DGUY

My Feedback: (9)

I am sure this has been asked, is it possible to run this engine without exhaust pressure. It is not pumped.
thanks
Derrick

HighPlains

My Feedback: (1)

Most likely it will run ok, but you may need to choke the carb down slightly to improve the fuel draw.

tacx

My Feedback: (1)

I have three OS 91s. One is one a 60 size Big Stick. While flying one day I landed and when I got the plane back on the stand I noticed that the muffler pressure line had come off. I never noticed any change in engine performance.

Gray Beard

My Feedback: (-1)

Depends on the tank location. I have had it go both ways. If the tank is close to the engine I have gotten away with it when I forgot to put the line back on. With another plane it would only idle with it off and didn't run well once in the air.

DGUY

My Feedback: (9)

Thanks guys, the tank is right there behind the carb and just a touch higher than the carb fuel inlet. Oh it is a 91 FS too.
thanks
Derrick

tacx

My Feedback: (1)

Derrick,

Should be easy enough to check before flying. Make sure when checking on the stand that you hold the plane vertical to make sure it will get fuel while in that position.

Tom

DGUY

My Feedback: (9)

I ran it today at the field. It ran great up 3/4 throttle. After that it would begin to die, and quit. Guess I will have to add a pressure nipple somewhere.
Derrick

Gray Beard

My Feedback: (-1)

They will do that plus they loose pressure at idle and low speed. The nipple can go anyplace around the mufflers center. Very easy to install but I have been known to over tighten them, I tend to forget they are hollow.

DGUY

My Feedback: (9)

Gene, can the pressure nipple go in the exhaust manifold it is the stock one that would be on the 91 fs. I. Was doing away with that massive muffler and that looks like it is a foot long outside the cowling.
Derrick

Gray Beard

My Feedback: (-1)

Derrick, I just tried to do away with the muffler on one of mine this summer. The headers on the 91 tend to break a lot more then on the 120 due to the threads being cut so deep and thinning the wall thickness down too much. I have mentioned this to OS and they said I needed to support the muffler. Anyway, I tried a couple of headers {broken} that I had on hand and placed a nipple on the aft end and it didn't work for me. Then I tried one more into the center and it too didn't work. I broke down and just bought a new header {Magnums fit} and put the muffler on, all the problems were then solved. For questions like this the best source is probably asking Bax in the OS support forum. I just couldn't get it to run without the muffler. Could have been an operator malfunction? When I make up different types of mufflers or headers I usually use the pumper or a YS. They have no pressure from the muffler. The OS is a vented tank.

JollyPopper

My Feedback: (6)

If you don't mind buying from Hobby King, they sell a muffler for an ASP 91 four stroke that is the same thread. Muffler and header pipe are $10.14 before shipping.

dbacque

My Feedback: (3)

I flew a .91 for almost 5 years. When I noticed less than perfect reliability, the usual cause was the muffler pressure line to the tank was disconnected. Weak idle, dead stick, poor transition or something. Why would you want to eliminate a simple way of increasing the reliability of a great engine? I would never run this engine without muffler pressure, it's just one extra piece of tubing (three if you use a Tee and a fuel dot as I do) and it does so much for your performance that it's silly to consider eliminating it.

Many years without a dead stick except when the fuel lines finally decomposed in the tank. I can't argue with that!

The .91 is as reliable as the day is long if you run it as OS recommends. And they recommend muffler pressure. It's a bullet proof setup.

Dave

DGUY

My Feedback: (9)

Dave, I am not trying to do away with pressure to the tank. What I am trying to do is eliminate that huge muffler hanging outside of the cowling of a scale plane. You are correct with muffler pressure the OS is bulletproof or has been for me. I have already had thread problems in the head because of that HUGE muffler!
Derrick

Gray Beard

My Feedback: (-1)

I found the header I needed on Amazon. With the points I had from buying books it only cost a couple of bucks. I never gave HK a thought but next time the guys get together for an order I may get into it and buy a couple new headers. I used to find them at my LHS on the shelf and have a couple on hand but it was sold and the new owner doesn't stck like it used to be. Something I didn't try was adding one of the exhaust deflectors onto the header. In my case I wasn't trying to get rid of the muffler, just trying to get the engine to run without one to avoid buying a new header until I got the YS rebuilt that went into the plane. It didn't work for me though. After I got the new header and put on the muffler it was fine, a few weeks later I rebuilt the YS and all was better. Until that dumbass married to my wife forgot to pull out the antenna. Now it doesn't mater and I need a new cylinder for the YS.

