Strange Plane Problem (fixed?)

[Phil Coopy]

I almost feel srupid presenting this problem after flying models for 55+ years, but I am stymied.  Suddenly my Vector 40 is some how losing fuel to the point that sometimes I cant make the sequence on 4 1/2 ounces of fuel.  When I land there is fuel and oil all over the  outboard side of the plane.  I have checked the plumbing and its OK. At first I thought that the LA40 was losing fuel out of the front of the bushing, but I have had two more engines in there and one of them is practically new.  Besides when you look in the engine compartement there isn't much of the fuel spattered in there.  My first guess was that it was spraying out of the venturi cause I have seen RC engines do that.  My next guess is that it might be dumping out the uniflow tube.  Problem is that none of this stuff happens unless the plane is in the air.

I am using the following:
LA40 with low compression head and long venturi with ST NVA and OS#A3 plug.
Vector 40
11x5 Zinger Pro prop.
10% nitro, 22% lube.
Launching at 8400RPM

Jump right in here anybody,

Phil

[John Miller]

Try hooking the uniflow up to muffler pressure.

[Willis Swindell]

Check and see if the plane has gained weight. I had a Time Machine gain about 8 oz. and the engine was using more then 1/2 oz more fuel.
Willis

[Steve Helmick]

I had a metal tank that leaked about an ounce per flight, on muffler pressure, and replaced it with a Sullivan clunk tank that leaked even more (made about 3 laps & quit!). Finally put on a Hayes tank and solved the problem completely.  Steve

[Dennis Adamisin]

Any other symptoms - have you had to re-set the needle, is it running dependably?

Are you sure you are putting 4.5 oz in it?  I seen overflow tubes that broke off inside a tank and gave a "false" fill.  Also possible if you have a bottle tank and the tube has rotated.

Sounds like some kind of fuel siphoning problem - I second John's suggestion

[Howard Rush]

If it does this with different engines, one wouldn't think the engine is the culprit, and if the engine compartment is dry, one wouldn't suspect the fuel hoses.  I wonder if it has to do with the flow field around the cowl near the engine intake.  You might try flying with the cowl removed.

[Garf]

You need to pressurize the entire fuel system. Fill the tank, plug the fuel line at the point that it attaches to the engine, plug all other vents but 1. Connect a syringe or fuel bulb to the remaining vent, and pressurize the system. You would be amazed how many leaks you can find that way.

[Joe Yau]

When I land there is fuel and oil all over the  outboard side of the plane. 

Phil

Another possibility is the plastic back plate coming loose maybe? then fuel will be dripping off the cyl head when running.

[Phil Coopy]

Problem fixed. Cause still not known. After deducing that the only place the fuel could be coming from was the uniflow tube, which is open and pointing forward, I installed one of those little yellow rubber caps that come with a new Brodak tank on it, with only a pinhole in it for venting. Voila! Fuel leakage stopped and the run is normal, fuel consumption is back to normal,  with no mess on the side of the plane. Still, I dont know all I understand about this.   During the process of elimination last week I changed to a larger tank (4 1/2 oz) but that only allowed me to get all the way through the sequence by compensating for the excessive fuel consumption. Now I,m back to 3 1/2 oz per run again, but the longer thank seems to get a better break, so I'll leave that in.  I am running a longer venturi than stock but I went back to stock and it didn't solve the problem.  I am still failing to understand the dynamics of this whole situation, especially since it appeared quite suddenly.  Something changed but I can't seem to track it down.

Puzzled,

Phil

[John Stiles]

Problem fixed. Cause still not known. After deducing that the only place the fuel could be coming from was the uniflow tube, which is open and pointing forward, I installed one of those little yellow rubber caps that come with a new Brodak tank on it, with only a pinhole in it for venting. Voila! Fuel leakage stopped and the run is normal, fuel consumption is back to normal,  with no mess on the side of the plane. Still, I dont know all I understand about this.   During the process of elimination last week I changed to a larger tank (4 1/2 oz) but that only allowed me to get all the way through the sequence by compensating for the excessive fuel consumption. Now I,m back to 3 1/2 oz per run again, but the longer thank seems to get a better break, so I'll leave that in.  I am running a longer venturi than stock but I went back to stock and it didn't solve the problem.  I am still failing to understand the dynamics of this whole situation, especially since it appeared quite suddenly.  Something changed but I can't seem to track it down.

Puzzled,

Phil

Could that have been the siphon effect of the air going by the uniflow tube

[Charlie Pate]

i had this problem one time.
Long story short;I had placed the vents on the left side of the body (outboard)why I don,t remember.
Put vent /fill  tubes on inboard side on plane .fuel usage was as should be. Engines ran great either way
with no fuel on plane,just would use too much fuel . Tried multiple engines (same brand). when I moved the vants to the inboard side ,everything was OK.
Uniflo or plugged overflow was not in use at this time.   

[Phil Coopy]

Charlie brings up a good point. Should the vent pipes always be on the inboard side of the fuselage?  Could be part of my problem 'cause mine are on the outboard side.  I can change them pretty easily.

Phil