Lancair-RCU

Im setting up my first glow powered model (not first model ever though ) and have bought a pitts style muffler (J'TEC JT-40PB) to keep it all clean inside the cowling. My question is the new muffler doesnt have a pressure fitting to run a line to the fuel tank to pressurize it, like the supplied OS muffler had, is this essential ? The model is a blackhorse models CAP232 so it will spend time in unusual attitudes. Should I be concerned enough to get a pressure fitting for it and drill and tap it in ?

Lancair

Bison

My Feedback: (1)

If you don't have tank pressure your engine won't run as reliably in the air. I have an OS61FX with a Brisson Pitts muffer that does have a pressure tap. I am using it to fly a Super Chipmunk and I don't think the OS61FX would be as reliable as it is in the air without tank pressure. Drill and tap the muffler in a convenient location and use the nipple from the OS46FX's standard muffler. It would be a good idea to practice on a scrap piece of aluminum and test the fit before doing it to the muffler. A couple of years ago I was going to use a J'Tec muffler on a OS160FX and it also didn't have a pressure tap nipple. I was going to tap in the nipple from the standard OS muffler but I decided to switch to a gas engine.

tjmra

I have just installed a JTEC muffler to an OS46 (funnily enough in an ESM Super Chipmunk). Engine is inverted so the the exhaust was rotated as well.

I drilled a small hole in the curved section of the muffler (opposite the exhaust pipes) and inserted a Dubro pressure nipple. These can be fitted by drilling a hole and then cutting thread using a suitable tap and screwing in the fitting OR by drilling the hole a fraction larger and fitting a nut on the inside of the muffler. I did not have a tap but I drilled a hole, managed to screw the fitting in and put a nut on the inside as well!

The whole engine is cowled so I used a remote filling device, when the tank fills the excess fuel goes up the pressure line and is directed straight out one of the exhaust pipes so the engine does not get flooded.

I did not use the strap provided to hold the muffler to the engine, but used the alternate method (noted in the instructions) of drilling the mounting face and bolting the muffler using the original engine muffler bolts. I used two locknuts per bolt.

Time will tell if I have disaster waiting to occur but it seems OK for the moment.

Lancair-RCU

Well I ran the engine for the first time today. First run I was having trouble getting full throttle without the engine spluttering so I was suspecting I will need the back pressure nippple. Second run (each only 5 min or so) was much better, full throttle was achievable. I need to redo my servo to throttle settings and give it another try, this time lifting the nose to vertical to see what happens with the non pressurised fuel feed.
I havent run a glow engine for about 15 yrs so it was a re-learning experience for me. I couldnt get the overly rich engine note (4 stroking as they call it) as per the OS supplied instructions, the only engine note change I could get was by going almost too lean (about 1 turn), which I did only very momentarily on this new engine. I went to about 4 turns rich and it still seemed the high pitched 2 stroke, even at idle, just more smoke.

My J-Tec Pitts style muffler (JT-40PB) has two screws through right through it, one could be removed, cut shorter and a pressure nipple fitted in the remaining hole, it would need to be drilled out though, the JTec has a metric thread whereas the OS muffler nipple is a more coarse imperial thread I think. Ill see what happens tomorrow.

Lancair

Lancair

Lancair-RCU

OK the engine will not run in a nose up attitude so Ive had to install a back pressure nipple, which didnt go so good, but its in and will probably work. I didnt have, nor did any local supplier ahve a 6/32 tap to tapp the hole out. My LHS guy gave me a hardened steel bolt in 6/32 thread, told me to cut two v notches in the end of it and use it to tap the thread. A test in hard balsa proved it would work, however in doing it onthe aluminim mufler the thread it cut is chewed ou ton the top half and basically only got about 1.5 turns of decent thread. With some heat proof thread locker its in and tight but . . a less than ideal job IMHO.

Ive seen now and considered another problem I may have.

This damn muffler my LHS guy sold me isnt designed for the OS engine, I knew that but he assured me no problems. The inlet to the muffler is roughly HALF the size of the exhaust outlet on the engine, maybe a bit more than half but smaller none the less. Im no genius at engines thats for sure but SURELY having an exhaust inlet thats a LOT smaller than the engines outlet is going to cause some difficulties in tuning it to run properly ? EDIT: Just measured it, the OS engine standard muffler supplied, has an inlet 28.5mm X 6.5mm, this JTEC muffler has an inlet 17.5mm X 6.5mm.

Right now Im pretty much fuming at the guy, Im ready to take the muffler back and DEMAND my money back, or just shove it up his exhaust, then order from an online shop a pitts style muffler designed to work with the OS46FX engine.

lancair

Lancair-RCU

After some searching last night, it seems as though the size reduction in muffler inlet or manifold outlet, whichever way you look at it, wont have as drastic an effect as I first thought. Maybe a few hundred RPM difference. Ill reassemble it today and test the pressurising of the fuel tank.

If anyone has any thoughts about the muffler inlet size reduction or actual experience with the same problem please let me know. I know my LHS guy is going to say Im being worried over nothing but . . .

Lancair

al

From JTEC,

There is nothing wrong with the muffler or the shop that sold it. It does require you to install a pressure tap.

We do not install pressure taps into our mufflers due to the versatility of many of them.

Most modelers use the vent line(pressure line) as there overflow. This is one of the reasons we do not install pressure taps, So the modeler can access the line for over flow. Put it where he wants to because of the model

Take a pressure tap out of one of your old mufflers or get one from Dubro and place it in a spot that you can get to for your overflow line. Keep it as close to the exhaust port as possible, don't mount it low where unburned fuel can find its way back into the tank.

Now remember you bought a JT-40PB. This is a universal bolt on muffler that is designed to fit many different engines. The key word here is Universal. It will work better on some engines than others.

As for the OS 46 FX it should work well with pressure. You may have wanted to consider the JT60PB. The 60PB may be a better fit for the power of the 46FX.

Al Young
JTEC
845-361-3660
[email protected]
www.jtecrc.com

Lancair-RCU

Thanks for the reply.
I have installed a pressure nipple in the muffler and all is well. The JT40-PB was the only one he had in stock, and in hindsight I was impatient and expected to get it in the air much sooner. Maybe I wouldve been better off waiting, as I still havent flown the model, or maybe not, as I do like the muffler. Its very compact and the fact theres no big metal tubes protruding like on other types I like as well, the supplied rubber extensions work well, just sticking out the bottom edge of the cowling. A friend has commented on how quiet it is compared to other OS46FX's he's heard.
I do wish someone had told me to keep it as close to the exhaust port as possible though, I put it at the far end of the muffler, about as far from the exhaust port as it could be. Oh well, its done now and supplies enough pressure to fix my fuel flow problems. As for access to it for the overflow, is there any major drama with doing as I do, I know my tank takes around 40 cranks of the hand pump to fill, so near that number I slow down pumping and watch the exhaust outlets, as soon as I see a trickle of fuel, I stop and wind back a few cranks to suck a bit out. Hasn't caused any problems with the running in and taxiing around Ive done so far.
BTW Ive ordered a new engine for this particular model and a new Pitts style muffler designer for the 50SX, so the Jtec muffler will be a spare part sitting in my tool box now unitl I get another model for the 46FX that needs a pitts style muffler.