O.S. Engines Support Ask Bill Baxter your questions about O.S. Engines.

OS91FX mixture setting

Old 04-17-2007, 01:09 AM
Senior Member
Thread Starter
Piet Le Roux's Avatar
Join Date: Apr 2005
Location: BloemfonteinFree State, SOUTH AFRICA
Posts: 174
Likes: 0
Received 0 Likes on 0 Posts
Default OS91FX mixture setting

Dear Bill Baxter,
I am running an OS91FX with a tune pipe, APC 14X8 and 15% Cool power. My mixture setting is about 3 ½ turns open (+- 200 rpm down from peak). The manual suggest a setting of 2 – 2 ½ turns. Does this suggest that my pipe does not have enough back pressure? I see that the manual suggest a 3 to 4 turns for a 61FX with a pipe instead of 11/2 to 2 with a standard muffler. How does one know if your pipe has enough back pressure?
Old 04-17-2007, 10:09 AM
Senior Member
Join Date: May 2005
Location: p, ANDORRA
Posts: 283
Likes: 0
Received 2 Likes on 2 Posts
Default RE: OS91FX mixture setting

I think it possibly a good idea to mention height above sea level and temperature as the region might be 2000 metres above sea level prone to temperatures exceeding 40C

Nitro requires extra needle
example 100% nitro would require some 9 times as much fuel to do the job and would require a needle valve open a lot to supply that fuel input
15% would require some extra opening usually close to double as you double your fuel demands and coupled with high altitude and temperature would also require extra needle

Not sure for back pressure but might be something to do with 2000 meters above sea level

Hope that helps a bit

balsaeater EX jo/berger now Eurolander

Old 04-17-2007, 10:30 AM
My Feedback: (11)
Join Date: Apr 2002
Location: Monticello, IL
Posts: 19,483
Likes: 0
Received 6 Likes on 5 Posts
Default RE: OS91FX mixture setting

If your engine's running correctly, then the needle setting is correct, and your engine's getting the proper amount of fuel. That means your fuel tank's getting enough pressure. The needle setting can vary greatly, depending upon conditions and installation. With a tuned pipe, especially if you're getting a good power increase, your engine will use more fuel. That means that the high-speed needle has to be opened more.

The settings recommended by any engine manufacturer, including O.S., are starting points of reference, and what you end up with will depend upon the propeller, fuel, exhaust system, and field elevation.

Again, if your engine's running nicely at the settings you have, then your engine is set very correctly.
Old 04-17-2007, 05:29 PM
Junior Member
Join Date: Feb 2006
Location: ConcepcionBio Bio, CHILE
Posts: 20
Likes: 0
Received 0 Likes on 0 Posts
Default RE: OS91FX mixture setting

Hello guys, sorry about using your post Piet, but im runnnin an OS .91 FX with a Bisson Pitts Style Muffler. It has about 3 turns, and its going fine, but I think that the transition is not the best it can get. Im using Wildcat 15/16. Im using the factory setting of the low speed needle, and im at sea level, with usual humidity in the air, APC 14x8. Also I think im having little heating problems because of the cowl, but I've solved it now, making bigger holes for the air to escape, gonna try it when rains stop.

Any suggests about transition? Ive thought about the low speed needle, but I have not touched it, because I dont wanna do something without being sure.
Thanks you so much!

Old 04-18-2007, 09:49 AM
My Feedback: (11)
Join Date: Apr 2002
Location: Monticello, IL
Posts: 19,483
Likes: 0
Received 6 Likes on 5 Posts
Default RE: OS91FX mixture setting

Throttle transition is set with the low-speed needle, but there's also some interaction with the high-speed needle.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are On

Thread Tools
Search this Thread

Contact Us - Manage Preferences Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.