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What causes air bubbles in the fuel line

What causes air bubbles in the fuel line

Old 09-04-2012, 02:55 PM
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Default What causes air bubbles in the fuel line

I have a losi 350 that's been giving me all sorts of tuning problems the last few days. I sealed the engine and carb and that helped a lot got it almost all tuned today but I am still a little rich on the hsn, I make a few wot passes and it's great then all of a sudden I can hear it popping like its lean the air pockets pass through and I am blowing a blue cloud again. Sometimes it will pop and die out and when I get to the car I can see the fuel flowing back into the tank. Any ideas the car is a losi 2.0 rtr about a week old
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Old 09-04-2012, 03:17 PM
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Check you tank pull it out use the sink method. CLose off a opening blow in the other while the tank is in the water. Watch for bubbles another potential issue would be your lid. The lid springs go bad over time it doesnt add the proper tension on the lid making for a poor seal. Check your lines, I change mines every month.
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Old 09-04-2012, 03:19 PM
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Also dont screw the tank down hard. My tanks are floppy loose.. The vibration sent through the mounts and into the tank will cause a lot of bubbles.....
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Old 09-04-2012, 03:21 PM
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Originally Posted by JamminSP View Post
Also dont screw the tank down hard. My tanks are floppy loose.. The vibration sent through the mounts and into the tank will cause a lot of bubbles.....

Thats something to try , had my tank off the other day before this all happened
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Old 09-04-2012, 04:24 PM
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Make sure the clunk is almost touching the back, but can flop up to the top when you roll over. It should sit in the sump. If the fuel line is pulled out it will suck tons of air every time you go WOT and the fuel pushes to the back, but will look normal when you stop to check it out.
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Old 09-06-2012, 05:59 AM
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I checked the clunk last night and it was in the middle of the tank , moved it to the sump. Hopefully that was the problem. Thanks
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Old 09-06-2012, 04:03 PM
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Okay I have tried all of the suggestions here and still having the same problems. I can get it to run great for a few minutes them it will die out. There will be no fuel in the line. I actually have to blow into the line coming off the pipe to fill the lines back up because the engine won't suck it up. Then it will run fine again , there's no ryme or reason as to when it happens either some times wot and others at idle. I really think its in the engine, because I can blow fuel up to the carb with the car off and leave it for hours on the bench and the fuel stays in the line. The only thing I haven't checked is the front bearing , is there a way to seal that or do you just change it?
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Old 09-06-2012, 04:15 PM
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The description for the carb on the engine is "Two-needle slide aluminum/steel". Unlike composite carbs, it may be susceptible to vapor lock (overheating). Try another carb, if available. About the only thing you can do is to run it richer?cooler.
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Old 09-06-2012, 04:50 PM
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That could be since it only happens when it's hot. But I am only running around 180 now. Maybe I will try another carb. Thanks
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Old 09-06-2012, 04:59 PM
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If you fill the tank completely does it still act this way?

I'm still suspecting fuel line or a seal somewhere if you have now fixed the clunk position. The motor does not ever "pull" fuel, it is pushed from the exhaust pressure with a good seal. If it does it when the tank is full, the problem should not be with a hole in the fuel line inside the tank. Pull the fuel line off the motor nipple, pull open the tank lid, and blow into the fuel line. Are the only bubbles coming from the clunk, or is there a hole in the line somewhere inside the tank or anything?
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Old 09-06-2012, 05:06 PM
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plain and simple a crap fuel delivery sytem , there are a few manufacturers that still have not addressed this issue , its sad as it is the arterial connection of the car and very very very important!!
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Old 09-06-2012, 06:03 PM
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Try a different pipe. I remember reading on rctech that someone was having similar problems and he found out his pipe was clogged up and not providing the necessary back pressure to keep fuel pumping to the engine.
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Old 09-06-2012, 07:09 PM
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Did you change the Carb if so try another motor that way you eleminate it
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Old 09-06-2012, 07:20 PM
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Originally Posted by AZTman View Post
If you fill the tank completely does it still act this way?

I'm still suspecting fuel line or a seal somewhere if you have now fixed the clunk position. The motor does not ever "pull" fuel, it is pushed from the exhaust pressure with a good seal. If it does it when the tank is full, the problem should not be with a hole in the fuel line inside the tank. Pull the fuel line off the motor nipple, pull open the tank lid, and blow into the fuel line. Are the only bubbles coming from the clunk, or is there a hole in the line somewhere inside the tank or anything?
The only bubbles are coming from the clunk, it only does it when the engine is hot. Fuel level hasn't made a difference.
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Old 09-06-2012, 07:31 PM
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The vapor lock makes sense because when the engine is hot if I blow fuel through the line it will actually be forced right back out. When the engine is cold it holds the fuel no problem. Once it gets hot and dies I have to blow into the pressure line while trying to start it it. The only thing that doesnt make sense is if it's a bad carb design why isn't it a more common issue. I could have just gotten a bad carb I guess. My factory hsn setting is 2 1/2 out I am about three turns out and it's running great at wot as soon as I lean it from there it starts popping.
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