Ok, I've done the searching, but apparently I'm still too dense to figure it out.

Which needle affects which? High affects low, or vice-versa? Should I adjust the high or low first?

I've been fiddling with my Brillelli 40 and it seems, on the ground, the transition is always good. I had the low needle less than one turn out today and I fear that may be too lean. I did eventually hit a spot where it didn't want to idle any more, and I backed away from that point. At that point, the transition was solid on the ground, but once I got it up in the air things were different. When throttling down on a downline, the engine is still idling well, but sputters when going back up on the throttle. It finally died on one of these manuvers and I had to deadstick it. Am I still too lean on the low?

When tuning, do you make all the adjustments on the ground, or make an adjustment, see how it reacts in flight, land and adjust again?

Thanks in advance.

---------------


Because the timing is controlled by the ignition system, you don't have to worry about running "too lean" as you do with a glow engine.

If the engine will run, the needle is not too lean from a safety standpoint. Throttling response, etc., is another matter and will require finding the best compromise.

High compression, highly tuned two-strokes can "hole a piston" if too lean, but our engines are nowhere near that state of tune when flown in non racing environments.

In flight requirements are what we should base our engine tuning upon. Who cares if it runs great on the ground, but terrible in the air? If the engine won't fly the plane, it is a useless lump of metal.

When an engine changes tune in the air, you are most likely seeing the air pressure in the cowl change and the carb seeing that change. It's a fairly easy fix.

On the carb you will have a plate with a small hole in the center. This plate is on the opposite side of where the fuel line connects. on saome Walbro carbs, this little hole can be on the edge of this plate.

Anyway, carefully remove the plate, sand the plate on the outside to get it nice and shiny, now take a short piece of copper tubing and solder it over the hole. The idea is to solder the tubing over the hole but not block the hole. Once that is done, remove any solder that may have gotten on the back side of the plate and replace the plate.

Now take a length of fuel tubing that is long enough to go inside the fuselage and attach it to this coppoer tube and run the line into the fuselage and just let it lay there.
Check the tuning and go fly

The idea of this is to fool the carb into thinking the air pressure has not changed. That little hole is what sense that pressure chage and changes the mixture accordingly

If you do not feel like doing all of this get an old Pepsi/Coke can and cut out a nice flat, say 3 inch square. Find 4 spacers or make up 4 spacers about 1/8 inch thick. Now remopve the 4 screws that hold the plate on, place the 4 spacers over the holes the screws came out of, take the square you cut out and place it over the spacers then get 1/8 inch longer screws with the same threads and screw the square you made over the plate. This will do the same thing as running a hose into the fuselage but it may still allow the carb to change depending on the airplanes attitude

Sounds lean on the low to me and I agree with Bubbagates on the pulse hole fix. I used a piece of carbon fiber on one of mine fo fix the issue, When I would roll or knife edge one way at 3/4 throttle or above the engine would sag. Too lean on the high will cause a sag or loss of RPM on a straight upline also but that was not my problem.

I would start over on both needles. The one marked "L" is the low and is nearest the cylinder. The one marked "H" is on the outside. Start out maybe 1-1/2 to 1-3/4 out. Engine should run decent but kind of rich. Being a new engine you want it a little rich on the high end for a few gallons remember. Anyway, lean the high for maximum RPM then richen it up a little. A little being maybe 1/32 to 1/16 of a turn. A screw drivers blade width. Now do lean the low end the same way and check for transition after you lean each time. You will also need to adjust the end points for your idle as you lean the low. Lean it until it stumbles on transition then richen it like the high. Again if the engine is new a little rich is good.

After you break it in assuming it is new, you can lean the high and the low using the procedure above. Remember that the low needle circuit affects the high. When you lean the high check the transition. When you lean the low check the RPM at WOT to see if it got higher which means the high got leaner.

This isn't real hard to do and just takes a little practice and experience. Try to find someone at your field who is experienced with gassers. Once you hit it right you will rarely touch a needle except a tweak depending on weather. Very humid here most of the year and hot. I will need to tweak mine again but haven't touched it in 8 months.

Oh yea, depending on location of your needles you may want to do this with the engine off. Some do it while running. You also will get many variations for tuning these carbs There is the Don's Hobby Shop way of pinching the lines etc but I have never been happy with the results.

Too lean on the low will also make it hard to start. After you run it a while check the plug too. It should be a light tan color if all is good but that will be a while on a new motor.

Tim

I believe you are talking about the static pressure nipple that the Brillelli's already have in place. I have a piece of fuel tubing attached to it and running back into the fuse. The engine is not currently cowled.

What I'm finding weird on this engine is that I can really lean out the low end and the engine still runs great, but it seems that the high seems to need to be much richer. I've got the top end to where it doesn't sag at WOT and it's loading on an upline, but it wants to die in the air when going from low to high after a downline manuver. I will try richening up the low to see if I can get that to go away.

Thanks

The go-kart guys really squeeze the lows because they don't care about transition. They run wide open all the time. We need the low to be smooth and reliable. Still sounds lean on low to me. Not sure I understand the "loading" on an upline though. Anyway, when the high is tuned right you should be able to pull straight up at WOT and never loose RPM and the plane should go out of sight. Again remember the low does affect the high. Also remember that the engine will lean some in the air so a little rich on the ground may be perfect in the air.