Kavan carb help

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Kavan carb help

Postby sfry » Mon Aug 31, 2009 2:44 am

I have read the way to hook up the Kavan carb on the MECOA web site, but still have a question: Can this carb be used for flying inverted? If so, what stops the fuel from being expelled from the tank thro the vent line and carb pressure release valve when running slowly inverted? On bench tests the engine cuts out!!
Many thanks in advance,
Stuart
sfry
 
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Postby Konrad » Wed Sep 02, 2009 2:02 pm

Stuart
Nothing stops the flow. This is the same issue with normal muffler pressure taps. At partial throttle the carb should have good fuel draw not needing the pressure. Not sure why your engine stops inverted. How is your tank set up and where are you taking the pressure from?

The Kavan carb came out before mufflers were popular (mandated). The pressure relief valve was used (needed) when we tried to use crankcase pressure (untimed). With a large enough hole (0.9+ mm) in the muffler pressure tap there is no need to use the pressure dump in the Kavan carb.

Friends don't let friends fly nickel,
Konrad
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Postby sfry » Mon Sep 07, 2009 12:10 pm

Thanks Konrad. First let me give you the link to the Kavan set up:- http://www.modelengine.org/forum/viewto ... 1755#11755.
I have not set up my fuel supply like they describe as I have taken the press from the crankcase. My set up is as follows: Tank has two vent pipes (one to top of tank and the other to the bottom), and a klunk piped directly to the needle valve. The crankcase press is piped to a "T" piece. One outlet of the "T" goes to the press release nozzle on the carb and the other outlet of the "T" goes to the tank vent pipe that goes tp the top of the tank. The second vent pipe from the tank is closed off after filling the tank.
This arrangement works really well with high power, good idle and excellent pick-up with throttle. However, when inverted there is a problem at idle because the vent pipe that releases the excess tank press is immersed in the fuel and neat fuel is expelled through the release valve on the carb. The engine then cuts out. This problem with expelling neat fuel through the carb release valve would also occur with the set up as described on the MECOA site.
Please note this would not happen on the normal muffler press set up as there is no press release valve in that set up. How should the fuel supply be set up to avoid this problem using the Kavan press carb that incorporates a press release valve?
Thanks Stuart :?
sfry
 
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Postby Konrad » Sat Sep 12, 2009 11:39 am

Try a T vent line so that at any attitude one of the Vents is open of a gas not fluid. As a gas is about 7 times easier to flow the vent exposed to the gas will vent the tank pressure rather than the submerged vent.
To aid in filling the tank the you might want to pull the line from the carb and fill though that or add a 4th tube

I find that the choke area of the Kavan carb is small enough that crankcase pressure is not needed. Muffler pressure is enough. As the vent flow is always in the tank (this breaks the siphon) for steady state operation I find that very little fuel is lost out the muffler while inverted.
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Postby sfry » Fri Oct 16, 2009 4:39 am

Thanks for your further comments Konrad. I have tried the T piece, but the engine still cuts out. The problem is that both pipes (upper and lower) fill with fuel when the plane is inverted and the tank cannot depressurise quickly enough so the carb floods. Realising this, I put in a 4th pipe with a float attached so that the end would always be in vapor. however, this proved to be unreliable as fuel still got into that pipe.

I think that it would be OK using muffler press as there would not be such a big pressure build up in the tank to release, but then you would not get so much power (which I needed in that model). So, I have bought an OS 90 FX. Plenty of power and no problems.

Thanks again. Happy flying, :P

Stuart
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