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Gas tank position - below motor??

Old 03-06-2017, 06:57 AM
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MarkPrima
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Question Gas tank position - below motor??

Hello,

I'm really confused with this issue. I just built a small airboat with a OS MAX 25FP and I mounted the gas tank (6oz.) below the motor but I am having trouble with the fuel siphoning and having to constantly prime the fuel line. Do people use something like a one-way valve in the fuel line?

How do you guys do this? I'm baffled.

Thanks
Old 03-06-2017, 11:22 AM
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Originally Posted by MarkPrima View Post
Hello,

I'm really confused with this issue. I just built a small airboat with a OS MAX 25FP and I mounted the gas tank (6oz.) below the motor but I am having trouble with the fuel siphoning and having to constantly prime the fuel line. Do people use something like a one-way valve in the fuel line?

How do you guys do this? I'm baffled.

Thanks
Having the tank below the engine will likely siphon back to the tank unless you clamp the fuel line or close the throttle completely. The airbleed carb may cause it to siphon back regardless.

When the engine is running, the fuel should not siphon back due to the fuel draw of the engine. The only way to combat the fuel siphoning is to mount the fuel tank behind the engine ensuring the center of the tank doesn't sit above the center of the carburetor (spraybar). Do not use a check valve in the feed line - you will have carburetion problems.
Old 03-06-2017, 01:01 PM
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Thanks, I may redesign my next air boat and have the fuel tank behind the motor as you mentioned. But I do like the looks of having the fuel tank below.
Old 03-06-2017, 01:09 PM
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Originally Posted by MarkPrima View Post
Thanks, I may redesign my next air boat and have the fuel tank behind the motor as you mentioned. But I do like the looks of having the fuel tank below.

Most engines will draw fuel fine when the tank is right under the engine - any more than that and the engine may have issues.
Old 03-06-2017, 07:24 PM
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Originally Posted by MarkPrima View Post
Thanks, I may redesign my next air boat and have the fuel tank behind the motor as you mentioned. But I do like the looks of having the fuel tank below.
Ooohhh. A home designed airboat. Do you happen to have pictures? Would like to see it. Thanks.
Old 03-07-2017, 03:21 AM
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Originally Posted by NEW222 View Post
Ooohhh. A home designed airboat. Do you happen to have pictures? Would like to see it. Thanks.
Yes here are my build photos. It is shaped out of pink insulation and then sheeted with oak veneer.





Here are plans for my first electric air boat - I still run this one - goes good in show - flyingblog.primaveraphoto.com/snow-water-bug-prototype-and-plans
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Last edited by MarkPrima; 03-07-2017 at 03:57 AM.
Old 03-09-2017, 07:44 PM
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That looks very nice. Congrats on the build, and thanks for sharing.
Old 03-09-2017, 09:51 PM
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you still have enough space to put the fuel tank behind the engine. having the tank down low will not only effect starting and running issues but also you will get lean runs which is no good for the engine.
Old 03-10-2017, 12:14 AM
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Originally Posted by crapshooter View Post
you still have enough space to put the fuel tank behind the engine. having the tank down low will not only effect starting and running issues but also you will get lean runs which is no good for the engine.
There's no issue with the engine drawing fuel with the tank underneath the engine. I have done some testing with a fuel tank 6" below the engine and.3" behind it and the engine ran well. You have to compensate with the needle settings a bit. Lean runs happen because of improper needle settings. The FP engines have tiny Carburetors, so they draw fuel well. Other than fuel siphoning when the engine isn't running, there should be no other problems.

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