I stopped working on the carb. last year when I realized the needle valve did not fit on the float. I just assumed I had a defective part from Fresno. After talking with them we figured out all of their parts were made to the same wrong dimensions. I returned the needle and seat and they returned my money. I then purchased new parts from El Reno. I knew as soon as I opened the box they were the same parts. Aircraft Spruce also sells the same parts. These are not factory parts and they do not have PMA for these parts. If the pin on the float were well worn it would work but the parts would not be acceptable, for use on a certificated plane, based on the Overhaul Manual. Item 54 is the dimension for the slot in the needle. It should measure 0.128" - 0.130". The pin on my float (item 52) measures 0.126" which is the high end of the new tolerance which is great because it has lots of wear left in it. To solve my problem I took the Factory made Steel Needle and Brass Seat from the carb. on our Fly Baby, exact same carb.
To check the slot size I used a 1/8" drill which measured 0.1245" and a #30 drill which measured 0.1285". The 1/8" drill falls in the slot with almost no clearance. The #30 drill won't go in at all. Since the minimum slot width is 0.128" this drill should have almost fallen in even at the minimum, it's not even close. When I measured the slot with the verniers I got 0.1241" which is undersized by twice the allowed tolerance.
Fortunately the pin on the Fly Baby float is worn and the needle fits with the required clearance. There is no requirement for a homebuilt to meet the overhaul manual so I'll use it.
You may need to lap the steel needle into the brass seat with fine polishing compound. Do not use valve lapping compound. To check for leaks on the test stand you let the needle hold the head pressure for 20 - 30 minutes. A way to test the needle before you go to all that trouble is to hold it upside down, suck a vacuum, seal the hole with your tongue and see if the needle stays in you're probably good to run the actual test. If not do a little more gentle lapping. This works better with the seat in the carb. casting but it's easier to see what I'm doing this way.
To test the needle valve, and to set the float level you need to be able to apply a specified head of pressure (1/2 psi) while holding the carb. level. Then you just measure the depth of the fuel below the top of the casting and change gaskets under the seat to get the correct fuel level. It's simple enough but you need some sort of a test stand.
I started with a piece of plywood for a base to hold the carb. To get the carb. to set securely on the board I drilled 1/4" holes for the studs which allowed the carb. body to set on the face which bolts to the air box. I made a paper pattern for drilling holes for the studs using the gasket and then located the drain plug which sticks down past the mounting surface.
Here the holes are done including a slot to clear the drain plug safety wire, rather than take it back off. It sets flat and solid on the base board. The studs fit snug enough it doesn't move around.
Next I needed some feet to level the board. I drilled 3 holes for the 3 prong tee nuts and used some 1/4" carriage bolts for the three feet. Three feet make it easier to level. You level the 2 across the front to level it side to side and the adjust the back one to level it front to back.
For a reservoir I found a bottle for watering pets at Tractor Supply. I had to remove the little valve in the outlet tube, which allows water to come out when a pet touches it. It is made to clamp to a cage so I made a block of wood which I attached it to. The block has a hole drilled in it so it can slide on the threaded rod to adjust the height of the column of fuel. Using Av Gas at 6.01 lb/gal I needed a 19.26" column of fuel to get 1/2 psi pressure at the needle seat. The nut under the block of wood allows the bottle to be held at the desired height.
The metal piece with the red line on it is a gauge for controlling the fuel level in the bottle. The top of the red line is the height of fuel needed as the needle seats to shut off flow into the carb. The nuts for it are tightened to prevent it moving along the 1/2" threaded rod.
To control the flow to the carb. I mounted a shutoff valve and a tee with a drain valve. The drain valve allows the carb. bowl to be drained between tests. For the carb. on the Cessna the hose routed under the board. For the Fly Baby it routed on top.
I ran the test on the picnic table outside. Using gasoline in the attic didn't seem like a good idea. The first step was to level the carb.
I wanted to have the fuel head level correct as the needle closed so I needed to figure out how far above that level the fuel needed to be at the start of the test. You can see the level droped significantly filling the bowl and the section of hose after the shutoff valve.
The depth to the fuel (13/32" +/- 1/64") was measured at the far end of the bowl just to the left of the seam in the float.
The rig worked well and the carb. is finally done.
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