DGUY

My Feedback: (9)

Gene, I got an idea that would be put a nipple in the metal exhaust manifold. Then add the silicone deflector and restrict it down with a ty-rap to build a little back pressure in the pipe. What do ya think?
Derrick

Gray Beard

My Feedback: (-1)

That was my thinking but I would try it with and without necking it down. The other thought was some of the longer tubing used for tune pipes like I see advertised in the Central catalog?? It is just a thought and something I didn't try. I had to get that plane back in the air faster then I really had time for. That and I don't really have the place to be working on engines any longer.

DGUY

My Feedback: (9)

Well I got it all fitted up. Just need some decent weather to get outside and give it a go.

Gray Beard

My Feedback: (-1)

Worth a try, I know what you are trying to do and it's all I could think of. I was just outside and we have a cold front that has moved in. Myst be in the low 50s or high 40s. I don't do cold!! I do laugh though, my buddy in NY was telling me this week it was -16 at his place without the wind chill factor. My brain doesn't even comprehend anything that cold. Been in cold like that but only because of the Army, never had any thoughts of living in it. My daughter wants us to move to Michigan, they have a small resort on the lake, no thanks. It's been in the low 70s all week so we have been outdoors a lot, just got a bit spoiled. The teaser before spring.

DGUY

My Feedback: (9)

Yes Gene I understand I don't do cold well either. It is 10 this morning, coffee sure is good. I do have a indoor shop kinda, it is in the basement. Got a 4 star I want to finish. I will post here if the I got the engine figured out or if it is back to the drawing board.
Derrick

JollyPopper

My Feedback: (6)

I moved to Arkansas thinking it was a southern state. The temp this morning was zero. So much for that southern state BS. Anyway, the OS 91 has a nipple in the back plate that produces good pressure with every other revolution. Pressure is pressure. Try hooking a line from that nipple to the tank. My guess is that it will produce as much pressure as the muffler.

Gray Beard

My Feedback: (-1)

Max, the pressure relief valve is on the older models, it was like a blow by on older cars and there is a lot of gunk that can flow from that nipple. A lot of crud you don't want in your fuel tank. OS did away with that then installed a case nipple that runs to the carb manifold, sort of like the original smog control. I had a few of the older models and liked the case pressure relief nipple. I have never tried it but I would think you could plug the nipple on the intake and use the case nipple for pressure? That is sure an ask BAX question in the OS support. The newer engines case relief is higher up and forward so maybe it doesn't blow as much crud out the tube?? I did once use that nipple on a fuel tank on an OS 70fs and it worked but I had a lot of gray crud in the tank after time.

DGUY

My Feedback: (9)

The OS 91 I have has a breather nipple for the crankcase that I have routed to a nipple on the intake manifold. I thought about running tank pressure of the crankcase but heard about the crud in the tank problem. Oh I hate cold weather by the way. My bones don't take it that well anymore.
Derrick

Gray Beard

My Feedback: (-1)

was just out in the shop making up some battery boxes and pulled out one of my 91s. I put a dot where the old pressure nipple was located. In that location I know it put gunk into the tank. The next model engine they put the pressure nipple over the cam and ran it into the top of the intake manifold. I have never tried to use this newer engines pressure for tank pressure. Not sure what would happen so it would be something I would ask Bill Baxter about. I know it isn't designed for the use or if it would blow crud into the tank or not. Not even sure how much pressure it would produce that far forward on the engine in that location.

LesUyeda

"I put a dot where the old pressure nipple was located".

I think that was not a pressure nipple, but rather a crankcase "breather" to expel the oil left by blow by. Since, unlike a two stroke, the crankcase pressure on a 4 stoke goes both positive and negative, the positive pressure is minimal.

Les

DGUY

My Feedback: (9)

Gene, my engine is just like the one in your photographs.
Derrick