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Fuel tank position too high

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Vecchio Austriaco18/01/2010 22:21:22
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Hi there
 
after flying for some years around with different small electric aerobatics I would like to go for IC again. (Had a trainer with 2-stroke engine some 25years ago) So I purchased a Graupner Extra 300, 1600mm wingspan, and a chinese (budget!) 4 stroke engine. Its a P.H. FS91AR, looks exactly equal to Magnum XL .91RFS.
So far so good. The best position for the engine seems to be inverted. If I do so the high speed needle of the carburator is approximately 2 cm below the center of the fuel tank.
Question to the experts: Will this lead to problems? Flooding of the carburator?
thanks  VA
Engine Doctor19/01/2010 13:06:38
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Hi Vecchio. It may cause problems if you leave the model standing for some time but I have used similar set ups and have not had any serious problems with flooding .You could fit a fuel regulator that will solve the problem . I believe Weston UK supply them .
 I did have a prototype Corsair some years ago that had the tank much higher than the main jet and this really caused problems . I solved this by using an carb extention that I believe was in an article or letter in the RCM&E . It involves a plate made to mount on the back plate screws of the engine  . The carb is mounted on this at the correct hieght and a alloy extention made to fit between the carb and the manifold . It worked extremely well but you really need the use of a lathe to make the extention tube . I used tufnol to make the extention tube . Best to try it first and see if any mods need to be made .
Good luck.
E.D.
Vecchio Austriaco19/01/2010 13:47:06
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Thanks ED.
I will break in the engine on a test bench before installing it to the plane and I will install the fuel tank exactly in the position as it would be in the model. If it will not work I can start thinking to move the carburator higher up. As I am happy owner of a lathe it should not be a big problem to do so.
VA
Vecchio Austriaco25/01/2010 12:43:34
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Looks like I have to remove the dust from the lathe. Tried the engine on the bench with the fuel tank position as described above and at the moment I opened the needle the fuel started dripping out. The engine was on the bench in regular position so on this type of engine the carburator is inverted (air inlet pointing down).
I will make the extension and try the engine in its final position on the fuselage.
VA
Paul Adams25/01/2010 13:07:56
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 I have a similar situation with one of my planes. The main problem I have is that the tuning of the engine is different depending if the model is right way up or inverted. If I tune with the model right way up as soon as I go inverted it leans out and strops. Alternatively I can tune with the model inverted in which case the engine runs rich the correct way up. I would not have thought that it would have made much difference as the tank is pressure fed from the exhaust, but as the model is meant for 3D flying, I am often at low throttle when just flying around, so I assume the exhaust pressure is fairly low in this situation.

 I would be interested in finding out how the card extension works as it may cure my problem as well.

Vecchio Austriaco31/01/2010 23:08:50
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Some news from the carburettor extension. Finally I made one. I moved the carburettor 20mm down (which means up as the engine will work inverted).
 
if somebode is interested how - have a look at the drawing and the pictures.
 

The sheet metal piece (aluminium 1mm) fixes the carburettor in its new position. I wanted to use O rings to have a tight connection  - but in my collection I could not find the right diameter. So I made the tube a second time - without the o-ring gap but with a 1 degree cone. I wrapped some teflon tape around the tube which connects inlet chamber and carburettor and pressed the extension tube on it. The other side works with the original O-ring of the carburettor.
 
 
Side view - the extension tube is in the middle. You can now see why the sheet metal which holds the carburettor in its new position needs to be bent as shown in the drawing. 
same view - carburettor not yet in place.
 

 
View from the back of the engine. One change I had to make was to turn the carburettor 180 degrees as the screw on the left side of the carburettor housing was touching the engine case. way. The needle is now on the other side of the engine.
 
As I am not very far with builing the plane where the engine will go to this is not a problem, I can easily change the position of the accelerator servo push rod and until now I did not touch the cowl. So no holes in the wrong position
 
So far so good - next weekend I have to test the engine.
 
VA

Edited By Vecchio Austriaco on 31/01/2010 23:15:49

Engine Doctor02/02/2010 15:49:48
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Helolo Vecchio . Your conversion is coming along fine . I used 2mm ali plate to mount  the carb on my engine as vibration could take its toll , but I extended it further so angle was decreased . 
Regards
 
E.D
Ed Darter04/02/2010 12:57:19
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the only thing I would say is are you sure the tank position is what caused your problems ? did it all work fine if the tank was positioned as it should be ? Just a bit concerned that you may be goign to all this trouble and the carb position is not actually the root cause of your problem....
Vecchio Austriaco04/02/2010 20:11:54
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Ed,
I had the engine running alredy on a testbench with the recommended tank position - and it ran fine. My doubts were starting when I saw the difference between carb and tank center on the actual model - so I did another bench test this time with the tank in 2cm higher position - and - surprise- the carburettor got flooded. I will retry with the extension in the model - but for that I have to finish the fuselage first as I dont want any fuel or oil on the balsa before covering. (It's an ARTF but I change design-so currently without foil overing)
VA
 
Vecchio Austriaco17/04/2010 18:51:07
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Just a final one to this thread -
 
The plane this engine was bought for is finally flying - the extension works problem free - no flooding, no wrong air intake. Engine starts within 5 seconds and works just problem free. I am happy that I decided to make this little alteration as I think with a carburettor which constantly gets flooded the starting procedure could be rather annoying.
VA
Roberto Gava19/04/2010 18:15:56
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67 forum posts
I've the same problem wayting for a solution on my Phoenix Tucano.
The problem is that it has a two stroke engine inverted position and it is flooding as soon as I fill the fuel tank.
I think in my case the only solution is to lower the tank. But it involves cutting F2 former that is difficult to reach without making "mayor surgery" to the model.
If someone has some suggestion. I'll appreciate it.
Thanks.

Edited By Roberto Gava on 19/04/2010 18:17:14

Engine Doctor20/04/2010 15:12:58
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2511 forum posts
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Hello Roberto You could  try fitting a fuel pump / regulator . these work with pressure from the crankase and will only allow fuel through when engine is spun or started. Weston UK in Teynham England sell them under their own bran but there must be other suppliers who can help . Another benefit is that the fuel tank can be moved ont the Cof G so no change in flight characteristics with changing fuel load .
 
Good Luck
E.D.

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