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Exhaust Pressure.

Old 09-24-2017, 01:20 PM
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RC_Fanatic
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Default Exhaust Pressure.

Many glow engines (remember those ) use exhaust gas pressure to the fuel tank to help feed fuel. How much pressure (psi) is applied to the tank this way? I'm sure it varies with engine speed but I'm trying to find out how much pressure to apply to the fuel system to find a leak.
Old 09-24-2017, 02:31 PM
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Not much. 1 or 2 psi.
Old 09-24-2017, 03:36 PM
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That's about the same pressure as a normal person can produce. That was the point of my question. Can I blow hard enough to test the fuel system for leaks? I did consider the effect of pressure peaks but I think the line from muffler to tank is long enough and thin enough, and the tank volume sufficient to damp out the pulses.
Old 09-25-2017, 03:49 AM
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Just replace your fuel lines and eliminate that option as the source of your problem.

If it fixes the issue, you're done. If not, you know to look elsewhere.
Old 09-25-2017, 08:48 AM
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I have measured exhaust/tank pressure by making a manometer. I used a long fill line and ran it up a yardstick and left it open at the top. I checked three or four engines, both 2 and 4 stroke.
They all gave about 16" of glow fuel at full throttle. I don't know the density of glow fuel (suspect it is less then 1g/cc) so can't convert that to psig. At idle the pressure is essentially zero. Pressure increase was exponential with rpm's which makes sense with equations for flow through an orifice(the muffler outlet).

I check for tank leaks by submerging the tank under water and can blow plenty hard enough to see bubbles.

Chuck
Old 09-25-2017, 11:02 AM
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Thanks, Chuck. That's exactly what I was looking for. 16" of water is about 0.578 psi. Glow fuel, mostly methanol, is less dense than water so I estimate the 16" of glow fuel is about 0.48 psi. I can blow a lot harder than that!

The fuel system was tight. The problem was OSF (Operator Skill Factor). I'd left the throttle open while refueling and some fuel ran out the carb and into the cowl. Verified that as the problem this morning. Fortunately, I did not have to fix the fuel tank as I'd done that a couple of months ago and it is a hassle to get the tank out and in, not to mention bending the lines into the tank.
Old 09-27-2017, 09:43 AM
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I have used an old syringe to pressure test a fuel system. I used weights to hold it under water, Pressed the syringe down, saw the tank expand, tiny bubbles, no not the ones Don ho sang about, came from under the cap for the fuel tank. I think they were so small I may not have heard the air escape or felt it.


Buzz.
Old 09-30-2017, 11:56 AM
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Ya, a syringe works good. I use a fuel bulb, but no one knows what they are any more. Don Ho. Wow, that's the 1960s? I guess I remember that, but wow.
Old 09-30-2017, 01:31 PM
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Default Holy cow

KISS principle keep it simple ......just blow in the tube while you hold the tank under water... No need to over engineer stuff... But if that's what you're into have fun
Old 09-30-2017, 01:34 PM
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Have fun whether you like it or not.
Old 09-30-2017, 04:10 PM
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Good discussion guys!

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