PA_Keith

OK, so I have a plane/engine that was donated to me... I don't know the history or if the engine/muffler combo ever worked right. The engine is an OS 61-FX, and the previous owner fabricated a custom muffler to fit within the fiberglass cowl of the airplane (see pic).

After having some trouble, I got a new OS 75-AX (same bolt pattern) and tried it with the custom muffler. Both engines run OK at low RPM with the custom muffler, and as soon as you get to >9,000rpm they cut out suddenly. I then tried running with the new factory muffler from the 75-AX, and it ran flawlessly (see pic of new muffler).

So, it appears that the muffler is the problem... how can that be? There are no obstructions in the muffler (I can see right through it from the plenum to the outlet port). The muffler does not appear to seat completely flat when I start bolting it to the engine. I have to crank down the muffler bolts pretty good to get the muffler to go down flat... but it seems to seat pretty well when I do that.
It seems to be 1 of 2 things:
1. the muffler doesn't get all the way seated, and at high rpm it vibrates and leaks a lot around the engine outlet. Would a leaky muffler even cause this kind of behavior (only stops at hi-rpm; cuts out suddenly like a sudden vibration kicks in)?
2. the muffler is not the factory design and something about it is not compatible with either engine. Not enough backpressure? Too much back-pressure? Like I said, I can see right through the muffler so there are no restrictions.

Is there a way to tell the difference? To try something to find out for sure?
I would run the new engine with the factory muffler, but the airplane was built around the custom muffler and significant mods to the firewall would be required to get the factory muffler mounted (even protruding outside isn't easy!).

Any advice! I hate to mod the airplane if I can solve the problem with a better RTV gasket or something similar.
Thanks,

1QwkSport2.5r

Inadequate muffler pressure to the fuel tank can cause the engine to run over lean and quit. A leaky muffler mount will allow pressure to leak down... The flange on the custom muffler is much too thin - this could warp from the heat and leak; as it’s already noted it doesn’t bolt up flat without extra force, this tells me the custom muffler is the problem and needs some work to work properly.

PA_Keith

Ah, that is at least a reason why some vibration and leaking would cause it to stop. I will try to run the engine super rich and see if there is a difference... and I will try to put some RTV/gasket on there and seal it up.

GREG DOE

Just as a test, and not a long term fix; try a gasket, or silicone sealant. I think your guess that it is leaking around the muffler flange is probably the source of your problem. Good luck, and let us know if you fix the problem.

Hobbsy

My Feedback: (102)

Bisson is always a good place to research muffler questions.

Picture #1 is Bisson .61. .75" X 2" X 3.125"
Picture #2 is Bisson .75. 1" X 2.25" X 3.75"

gyrocptr

Muffler outlet aligned with engine exhaust port? WAGuess: high-speed exhaust at high throttle passing straight thru to muffler outlet lowers pressure at the muffler pressure tap. Most mufflers force the engine exhaust to change direction one or more times before exiting muffler. As velocity drops pressure rises.

PA_Keith

Interesting... I was wondering if the muffler tap might be at a low-pressure point of the standing wave inside the muffler (causing loss of back-pressure for the fuel). But if the exhaust is passing straight through, then there wouldn't be much pressure at all. Can I insert a diverter of some kind inside the muffler? Will that make too much restriction to the exhaust flow?

I finally got some RTV and I'm trying the RTV gasket first to make a better seal.
Next, I'll try changing the exhaust flow somehow and get some data.

Thanks for all the ideas!

Hobbsy

My Feedback: (102)

Example, this is a .60 sized muffler by Bisson, I made the adapter to use it on a Fox .60 or .74. It was one of the best sounding muflers I ever owned. A nice low, soft purr. Note two offset baffles.

gyrocptr

How about "attaching" a 90deg bend/extension in the muffler outlet tube?

PA_Keith

The suggestion from gyrocptr has the added advantage of using something I already have (the extension)! I'll run a few experiments this weekend and post the results.

Does anyone know how I might create the muffler feed pressure to the fuel tank without using the muffler? If I had a way to apply constant pressure to the fuel tank not connected to this muffler, then I would find out if it's this muffler that is causing the problem. Can I just blow into the tube when I open the throttle? Is that enough pressure?

the Wasp

My Feedback: (1)

if your engine is leaking exhaust around the Flange you should see residue from that on the engine and muffler's Flange, have you seen that ??

that would probably work but it's not something most of us would do, besides, when you stopped blowing you most likely will get fuel forced back out into your mouth or face, please don't do that

Jim

PA_Keith

Any better suggestions? I could run a second engine and use that to provide the back-pressure. I'm doing all this on a stand so I can whip up whatever might help... I'm not sure how to provide a constant source of pressure to confirm that my non-standard muffler is the issue.

Maybe I should try running without the muffler at all? Will it run with no back-pressure? If my suspicions are right, then it's the sudden loss of back-pressure that kills the fuel flow and kills the engine. What happens if I run without a muffler and without any back-pressure connected? I've never tried that...
(Sorry for all the questions... I'm going into un-charted territory, for me, so I want to do something simple to debug the problem without introducing more questions! Thanks for all the feedback!)

Hobbsy

My Feedback: (102)

The home made muffler looks to have about 150% the volume of the stock muffler, between that and having no baffle, it ain't gonna work. That's my take anyway.

