sboxer

My Feedback: (1)

My ryobi 725 from Harbor came with a Zama Carb. Unfortunately, the throttle arm on the zama is so short that I didn't think I could attach a nose wheel arm to make the throttle less sensitive (1/3 throttle on my hitec eclipse is full throttle on the zama).

On the spur of the moment I bought a Walbro WT-570-1 off ebay (see pictures attached). The carb is stamped 939 / wt570
According to the walbro tech specs at:
http://wem.walbro.com/walbro/product...m=WT%2D570%2D1
this carb has a 11.13 mm venturi.

I have 3 questions and help from the engine intelligensia is appreciated:

1) In general, is this an appropriate carb for my Ryobi
2) The fuel in and return valves are on the same size (i.e. no bent brass tube for fuel in). Which is which?

3)Was there a good way to attach a new arm to my zama give the very short throttle arm?

Thanks!
Steve

sboxer

My Feedback: (1)

And for those of you keeping score at home, here are two snaps of my zama already mounted with the very short thottle arm and temporary linkage (clevis). How to extend that throttle arm?

w8ye

My Feedback: (16)

The big tube is the fuel supply line.

Plug the other line for without a check valve in the line it will suck air.

The carb should be ok

The problem with the throttle lever is that it will hit the back of the engine in the open position.

Enjoy,

Jim

mikenlapaz

#1 yes; #2 open the side cover with the single center screw and see which fitting feeds into that side of carb; that is the supply line from clunk in tank

Does the zama throttle shaft extend far enough on the opposite side to install a dura collar stop?
If so, soder a brass stock arm onto collar and rotate the carb 180 degrees. Modify the carb adapter plate to assure the pulse hole lines up to new location.

sboxer

My Feedback: (1)

On my stock Zama there is no throttle shaft at all on the other end.

From what I can see, the throttle arm on my Zama does not hit the back of the engine before it hits full throttle.

smallplanes

My Feedback: (3)

The arm on your carb should be the the same length as the arm on your servo. If from the center the shaft of your arm on your carb is 1" then from the center of the servo arm should be 1" . If you have it this way your throttle will work just right.


smallplanes

sboxer

My Feedback: (1)

My throttle arm is 1/2" and I can easily do the same on my throttle servo's arm. However, is that going to fix the issue that 1/3 throttle on my transmitter equal to full throttle? I thought the purpose of extending out the throttle arm and moving the linkage in on the servo arm was to "de-sensitize" the throttle response.


SB

smallplanes

My Feedback: (3)

Well ive built a few planes lately and i always may the carb link and the servo link the same length. It will then move with every click of your radio. The same all the way tru. So far it has worked for me every time. This also applies to all your control links tru out the plane. Try it i bet it will fix your problem.


Thanks
smallplanes

mikenlapaz

Your servo has 120-140 degrees of max rotation. You want to use as much of that rotation as possible. (shorter servo arm)
Your carb is probably going 75-90 degrees from closed to open position. (longer arm on carb utilizes that full servo rotation better giving more resolution (finer tuning on the stick.)
Short connection at servo and a longer arm on carb works for me.

Smallplanes system works but I think he probably is not using as full of range on the servo (endpoints)

The longer arm may have a clearance problem with the back of engine. Mine did on on carb. I made an additional carb spacer out of some ac ply and 30 minute epoxy. Two pieces of glass with some 'vaseline brand release agent' made the mating surfaces nice and smooth.

A search in engines for something like carb linkages can give you a bunch of info practical and technical.

AnthonyH

My Feedback: (16)

I have a similar carb to the walbro in the pic. The big tube is the fuel supply line, but what is the purpose of the other tube? Is it for a primer bulb? That is my guess since it comes in on the other side of the inlet needle valve.

speedmenow

My Feedback: (2)

It the tube on the other side may be for external pulse pump for fuel.