I have picked up a OS40la and a 35fp rc engines. Both new. What size venture is recommended for them when I convert them. Anything else I need to know would help. Thanks   rog

I have a lathe and material to build these. Also have a 46la stunt. Same size venture work on the other 2?

Geoff; Thanks for the info. A little research turned up 7MM for OS20 venturi. Have built two and will get them installed when NVA's get here.  Thanks again  rog

Roger, I've converted a few FP 20s from RC using the venturi that Tower sells for the FP 20/25 and, despite what their technical dept says, every one of them has been 6.55mm ID—as measured with a vernier that has never been dropped. I've also bought some LA 46 venturis from Tower and they all measure 7.1mm.

Some applications use the 17/64"ID (6.75mm) for the LA 46 and there have been reports of the FP 20/25 venturi working to tame the FP 40. I have only used the FP 20/25 venturi in FP 20s flying Goldberg Shoestrings. I tried the FP 20/25 venturi and a 6.75mm venturi in an FP 40 in the 59oz model—both unsuccesfully—and I tried the 6.75mm venturi in an LA 46 in the 59oz model. It was OK but the stock 7.1mm venturi produced a better result.

The model was 565 sq in and weighed 59oz. That may have been a factor.

I can only guess what the result will be with a 7mm venturi in an FP 20.

Brett I am putting these in an 40la and  35fp .At 7mm am I close enough?  thanks rog

Brett I am putting these in an 40la and  35fp .At 7mm am I close enough?  thanks rog

     I don't know for sure - what did the CL version come with originally? And more to the point, for which airplane? 

   Brett

     

Depends on plane. Same with extra head gaskets. Lightweight sport/stunt planes like the Midwest Magician and Twister should work well with the smaller .257 venturi in either the LA40 or FP35. In a Tower 40 (FP40 clone, but better materials and better machining) I had a .272 venturi (Lee Machine, he makes most in between sizes .257-.283) in a Light Weight Midwest Magician. 36 ounces. Walter Umland wing kit, fues made of wood as light and stiff as I had. Excellent flier. Very well matched engine run. 2 or 3 extra head gaskets, stock FP click needle valve, tongue muffler. This past season I pulled out a $10 flea market Prowler to work with. 46 ounce plane, much thicker airfoil. Power was a Tower 40, same set up as the Macigician. It would pull the Prowler but needed to be set to a leaner 2-stroke, the result was no break. Not ideal. Dan Banjock and Mike Palko insisted I go to a bigger venturi. The .283 improved the run. More power, more authority over the top, but still not much break, if any. Mike and Dan then got on me to remove head gaskets and up compression. OK. Did that. Now I had a break, good power, very nice match to this plane. Fuel, 10% Powermaster, 22%(50/50). Needle was not critical. Power delivery the same flight to flight. I did some further messing around. I had the actual engine from the Magician tuned as it was when flying that bird. I bolted it to the Prowler. It behaved the same as the other Tower 40. It worked best with the bigger venturi and one thin head gasket.

The large venturi works, can be preferable to the smaller venturi, depending on weight and drag of model. Engines need to be tuned to the model. Runaway is often the engine running at the rpm where it is tuned. The tune a function of many factors we can manipulate on our simple (no carb, for instance) 2 strokes. The tuning elements we can change are venturi size, fuel, prop, muffler type and restriction, head gaskets. Frustrations are intense when we attempt to make an engine run at a slower speed than is inherent in the tune. FPs and LAs 35/40 have a false seeming sweet spot. They can be set to a rich needle on the ground, a wet 2 (almost 4) to leaner 2. The back and forth switching as the nose is pointed up or level, repeatable on the ground. Sounds sweet on the ground. Problem is that once the model is flying engine variables change. The sweet break turns into a runaway as the model flies. The engine has heated up and assumed the inherent tune. If speed is too high, time to fit a smaller venturi and set the needle leaner. (See above.) The false sweet spot, setting FP 35/40s and LA40s to the false rich setting, is a common problem leading to endless frustration. Engine is set too rich on ground. Needs to be run leaner, a  needle setting that can be repeatable from flight to flight. A needle setting that can keep the engine running in a similar way throughout the flight. If model flies too fast, go to a smaller venturi, or a prop with less pitch.