Sunday, October 27, 2013

Carburetor Float Level Setting

 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.

Saturday, October 26, 2013

Baffles Are Done

The baffles are all repaired.  Except for a few small pieces they are original Factory parts.  After I cleaned them and made a few small repairs I primed and painted them with Aluminum color Engine Enamel.  They clearly look painted but so much fresher.

Before I can install them I need to make new retainer wires and springs.  There are 3 different length wires and I only have 2 so I need to do a little research to figure out the length of the 3rd one.  The long one is obvious where it goes but I don't know which of the 2 short ones I have.  It also appears I'll have to remove the intake tubes on the engine to install the scoop pieces which go at the bottom of the crank case.

Friday, October 25, 2013

Installing Engine Baffle Rub Strips ( Seals)

 Installing the Rub Strips was simple but slow work compared to stapling them on in the factory.  I used .041" stainless steel safety wire and a variety of pliers to cut, hold and form the staples.  To cut the seals I used scissors and a knife.
 I started with the simplest pieces which were on the front of the scoop below the crank case.  Because of the sharp corner bend I cut most of the way through the felt, leaving a little of the felt and the bottom fabric to hold the 2 pieces together at the corner.
 To locate the staple holes just hold the part so light will shine through and you can see the holes very easily.  I then just pushed heavy duty straight pins, like you use for sewing a fabric envelope, through the fabric and holes.  This held the strip in place and created holes for the staples.

 To make the staples I cut off pieces of safety wire with the end at 7/8".  The wire is slightly longer than that because of the shape of the diagonal cutters.  Because there were so many wires to cut I used a square to create a stop to speed up cutting.

 The legs of the staples are spaced with a gap of 3/8".  I was planning to make a tool to form these but found I had a pair of vice grip pliers which were the perfect width to form the legs.  You just use the table top to push the legs over to 90 degrees.

Perfect staples for holding the strips.

 I found it easier to first push the staple through the fabric and then through the holes in the baffle.  I also worked one staple at a time.

The legs sticking through the baffle were bent enough to hold the staple and get a good start on bending them flat.

 I used another large pair of vice grips to finish the bends flat and tight.  The joy of having lots of hand tools.

 The strips are as secure as the originals and easy to replace when they eventually wear out.

 The felt sets nicely on  the flange on the baffle ready to rub against the cowl to seal and allow everything to slide around when the engine is running.
 The bottom scoop seals are done.

Here is a more rounded corner on the horizontal baffle around the cylinders.  I like how these came out.  I still have the large rear baffle to finish.