How large can I open the stock, “.250”, venturi with a stock Brodak nva. for stunt flying?

      Tia,

      Frank McCune

I have the prototype Brodak 25 in my possession.

The venturi is .250 and it worked perfectly for stunt.
But you can probably open it up a bit if you want to experiment.

Bob Z.

What size is the spray bar? I only have Brodak 40's. I've not owned a 25

  If you want to boost the power of your B-25 or B-40, modify the muffler for a straight out the back exhaust exit with the stinger hole about 1/2" I think. I was disappointed in the performance of my first of two B-25' s and after posting about it Tom Hampshire had me send mine to him  He modifed the exhaust and what a difference that made! Tried the same muffler on my B-40 model and it had an explosive break with that muffler. The B-40 was very good as it was, but was almost uncontrollable with the more open exhaust.  The stock muffler is pretty restrictive. You basically cut the back end of the muffler off, then machine and press, JB Weld, or TIG weld the new piece in it's place. I haven't tried simply opening up the standard muffler stinger as of yet.
   Bob, have you any pictures of what T om does to these?
   Type at  you later,
  Dan McEntee

Hi Bob;
   Send me your email address in a PM and I'll email you some photos of the muffler. Then you can post them . I still haven't learned what I need to do that yet. It's been a while and I am getting older but I do believe it was Tom Hampshire that modified mt muffler, and then I did a few for myself to experiment with. With out the modified muffler, the B-25  I had would not make enough power to fly a Ringmaster, no matter what prop I tried. Afterwards, it was the equal to an LA.25. This B-.25 engine I have was from the first run I believe. Put the same muffler on a B-.40 and it becomes a monster!
  Type at you later,
   Dan McEntee

     Hello Brett,

     Thanks for the reply.  However, I did not want to hear that news.  This engine was always a dog.  I tried to fly it on a Clown but it was futile.  Just plain gutless.  Yes, I tried various props and still no power.  It is on a Sterling F-51 Mustang, 52’ lines and turning a 9-5 wooden prop.  The fuel is 10n, 11% syn., 11% castor oil.  The engine is very easy to start but after that...  It is run in a wet 2 cycle mode that is very smooth.

     Be well,

     Frank McCune

     Hello Brett,

     Thanks for the reply.  However, I did not want to hear that news.  This engine was always a dog.  I tried to fly it on a Clown but it was futile.  Just plain gutless.  Yes, I tried various props and still no power.  It is on a Sterling F-51 Mustang, 52’ lines and turning a 9-5 wooden prop.  The fuel is 10n, 11% syn., 11% castor oil.  The engine is very easy to start but after that...  It is run in a wet 2 cycle mode that is very smooth.

   I agree that this sounds very weak, much weaker than I would expect, and not nearly as strong as a OS-25S (baffle piston from 70's) or Fox 25/29. My 25S was/is no powerhouse, but it's stronger than this.

   As always - what is the provenance of this engine, did you get it right out of a box from Brodak, or did you get it from some other source?

    Brett

      Hello Brett & Dan:

       I am running a stock Brodak chip muffler that came with the engine.

       I bet that Brett has already guessed that I purchased the engine used.  It came second handed from a “friend” who bought it from the estate of a fellow club member who had passed.  The plane ,Super Clown, looked very well built and for $50.00 I took a chance.  It was not one of my better choices. Lol

      I have been told many times that the B25 are a real dog.  I should have heeded the advice.

      Thanks for the advice.

       Frank McCune

    Hi Frank;
    I sent you some pics from my phone of the modified muffler along side a stock unit. For those that never read my posts, like the ones in this thread, #6 and #8, I have a brand new (at that time) Brodak .25. I mounted it on a fairly light Ringmaster after careful break in per instructions, and no matter what prop I put on it I could not get enough power out of that engine to competently fly the old time pattern. I had a LA.25 on the same model as a comparison. I tried every prop in the book. The engine did not run bad, it just didn't make any power. It would go where ever you needled it, but would not make enough power to pull your hat off your head. It was like the proverbial Eddy Murphy "banana in your tail pipe!"
   I commented on it on Stuka Stunt and on here I believe at that time, and I got a message from Tom Hampshire to send him the engine. I had already examined the engine pretty closely and could not see anything obvious, so I marked up all the internals very subtly, and sent it off to him. A few weeks later I got it back with the muffler modified and a nice note from Tom explaining that the muffler, although the same as on the B-.40, was a bit restrictive, and had some before and after RPM data to accompany it. I put the engine back on the airplane and it was like a different engine, but I could see all of my markings on it. In fact I don't think he touched the engine at all, just machined a new , straight through back end with a bigger stinger, cut the back end off the muffler, and pressed the new back end on. The engine didn't sound any louder either. It would still needle fine, and use about the same amount of fuel. That was one of the things that bugged me about it at first, was that it was using fuel, about the same as a LA.25.
  I made my self a copy of the muffler from a spare stock Brodak muffler and tried it on the Brodak .40 that is in my Twisted Shark model, which has a early run B-.40 that has been an excellent  engine from the beginning, with a good ability to needle where ever I wanted, good power to pull a high 40's ounce airplane, and a nice , subtle break in the maneuvers. With the more open muffler, it had a break that was explosive! It seemed like the model doubled in speed. I didn't try to tame it in any way, as the answer was to simply put the stock muffler back on, and it was back to it's old self.  So, there is something to the muffler being restrictive. I haven't tried just cutting off the stinger on a stock muffler and opening up the hole to around 1/2", but it is something to consider. I don't think you here much about the experience that Frank and I have had because it seems like most guys use these for racing and don't use the muffler. That is why I had an easy time of finding a muffler when Brodak didn't have any to sell.
   So Frank is not the only one to experience this, but I have no idea how to explain it, but opening up the exhaust does help it quite a bit. And if you look down the venturi, you see w hole lot more spray bar than opening, and I bet opening it up .005" or .010" might make a difference also. I might try that some day just to see what happens with the stock muffler.
  Type at you later,
    Dan McEntee

Interesting. I guess I never noticed the gutless thing because we don't run mufflers?? (I mean, we totally do....  )