210cc Solo engine

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Postby JPXman » Mon Jun 25, 2007 1:26 am

perhaps we should cure the problem in stead of address the symptoms? the only thing remaining as problem sources in your system is then:
1) the head that the pump faces as the tank is so far below the pumper carb
2) the pumper carb

since we can't change #1 that's a non-starter.

the only thing would be to change carb types with one with a better impulse pump. could it be the pump itself, when facing the pressure head it has to suck causes back pressure on the one side of the pump cycle and it causes the gas to dance up and down a bit in the line causing bubbles?

is this why when the electric fuel pump is on right outside the tank, pressurizing the line, the problem disappears?

were all sources of gas line vibration eliminated?

should we try a rotax 185 carb on your solo 210 and see what happens?

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Postby lazairiii » Mon Jun 25, 2007 11:41 am


Just a couple of thoughts here. Note that this is what I do for my 185. Think about it, if you have a good solid line burried in fuel that runs all the way up to the carbs with no splice or inlet for air, then where is it coming from...it's coming from somewhere right? You just need to eliminate every spot that could possibly be causing this problem. And that included splices. How are you guys attaching the fuel line to the carb? I happen to like to used a DBL wrapped .032 safetly wire at the carb that is twisted tight to ensure no air leaks. This is a spot that you could look at carefully as if the above is followed, would be the first spot air could get in right?

#1 Check to ensure all carb bolts are tight, gaskets are not leaking and not drawing any air from anywhere.

#2 Make sure your fuel line is not cracked or does not have any small pin holes. These WILL draw air.

#3 Run with 1/4 tank or more of fuel. I have noticed in the last few weeks of flying that if I get down to a few inches of fuel in the tank, I start getting bubbles in the lines. I always run clean fuel and oil that is NOT older than 2-3 weeks either. I'm burning 5 gallons a week right now thought Heehee!! The only time I get any bubbles what so ever (and these bubbles are limited to a few inches below the carb) are when I get low on fuel. Pressure between the tank and carb??? Who knows.

#4 Be sure BEFORE you start the engines, that you draw the fuel up into the carbs and get rid of any air in the lines. On the 185's you can rock the props back and forth at the pump position of the rotationa and it draws up nicely. I do this each cold start to ensure fuel is all the way into the carbs and air is gone before I ever pull the rope. I don't do it so much that I flood the carb, but enough to eliminate the air in the lines that has settled back down the lines from sitting. This may also be why I'm not getting bubbles...who knows, it's just something I do and has been working well for me.

Just my observation and 2 cents worth for the day.
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Postby JPXman » Mon Jun 25, 2007 5:22 pm

an air leak at the carb fitting won't produce air bubbles in the line, but rather a strange running engine.

from what i'm hearing (and most importantly interested in finding the reason why) some air bubbles just appear out of nowhere! why is that?

i am usually pretty good at coming up with theories about stuff and then later proving myself incorrect - but i think it has to do with:

a) the pumping efficiency
B) the "pump head" ability
c) diameter of the fuel line

compared to most ultralights, our engines are WAY higher over our gas tanks than most. we have about 5 feet of "head" between a nearly empty tank and the carb.

i have to get back to work now, but i'll come back and write more on this....

Myles, perhaps we should try the skinnier fuel line like you used back in the 80s?

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Postby daffy1029 » Mon Jun 25, 2007 6:05 pm

Hi Jpxman,
Well I have had enough, I am going to try the electric pump and hope it can take the 18v instead of the 12v it's designed for. I spent most of Sunday trying everything under the sun and now I am convinced, it's better to push fuel up, rather than sucking it up. Daffy
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Postby lazairiii » Mon Jun 25, 2007 6:09 pm

Ok, but I've seen loose tubing fittings around the inlet produce air bubbles a few inches down the line. I agree it's probably from a rythmatic pumping action, but still, air HAS to be getting in from somewhere. I'm pretty good at trouble shooting too. Now the difference may very well be the different carbs, engines and fuel lines...I was just giving you what I know to be good info on 185s.
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Postby lazairkid » Mon Jun 25, 2007 10:23 pm


Can you send us some pics of your fuel line set up.
The in tank fuel filter is submerged in the fuel so we know
it can't be making air.
How much fuel is in the tank!!!!!
Air has to be getting in the line from a pin hole in the line
or something if there are no connections between the tank and the engine.
Did you try all new tubing or putting the tubing you have under pressure
to check for pin holes.
I may be wrong but I can't see the carb pump being the problem.
If you have the electric pump just after the tank like I do you need
to safety wire the connections with 2 to 3 wraps and then twist.

