65cc echo conversion, hda 16.6 mm carb. I can adjust the carb for a good top end and smooth idle. From idle up to about 1/2 throttle it runs rich. I can not adjust he mixture screws for a smooth midrange and full speed/ decent idle, it goes lean , top and bottom end, with a smooth midrange. I bent the pop off lever to meter less fuel. Bend it more? Is the carb too big? It runs real smooth with a smaller carb but the top end looses 500+ RPM. It is making good top end power spinning a MENS 22x10 at 7000. The plug shows a rich mixture. I am running mid grade gasoline.

I know props are expensive but you might try a different prop load?

The bigger engines do not usually turn as fast as the smaller engines.

I second Jim's advice. Try a larger prop first and if that doesn't get you where you need to be you may have to start changing the rocker spring under the metering diaphragm until you get it right. There are a number of them available; they are typically sized for a particular carb/engine combo and they control the delivery curve to an extent. (What they actually control is the pop-off pressure on the regulator) That's why there are so many Walbro's with the same venturi size. They are custom calibrated to the specific engine the were intended for and also for the load the engine was designed to operate at. Change the load (think prop size here) and the calibration changes accordingly.

I had a Saito 72 on a 40 size Ultra Stick running a APC 13 X 6 prop and everything was fine.

I changed the prop to a APC 11 X 9 and the midrange was terribly rich.

Changed back to the 13 X 6 and all was forgiven.

Yep, it's a pop off spring deal.... That can take some experimenting to cure.

AV8TOR

You can stretch the springs or cut coils from them.

It is best to have some extras from old carbs laying around.

Walbro sells a kit with different springs in it. I'll look around and see if I can find it again.

Edit: OK found it. If you look on page 7 or 8 of the Walbro service manual you will see a picture of the spring kit I am referring to. It is listed in the manual as Walbro part number 400-606. It contains quite a range of springs.

Link to the service manual - http://wem.walbro.com/distributors/s...viceManual.pdf -

I put a much beefier spring in it. No a "Walbro" spring but much more tension. Same situation. 22x10 Mens should be pleanty of load? I am pretty sure I have a 24x10 but can't find it. If I do I'll try it.

Isn't there some trick about filing a notch in the butterfly? The notch lets in more air and leans the mixture.

The 24 X 10 is about max for that engine.

I picked up a MAS 24x10. It only spins up 5600 RPM for 24# thrust. It does have a smoother midrange with the big 24" prop though. So, loading it down did help but, at the cost of power. If I fly it with the big prop I will need taller landing gear.

It spins a 22x8 bambula to 8000, for 41# thrust, 22x10 MENZ to 7000 for 32# thrust. Apparently faster is better for this engine as far as thrust and RPM.

Is it common for MAS props to be badly out of balance? This one was a mess.

5600 is a better rpm for the engine

<table cellspacing="1" cellpadding="3" border="0" class="background"> <tbody> <tr> <td class="menudetail">400-606</td> <td class="menudetail">Kit - metering lever spring</td> <td align="right" class="menudetail"><font color="red"> <font color="red">$41.84</font></font></td> </tr> </tbody></table>
Walbro Price List
http://wem.walbro.com/walbro/compone...mp;FamilyName=
http://wem.walbro.com/distributors/P...k/PriceLst.pdf

The spring kit is a little cheaper here . . . scroll down

http://www.lawnmowerpros.com/Walbro-Parts.asp?Page=297

"5600 is a better rpm for the engine"

Glad to hear that.

How do I justify sacrificing 40% of my thrust when going from the Bambula to the bigger prop?

Below is a photo of a mod you can do. You should havea extra throttle plate in-case you cut too much. This has been show to work to get a leaner mid-range. I only recomend it for guys that have the time to read a little about this &amp; do not mind to think this out real good. It worked for me. Capt,n

Prop Power Calculator In Excell (.XLS)

http://mvvs.nl/prop-power-calculator.xls

Why is that to open these files you need to download a lot of crap to get any use of the file extension...why not use files everybody can open???????????????????????????/

What about trying different carbs? They can often be found cheap on E-bay. I don't think a 15 mm carb would cost you much, if any power, and would be worth a try. On this site/page, you can enter the carb number and it will give you a blow up diagram of the carb and the venturi size: http://wem.walbro.com/walbro/family.asp

I think the 22 x 10 carb should be a fairly good match for the engine, but you really won't know which prop is best until you fly it and see if it performs how you want it to.... Static thrust is only valid for 3D flying; particularly hovering. In all other flight modes, a prop will unload and give you more thrust than the static figures would indicate.

The modification Captain John showed is a good one that I have used many times, but be very careful not to get the notch in the butterfly too big or you won't be able to get the engine to idle low enough. The notch is directly in line with the holes in the venturi that feed fuel at idle and just above.

I sure wish Walbro would make (or have made) a carb with midrange adjustments....

Good luck and let us know how it goes,
AV8TOR

The .xls files are for running in an EXCELL spread sheet.

You can also download Open Office and it will run in open office. This is what I do.

Otherwise, you will have to do without and forget it.