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Fuel draw question/problem solved

I was having fuel draw problems with a new plane. The tank was a little far back which doesn't help, but the problem turned out to be the pitts style muffler. I read a post on rcuniverse about reduced tank pressure due to using a pitts muffler. They said to block one hole on the muffler. The outlets on my pitts muffler are the same diameter as the outlet on the stock muffler, but there are two of them. Seems logical that the muffler pressure supplied to the tank would be less. I blocked on outlet with a piece of dowel rod. I had to lean it out as it ran quite a bit richer. No dead sticks either, no matter what I threw at it.

If I were designing a pitts muffler for a specific engine, I would measure the cross-sectional area of the outlet on the stock muffler, then make two outlets on the pitts muffler half the size. Is this a correct assumption? Anyway, the engine ran great.

John VB

Reply to
jjvb
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The reason that these mufflers have such a large outlet area is that they have far too little internal volume and need to get rid of the exhaust before the pressure gets too high.

Reply to
Paul McIntosh

So is it a bad thing to plug one of the holes? The engine didn't seem to mind. You probably don't want the back pressure so high that it will not let the engine run like it should. There should be a happy medium somewhere.

John VB

Reply to
jjvb

I would think not. Reason being that these mufflers are made for a range of engines. So I suspect the outlet size is a worse case plus some type of scenario.

What I think would be better would be to use some aluminum tubing inserts to size down the outlet. You could add progressively smaller tubing until optimum engine runs were obtained. Sort of like tuning the exhaust.

Clean the muffler real well first and use JB Weld to hold the sleeves in place. Once the right size is finalized you can probably trust the JB Weld to hold things. Worse case is the sleeve will come out at the worse time and be lost forever. Or, you could drill a small hole through the side of each exhaust outlet and install a self tapping screw to hold everything in place. Of course, the screw can mess with the looks of everything.

Chuck

Reply to
C.O.Jones

The usual thing that happens is once you get enough pressure for the fuel, AND make it quiet, the engine suffers power loss and starts overheating. Some of the newer pitts mufflers are designed with more internal volume just for this.

Reply to
Paul McIntosh

I was thinking of sizing down the holes. My idea was to cap them both then drill holes in the caps. Sizing the holes until I had the engine running properly. Your idea of tubes would work also. If I can get the first one to fit fairly good.

Last night, during a flight, the engine started leaning out again. I suspected that the dowel rod plug had come out. It had. It was just supposed to be temporary and it was.

John VB

Reply to
jjvb

Paul,

Why would the engine overheat? Lack of mass flow through the engine? The muffler is from Bisson and seems to have more internal volume than other pitts mufflers out there. It is on a GMS 76. The engine is really running well. I'm very satisfied with it.

John VB

Reply to
jjvb

John, do you have any rpm figures for the GMS or other indications of performance? How does it compare for instance to strong .60? I have a GMS 1.20 (Bisson Pitts) and find it similar in performance to a pumped OS1.08 with the carb insert removed. I have been considering swapping out a strong running HP61for the GMS76 but wonder if it would really have any more punch.

I have cut the outlet size down >Paul,

John Hawkins - From Canada's Atlantic Coast

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Reply to
John Hawkins

I don't usually keep track of performance type info. However, with a 13x6 apc prop, the GMS 76 was turning over 10k rpm and still running rich. I richened it up a bit more to the high 9s as it is not fully broken in yet. It runs very consistently and hasn't caused any problems since I have cut down on the outlet area of the pitts muffler. I don't have any 60 sized engines with which to compare it to. It pulls my 66 inch edge around very well running quite rich. Sorry I can't help you much.

John VB

Reply to
jjvb

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