No, I have not got to fly yet. I was doing some changes to the engines
back plate when I found a cracked engine mount on the left engine
I'm welding up a new one and beefing up the bottom mount.
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Postby russell » Tue Jun 26, 2007 7:01 am

It's normally not air, but rather fuel vapor. It gets transformed from one state to another sorta like that mysterious tiny string of bubbles that eminate from the inside wall of a glass of beer, or when you shake a bottle of coke, the cardon dioxide is released from the liquid state. Or something like that...
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Postby JPXman » Tue Jun 26, 2007 5:16 pm

yes exactly russel, its fuel vapour.

for a liquid to turn to gas, you have to give it energy. you can either give it kinetic energy (motion) or thermal energy. away from the engine there are no sources of heat, so if the bubbles are appearing from a seemingly benign spot, then it must be kinetic energy being imparted to the gas.

vibration is a sure source of this. but what kind of vibration.... lateral shaking of the gas line, or is the gas shaking up and down in the line?

the more i've thought about this, i think the fuel line diameter might have something to do with this. we use 3/16" line, what do you lazairheads use? i know there is a size smaller - is there a chart somewhere that shows fuel burn vs. fuel line diameter requirements?

if you had a a gas can, and a straw 4.5' high - if the straw was skinny you could easily suck the gas up to your mouth and have that nice lazair taste for the rest of the day. but if the line was fatter, it would be harder to suck the gas up - because you have to lift a heavier amount of gas - it requires the same psi to suck the gas up, but its over a larger surface, requiring more power to pull the gas up.

so if your little pump was working really hard just to hold the gas up (and maybe the check valve relieves some of this load, but there is a moment in the pump cycle where the pump has to suck the whole fuel line's worth of gas up a fraction so the check valve can then close again while the pump pushes the gas through to the carb)...and if it was workin hard, perhaps it is pulling the gas up, and then ever so slightly the gas falls down the line before the check valve does its thing.

if the gas is going up and down in the line 5500 times per minute (with each compression cycle of the crankcase) then that also sounds like vibration - and perhaps this phenomenon occurs in the fuel line right where the weight of the gas becomes just a bit too much for the pump system to handle.

so for some it occurs just below the carb, others maybe 2 feet down the line.

george said he gets bubbles when his tank is low and the bubbles appear just below the carb.... so perhaps when the performance of the pump reaches a point where its "head pressure that it can hold" exceeds the fuel line length, that this pump jitter causes fuel vapour bubbles to begin in the line.

nice theory or what?

i think i may try a skinnier gas line if i can find a throughput per diameter chart. daffy did you try putting the gas can at the same height as the engine, and then lowering the can to see if/when the vapour bubbles appear?

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Postby russell » Wed Jun 27, 2007 9:48 am


when I first built my plane I used a 3/16" fuel line. I flew this for 8 years with no air bubbles. When I recently rebuilt my plane I decided to use a 1/8" line. The reason for me to do this was not in consideration of bubbles, but rather I felt that it would prevent a lean running condition or a possible periodical starving for fuel because as you stated the suction effort required would be less with the smaller tube.
This was a bad decision. This created a steady flow of tiny bubbles in both fuel lines which appeared to develop about half way between the engine and tank.
Your results may vary of course due to the difference in the types of engines.
Your suggested theory on the pulsating fuel sounds viable.
My kit originally came with a primer bulb and vapor would collect in a high point in the lines just over my head. Dale Kramer later came out with the recomendation to discontinue the use of the bulb and the bubbles went away.
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Postby flyalaz » Thu Jun 28, 2007 3:25 pm

Hi Guys,

Been busy lately, but I have been flying when I get a few spare hours. I have been using 1/4" I.D. fuel line for the last two or three years with no problems. Last year, I got rid of the primer bulbs, and installed shut off valves instead. If the plane has been sitting for a week or so (like it usually does), when I open my shut off valves, there are a few bubbles that run up the line. During run up and in flight, I play with the lines to get the bubbles up to the carb and out. The tach doesn't show any change, and I don't detect anything with my ears unless it is a big void in the tube. Once I have chased all the bubbles out the lines, they don't form again. My lines are ty-wrapped to the throttle cables, which run through adel clamps, attached to WSI stand-offs, and I guess they're pretty well isolated from vibration. They flit around a small bit in the wind, but the cables are stiff enough to prevent a lot of movement.

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