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 hard wood 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.

Given today's engines I agree, though things sure have changed over the years. I remember trying to find some extra HP from a Fox, McCoy OS Max, etc. to fly a slightly "overweight" airplane... not trying to throttle back on the power. Who said the 50's and 60's were the Golden Years?


Edit to add.

IMHO the best method for restriction would be the Bru-line or pantyhose approach. By increasing the resistance to airflow well above the venturi area, the laminar flow isn't disturbed as it is with stuff poked into the venturi opening.

The screw in venturi setup is very common in speed limit combat.  Gets the speed right at the limit, accurate and repeatable.  Tom H.

Hi

It has been done before many times, I have setup OS 30s  OS 35s, ST 40, 46, 51. , PAs of several sizes, HPs K&Bs and others over the past 25 years or so.
I also setup OPS engines as well as VF motors for Bill  Werwage with both screw in adjustable and standard true venturies.
Scott Bair  made many of his engines for testing that used screw in fuel post venturies  to adjust the venturie size.
However with all that being said, once you find the right size, it is better to make a true venturie this size, you will get more power and most times easier priming and better fuel draw.

below is one of mine that is screw in adjustable from one of my ST 49s from the early 80s,  A very easy way we used to make these was to use a OS 10 FSR  RC  carb  assembly and  machine a alum block venturie  and thread it in from the side with a locking nut used to controll the depth

Regards
Randy

Bob R:
Back to your 4-stroke set-ups.  Just how sensitive is it?  Do you adjust it instead of the needle?  Have you had any issues with the screw adjusting itself (ie wandering) due to engine vibration?

To Randy's point - once its adjusted do you forget about it?  Could you replace the system with a simple smaller venturi?

Hi Dennis
Yes once you get the size right , you never touch it again  Unless  you change something else that affects the  venturie size. The venturie being adjustable is small and simple. However if you are setting up another of the same..unknown.. motor  you need not make another adjustable, you just make one the right size.
I have seen others do basicly the same thing with an older K&B  2 piece NVA and venturie, and I have maybe a dozen more here

Regards
Randy

Bob R:
Back to your 4-stroke set-ups.  Just how sensitive is it?  Do you adjust it instead of the needle?  Have you had any issues with the screw adjusting itself (ie wandering) due to engine vibration?

To Randy's point - once its adjusted do you forget about it?  Could you replace the system with a simple smaller venturi?

Hi Dennis,

When I tap the hole I carefully run the tap in just far enough to make it a tight fit for the nylon screw. Standard taps have the first several threads tapered for easy starting and done carefully the nylon screw can't be turned without a driver.  I understand one can purchase undersize taps but I haven't looked into it. An undersize tap would make it easier.

BTW: The firewall mount actually uses two hardwood blocks, the photo was of my concept setup, I added another support closer to the venturi on the final version. The new ship uses the fuselage side as the outside support with a hardwood block about 1/2 inch from the venturi.

I haven't found it to be extremely sensitave, if it were a click setting 1/8 turn clicks would give you enough resolution to get a good setting. I have always readjusted the needle when I change the choke area just to be sure I'm not too rich or two lean, the needle is pretty critical on a four stroke.

So far I haven't found a need to change anything for "conditions".. On all 4 of the four strokes I'm running once I had it set for the prop/lines/laptimes they stay fairly stable. Last year I attended contests in 3 states with temperatures from 80 to 95 flying the Score/Saito 56 and didn't touch the choke or needle. I've heard some brag about never adjusting the needle but I never really believed it till last summer.

I would say if I had a way to figure out what the actual choke area was I could make a venturi that size and it would work fine.  To me the nylon screw doesn't weigh enough to make jumping through the calculation hoops worth it.