Hi,

I have an OS .46 FX engine that has served me flawlessly for many months in a Rascal 40. Recently I bought a Kyosho Pitts S2 and put this engine in the nose with a Bisson Pitts style muffler.

The engine still runs great and provides ample power for this plane, but I have to keep landing every 2-4 minutes to richen the mixture for vertical performance as it has a tendancy to start running lean in the 2-4 minute timeframe. I have never had to do this with this engine before. The fuel tank is well padded and isolated from the airframe, so I don't think it is foaming/bubbling.

Now I am aware that 2 stroke engines need the correct size expansion chamber and exhaust outlet to run at their best, which brings me to the Bisson Pitts muffler. The total exhaust outlet size is roughly 2 times the outlet size on the stock muffler, while having an obviously smaller expansion volume. As mentioned previously the engine has plenty of power.

My thoughts are that the Bisson muffler is not providing enough back pressure to properly pressurize the fuel tank, leading to lean running conditions as the fuel level decreases.

I'm wondering if anyone else has had a similar problem with the Bisson muffler. Any thoughts?

Thanks,

Dave

Plug one exhaust and the problem will be solved.

Exactly. If you do a search I recently had the same problem on my os 50sx. Bought a wooden dowel fuelprofed it with thin CA. Shoved it into one of the pipes and presto. Runs great. Thanks for the help everybody.

Do the slimline pitts mufflers have the same problem??
Ive got one for my OS 91 FX , but im breaking it in right now with the stock muffler.

Thanks for the replies. Instead of plugging one tube, I decided to flatten the tips of the pipes slightly to reduce the area. I have yet to try it though.

be careful. I'm not sure what those pipes are made of but you don't want it to crack. The advantage to plugging the hole is that it is very easy to undo. If you would like. Send me your address and I will mail you a piece of dowel.

Thanks Crash, but I'm pretty familiar with working with aluminum... Shaping, soldering, machining etc.

If reducing the size of the outlets works, I will most likely cut the crimped sections off and size the tubes down to keep both pipes open. I'm kinda funny like that.

Let me know how it works.

Will do.

I used telescoping lengths of K&S aluminum tubing that I RTV'd inside the stock pipes. Was able to sit at the field with a bunch of precut pieces of tubing and tuned the pitts muffler until the 46 FX would needle properly. For some reason I wanted smoke coming out of both of the exhaust stacks.
Rich

Rich, that's pretty much what I had in mind, except I'll solder the tubes in. The engine has enough power for the Pitts as it is, it's just I think the open pipes don't give it enough tank pressure.

I ran the 46 with the crimped down tubes yesterday, and found that it helped tremendously. It was fine from start to finish with no landings to re-adjust the mixture. Now I can work on reducing the pipe size with tubing.

It's too bad that more time is not put into designing such pipes to get at least a reasonable facsimile of useful pressures and back-pressures beyond tacking a bit of tubing and machined aluminum together. There is certainly a lot more science and experimentation involved in making a 2 stroke run properly and efficiently than has been expended in producing the Bisson Pitts muffler. The Slimline Pitts muffler would probably be even worse due to lack of an expansion chamber.

Oh well... Sometimes you just have to make or modify things yourself to get them right. It's all part of the enjoyment of the hobby.