GREG DOE

Try restricting the muffler outlet. Even one of those rubber extensions might help? As an example ponder this. The Nelson, and Jett Q40 racing engines run on muffler pressure, produce over 2 hp. and have a 1/4" dia. outlet in the muffler. It's true that those mufflers give a boost to the engine, but the take away is that a little more back pressure won't rob large amounts of power. You might also consider attaching a long piece of fuel tubing to the muffler pressure tap, fill it partially with water, and see how high the pressure will raise the water level in the tubing. This test might also reveal if the pressure tap is in a low pressure area in the muffler. If a restriction is the answer it shouldn't take very much.

gyrocptr

Engine should run fine without muffler. More power and very loud. Muffler is to reduce sound. Downside is that the muffler, without a tuned exhaust system, also reduces power. Upside is that back pressure in muffler can be used to pressurize the fuel tank and improve fuel delivery to the engine, especially when airplane is in nose-high attitudes where tank is lower than the carburetor. Engine makers often depend on muffler pressure to help deliver fuel and then increase the carburetor bore to regain power. Without muffler pressure the carburetor bore has to be kept smaller to create enough venturi effect to draw fuel when the tank is lower than the carburetor (when airplane goes nose-high).

Any way you've already demonstrated that the problem is the home-made muffler. Engine ran fine when you changed mufflers.

the Wasp

My Feedback: (1)

as GREG DOE suggested, you have 2 things to try, reducing the Stingers ID first, and or try remount the Fitting away from the flange and closer to the stinger, my thought on the fitting alone is, that where the fitting is the exhaust has little pressure-build-up there,

now, ask people (including me) why they do not like the tiny Slim Line .61 Pitts Muffler..

just know that you do not need say "a Pressure Wave" in a muffler to produce pressure for the tank>, you simple just need Pressure inside the muffler (we call it Back Pressure because it reduces Flow), and you get Pressure by putting exhaust into the muffler faster than it comes out (so maybe by you reducing the Stinger's ID it may work for you <try that first),,, note that an Ideal "muffler" for a 2 stroke would have no back pressure at all, <that would produce the same amount of power as NO muffler or No pipe at all, and that is why you see some people testing Prop RPM with no muffler/no pipe,
but so, as known, we do need pressure for the tank.


are you 100% sure ??> that the owner made that muffler ???? many companies that make "after market" mufflers will make a custom muffler for people,, so 1 thing you can do is you could send a photo of it to Bisson and Slim Line and ask if they made it and why they think it does not produce pressure,, the "chamber" on your muffler looks like a Bisson to me

Jim

the Wasp

My Feedback: (1)

PS, as a side note, just know that RCU is not showing some of my posts (s), even on the Saito page,,
I have PM'ed RCU and they have not responded back to me about the problem

Jim

Hobbsy

My Feedback: (102)

A properly sized muffler should serve two purposes, one make the engine quieter and two, accept a full,throttle exhaust stroke and then prevent the intake charge from following the exhaust out off the cylinder. It's a thin line and Slimline used too many one size fits all, for example the Super Tigre .61/75 worked well on the .61 but made the .75 run hot.

Lou Crane

My experience may not address your problem, but it might just...

As far as leakage where the muffler attaches, a way to reduce or eliminate the problem is to 'face' both surfaces at the joint, so that there are NO irregularities that depend on gaskets to seal. I've set up a 'flat table' to do this. smallish piece of plate glass - about 8 by 10" - mounted on a piece of formica counter-top, as a 'reference' base.

With care, the 'base' can hold fine w/or/dry waterproof sandpaper (water does well to hold it to the surface AND "lubricate" the engine or muffler flange while rubbing it on the flat plate.) Care to keep the entire surface evenly in contact, of course. Easily done. The mating surfaces can be done separately; minute misalignments should averaged cancel when the mount bolts are drawn down. I start with about 320 grit paper, and occasionally go above 600 grit to finish. Clean immaculately! Some muffler do not allow much way to do this.

I also cut and include a fiber gasket for between the pieces, careful to avoid any masking of the 'holes' between the parts.. Paper gasket material about 0.0025 works well, and any scorch or burn can be taken up by re-tightening the bolts. Check for that, the first few runs.

IMO, the location of the pressure tap is not significant. If there is enough "back pressure - as defined in a post above - to provide assistance to fuel feed. We only use the averaged difference between the high pressure spikes and the low, when we use muffler pressure.

The carb may be a place to look at. The old way to estimate a workable carb initial setting was to set it at WOT with NO effect on needle setting, and for low at where it takes a strong blow down a fuel tune to detect ANY passage. If it then runs at idle, play with the hi-speed settings to improve things. ...Always checking that the idle run and transition remain healthy. Balancing the twoshould get you to a serviceable run.

Just some thoughts from an old time Ukie who even likes a lot about the accursed Fox Stunt 35.

blw

My Feedback: (3)

Jim- you need to notify one of the moderators who can access your settings. The best method is to leave the Engines section and get back to the main menu of RCU. Scroll to the bottom and you will find RC Universe Support. Go there and you will find a subforum named Forum Questions or Problems. Go there and post a message giving the details of your problem. This is the fastest way to get an issue like this looked at and hopefully solved. Please contact me by Private Mail if you aren't contacted by another moderator to help you.

the Wasp

My Feedback: (1)

blw, thank you for the reply, I will do that,, just so you know I now have PM'ed RCU twice, but as you know it was from one of my posts

thanks again !!
Jim

1QwkSport2.5r

The admin of this site are not much interested in fixing problems with the site. Don’t expect much.

the Wasp

My Feedback: (1)

well, they sure don't reply to PMs !!

Jim

1QwkSport2.5r

Or forum bugs. I wish you the best of luck that someone can help you.

Hobbsy

My Feedback: (102)

Actually they do care, it's been a long time since there was an issue, Jim's should be an easy fix.