I have a low mileage "Big Art" OS35 from somewhere around 1977-79.  It runs well but I am not getting the power boost on the 4-2 break that I remember.  I confess to knowing as little as possible about engines and I don't have the budget to just go out and by something new.  As I see it there are three things that I can tinker with.  Venturi, Prop and Fuel.  Right now it has the a very short venturi with about a .125 center hole that flares out rather abruptly after about 1/8".  No idea what it flares to but it is nearly the size of the intake.  Looks a bit like a flat insert into a large stock venturi.  I have another tall one that is about .250.  I have read about 200 of the over 1000 posts related to Venturi Size and I could not find any mention of a .125 venturi, why it is so small,  or for that matter what change to expect if I went larger.

So, since all of the advice from the 200 posts I read was basically boiled down to "try a bunch of venturis till you find one that works" so I will ask "Where do I go to find a bunch of venturis."    Also is the OS35s the right model and if memory serves me right didn't OS use the same venturi for .25 through .40?

I can tinker with the prop and fuel once I get the right venturi.  Right now I am running a 10-5 Brodak prop and 5% nitro. 

This seems to be my last hurdle to getting back into the sport!  Please forgive me for asking what are basically dumb questions to those of you who have evolved with the sport and are comfortable with all the new technology.

No expert by any means but are you sure its 0.125" or 1/8th inch or approx 3mm? Only asking as an OS NVA is 3.5mm diameter so no air would pass at all? 0.250 sounds like a starting point then ream out till it does what you want

Sorry, I should have passed that on.  The .125 or so is only about 1/8 deep then it flares out.  Thanks for spotting it.  I will amend the original post.

Ken

double the nitro  to 10%, then you will  use a  280 to 301  venturi  size  with that motor, depending on what muffler you use, they came  stock with 301  diameter  venturis  but that was for use  with the  very restrictive  703 mufflers
use at least 25% oil, part castor

Randy

Thanks for the help!  This is exactly the advice I am looking for - where to start!  I suspected I might be under nitro'd and that mey ve the 1st thing I change.  I have one of the older very open mufflers on it.  I think Art manufactured them.  I have a venturi in an OS40 that is tall and about .250 or so.  I don't have the instruments to measure it.  Is it worth trying?  Finally - where do I go to buy a set?  Every place I look is "Backordered", "Out of Stock" or "Discontinued".

Ken,

I expect that Randy could help you out with venturis for a .35S.  I'm sure that Jim Lee (Vendor's corner or http://www.leemachineshop.com) could make you some to fit the .35S in whatever size you want also, but he's at VSC this week so you shouldn't expect any contact with him until at least next week.

I'm running an OS35s in my Polywog Chief. The venturi is .250 with OS NVA. I use 5% fuel with 25% oil 50% castor,50% Klotz tecniplate. The prop is a 10x6 .The plane is very light weight and fly it on lines 59'6" cl to cl. The plug is an Idle Bar type. The engine runs beautifully....Cheers,Skip

It ran well with it, so I left it in there .No technical reason.