wont prime. I will cover the exause and the fuel will go up but then it will shoot back down even with my finger still over the pipe. I have the lines zip tied. It only does it after it has been run. Any ideas as to why it does this?
 
Yeah I had the same problem on ny brother's Ntc3 and we couldn't figure it out. Could it be a setting on the carb?with regards to the fuel going back down even with your finger covering up the stinger, sound like you have a rip on your fuel line!
 
It can't be the lines. I took them off, pluged one end, and put them in water and blew through them. It is weird because it dosnt do it when it's cold.
 
You either have a leak in your fuel line or a leak on your carb. I replaced fuel line after fuel line on my savage only to find out that I have a torn o-ring on my hsn that was allowing air into the fuel line rather than a sealed system. I replaced the o-ring and she worked flawlessly.
 
It could be bad seals as mentioned by Pirata, or it could also be overheat on the engine. If the engine gets to hot it can vaporize the fuel creating back pressure down the fuel feed line. This would provide resistance to fuel flow in the proper direction.
 
Which fuel tank do you have?

Is the backpressure fitting on the tank or the lid?

If it is not on the lid, I am 99.99% certain this is your problem.

Let me know.
 
Originally posted by SkyMaxx
It could be overheat on the engine. If the engine gets to hot it can vaporize the fuel creating back pressure down the fuel feed line. This would provide resistance to fuel flow in the proper direction.
Click to expand...

I would richen your engine 1/8 of a turn. I think you are running the engine a little lean and when it dies the fuel is vaperizong as what skymaxx said. This is the first thing I thought of when I read this.
 
The tank is not the new one with the pressure thing on the lid. I dont think it is over heating, i am using the water drop test. And it takes a few seconds to boil away.
 
Sound like a too hot engine cause:

1. no problem when the engine is cold
2. problem only exist when the engine is hot

That indicate the fuel is vaporised by the engine and being push back to the tank. If there is leak, problem will occur in both case.
 
This is totally speculation... but couldn't cool fuel and air entering a hot engine expand in the carb and be pushed back through the fuel line before it reaches the cylinder to be combusted?

In any engine?

I have a bitch of time starting a hot engine and I have no leaks (to or from engine and fuel tank).

(TRX25, just over a gallon of 20%, anal about break-in)
 
Sounds like to much heat at the carb. Is is just like if the engine gets hot and you try to manually primethe carb and the fuel just boils back out the top. That heat is probably forcing the fuel back to the tank.
 
Yeah How does that affect the air filter? Is there any way to alleviate the heat at the carb..besides leaving it off?
 
i just read on a rescent thread that if your engine is loosing compression, this can happen. can you easily push the piston over top dead center (TDC) by moving the fly wheel by hand? if so, you may need a piston/sleave replacement. if the engine is new or has plenty of resistance when the piston reaches TDC, then I would have to agree with the heat issue. use a temp gun and make sure you're running at a safe temp (most will tell you 230-240 degrees)

Good Luck and welcome to the forum,
Rob
 
the compression is still fine. I had it tested not to long ago at my lhs. I am pretty sure it is not to hot. I am still using the caveman method of the water drop test:D, but i think i am going to get a temp gun very soon.
 
The air filter is not the problem. The problem may very well be that the mixtures are a little too lean. If they are set correct for your engine, then maybe it is just being pushed too hard before the point where it shuts down. Th engine will run leaner as the tank's fuel level gets lower. Thus, higher temperatures. Never run without an air filter, NEVER! You need to lower the engines running temperature, I'd suggest running richer to the point whee you may even get a bit slugish pewrvormance with a full tank of fuel. See how that works out and take things from there.
 
Does the engine really run leaner as the gas level drops?
 
Yes, I guess it is from less pressure in the tank and possibly from the fact that the fuel will be constantly sloshing (spelling?) around the tank. The lower the fuel level the leaner it runs, it is not a great deal more lean but it is some. Most people will set the engine with it running a bit richer than ideal with the tank full and some only tune with the tank at about the half way point.
 

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