Why not, adjustable choke area on a 2 stroke..

Last year when I was getting a handle on how to make my 4 strokes run I was adding layers of pantie hose to adjust the choke area, it finally became a real PTA when I got up to 8 or 9 layers. I said this is silly and stole an idea from the speed limit combat guys. I drilled and taped the venturi so I could run a 10-32 nylon screw through the side. By adjusting how far into the venturi the screw is I can easily dial in the engine run.

Wondering why we are not doing this with 2 strokes.. Hear of adding a Brue-Line filter, layers of pantie hose, swapping out ventures for the conditions and even Bill Willson who made a bunch of hardwood clips that he sticks in the venturi to dial it in.

The only argument I can think of is the screw somehow effecting the air flow.. I haven't seen any detrimental effects from the 4-stroke engines I'm running with a screw stuck in the venturi. I would think turning a screw would be allot easier than any of the above means to adjust the choke area for the conditions.

Just kinda makes me wonder why this hasn't became as common as something like adjustable horns. Can't believe I'm the first to use it on a stunt ship.



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That looks like a good idea for a rear venturi but for a front one (which most CL engines are) it might be difficult to find an anchor for the screw to be mounted on. Good idea though...

Ya pretty obvious the firewall mount wouldn't work on a typical two stroke, just posted the photo for reference. The 40 has a 10-32 nylon screw through the side of the venturi and would be the way to go on a typical two stroke.

I have put my speed limit engines on flight streaks and the like. (OS 25FP's) It works well, it controls the speed just like choking the engine down with a filter or smaller venturi.

It is kind of a set and forget it situation. Once you find the prop/fuel/venturi to get the right speed you don't mess with is much. Is is worth it? I don't know. I am not that fussy on speed and not that good of a stunt flier.

For me the hot setup is a largeish (1/4inch) set screw tapped into the side of the venturi. It is kind of sensitive, a 1/8 turn is a lot but it works, and doesn't stick out to far.


Just a sport flier
Dave Siegler
AMA 720731

Bob,
This is the venturi of the current world champ.
I don't read French but it looks like a small set screw directly across from the end of the spigot.


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Bob,
Phil Cartier does this on his 2 stroke engines. I saw it for the first time year before last at Brodak's

regards,
Rick "catdaddy" Blankenship

Doesn't an RC (Oh what dirty words) carb provide the same thing at perhaps a very light weight gain.

Paul.

Hey Paul,
I was thinking about brazing a horn on the screw, and then hooking it up the the servo on my RC plane

>Doesn't an RC (Oh what dirty words) carb provide the same
>thing at perhaps a very light weight gain.
>
>Paul.

less moving parts? more possible air leaks? People set in their ways? I have used RC carbs on sport planes with good results.

For combat I don't think a carb will work on a bladder, and the the needle will be too course, and of course more possible crash damage


Just a sport flier
Dave Siegler
AMA 720731

>less moving parts? more possible air leaks? People set in
>their ways? I have used RC carbs on sport planes with good
>results.
>

Maybe less moving parts - not sure. Most likely just people set in there ways (some call it tradition!). I would think air leaks would affect RC performance as well (and it does on some poorly manufactured/designed) carbs.

I certainly would not advocated it for combat - especially with a pressure system!

Paul

An RC carb weighs a whole heck of alot more than a 10/32 nylon screw. Plus you have to setup a linkage to hold the carb and adjust it. With a 10/32 nylon screw through the side it doesnt get much easier than that.

My Brother bought a 60 sized Belko Motor at the 04 WCs. Venturi on the bottom of the motor. The Venturi is threaded on the outside. There are 6 large holes around the side of the venturi. There is a cap that screws onto the venturi. So you just screw it on tighter to cut off the amount of air in and unscrew it to let in more air. The inside of the ventrui is never touched so the airflow and direction is not changed only the amount let it. Pretty darn cool design if you ask me!

Doug
Does that mean that the venturi throat remains untouched. You're just blocking airflow into the venturi? This is just like more panty hose over the opening. The set screw changes the venturi throat area.

Frank,

Yes it limits the air in but not the throat area. Throat area is untouched. The set screw is more of an obstruction in the throat area wouldn't you agree?

Doug
Back in the 70's, Addis Elmore, from the Dallas area, flew OS35's very successfully with a unique venturi arrangement. He had escentially a large bore throat area with a spraybar through it. The bore was really large, it wouldn't have worked without the rest of the system. The "rest of the system" was a small plastic disk that snapped into the top of the venturi. The disk had a small hole in the center. He had a bunch of disks with different size holes. He said he never changed the needle valve once it was in a favorable spot. From day-to-day or contest site-to-site, he would just pop out the disk and snap in a smaller or larger hole. It worked for him. The true "restrictor" venturi. As I recall, they were really small holes, like 0.100 to 0.125 ". Choked flow at work here?

>snip<
>My Brother bought a 60 sized Belko Motor at the 04 WCs.
>Venturi on the bottom of the motor. The Venturi is threaded
>on the outside. There are 6 large holes around the side of
>the venturi. There is a cap that screws onto the venturi. So
>you just screw it on tighter to cut off the amount of air in
>and unscrew it to let in more air. The inside of the ventrui
>is never touched so the airflow and direction is not changed
>only the amount let it. Pretty darn cool design if you ask
>me!
>

Would love to see a picture of that, sounds like it wouldn't be real tough to make.