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needle valve

 
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bobbypaulk(at)comcast.net
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PostPosted: Sun Dec 09, 2007 3:42 pm    Post subject: needle valve Reply with quote

All,
I have a Jab 3300 that has never run right since new. It was flooding at idle and blowing soot out the exhaust. I had to clean the plugs several times in 25 hrs. I decided to run a bench test on the needle and seat. I filled a Tupperware type clear bowl with fuel and sat it on the bench. I removed the Carb from the A/C and removed the Bale from the float chamber on the bottom of the Carb. I connected the fuel line to a regulated air supply of 5 psi with a calibrated 0 – 5 psi gauge only inches from the carb fuel inlet. I sat the carb in the bowl of fuel so that the top of the fuel would be even with the split where the bale was removed. I got a friend to increase the pressure slowly while looking for and listening for bubbles. The carb started passing gas (pun intended ) at 1.5 psi. I repeated the experimented with this by raising and lowering the carb out of the fuel and back in. the results were that the needle was not seating at a very low pressure.
I decided to polish the needle seat with a rounded wooden dowel and fine valve grinding compound. With light finger pressure I began turning the dowel and discovered a burr on the seat caused by the drill bit or tool used to drill the hole. I used a larger drill bit with very light finger pressure and turned it on the seat. I saw shiny metal around about 40% of the seat where the burr had been removed. I finished polishing the seat and the needle would hold bubble tight to over 4.5 psi.
If you are having problems with the engine running very rich at idle you may want to see if the needle valve seat is smooth.

Bobby Paulk
Zodiac 601 XL
N131BP
55 Plus hrs.
[quote][b]


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