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OS FS91 Surpass II pump issue

Old 12-02-2008, 10:00 AM
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Warnberg
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Default OS FS91 Surpass II pump issue

I have a new FS-91SII-P motor. I have run two tanks (Tower Hobbies 10%) through it for break-in as directed in the instructions with a APC 13X8 prop. After the break-in period and during break-in I found it very difficult to get the engine to idle, it was very rough at idle and from what I could tell very rich at idle. I then decided to set all carburetor adjustment to factory settings.

edit: This motor is mounted inverted if that makes a differance....

Here is what I found, the low idle mixture screw was set 3 and 1/2 turns rich, the book says for the pumped version 2 turns so I set it correctly, I left the high speed alone.
After setting the low speed mixture I found that I could not even get the engine to start. I richened the low speed mixture 1/2 turn and it still would not start, I went another 1/2 turn rich and started the motor. It would then idle decently but still on the rich side. After getting everything set and putting on the cowling I tested it one more... everything seemed fine.
Took the plane to the field the next day, started the engine and had issues with idle once again.... I'm stuck..

edit: I discovered one other thing, while at the field, after starting the engine when I removed the glow ignitor the engine would drop in RPM...

Why can't I set the factory 2 turn low speed mixture setting and get the engine to idle?
Could this be an issue with the glow plug?
I'm also about out of the tower fuel and will be switching to Wildcat Premium Extra 15%, will this help?

Thanks,
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Old 12-02-2008, 10:17 AM
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Default RE: OS FS91 Surpass II pump issue

A proper breakin will take about a gallon or so of fuel...maybe a bit more, maybe a bit less.

You cannot get an idle setting until the high-speed needle is properly-set. Here's how:

Your O.S. Max engine with pump should be set up in a slightly different manner than a non-pumped engine. Most modelers tend to get them set with the mixture too rich.

One sign your engine's not adjusted correctly would be surging at full throttle. This almost always means it's too rich. Also, the idle may be too rich no matter what you do to the idle mixture. If the idle is leaned a lot, but it's still too rich, the high-speed needle's too rich and needs to be properly set.

To get a good setting, you need to use a tachometer. Once the engine's had enough running to accept being leaned to peak RPM, use the tachometer to lean the engine to its peak.

At full throttle, lean the engine until the RPM stops increasing. Continue to lean the engine to the first sign of RPM drop or detonation. When you get to that point, richen to the point just before you had the RPM drop or detonation and leave the needle there. DO NOT lean to peak and then richen your usual 1/4-1/2 turn.

With a good-quality fuel, the pump system will insure that the engine receives enough fuel. The regulator in the system will make sure that the mixture stays correct as you change the throttle positions. Fly the engine to see how the mixture may change as the engine unloads in the air. You may have to lean or richen slightly, depending upon how it acts. Once you've found the proper setting, you should note the change from the absolute maximum lean position. Now you have a repeatable setting for your engine.

Newer O.S. engines with pumps have a broader range for the needle setting, but you can use the same method for all of them.
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Old 12-02-2008, 11:43 AM
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Default RE: OS FS91 Surpass II pump issue

Thanks for the info Bill....

One more question, should I be able to get a good idle now? or should I really wait until after a gallon has been run through the motor?
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Old 12-03-2008, 10:19 AM
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Default RE: OS FS91 Surpass II pump issue

If the engine will sustain running at full throttle when leaned to just rich of peak RPM, it should be able to accept an idle setting.
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Old 12-08-2008, 07:33 AM
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Default RE: OS FS91 Surpass II pump issue

Bill,
Thank you for all the help. I got it dialed in. With engine not running I set the idle screw to factory spec's (with carb fully closed), then set the high speed needle to factory specs as well, I also checked over the engine and found the glow plug to not be completely snug, it was almost to the point where I could have removed it without a wrench. I Switched fuel to Wild Cat Premium Extra 15% and took it to the field. I got some help and we started it up, set the high speed needle first then worked on the low end, one very slight adjustment on the low end of less then 5 degrees to get the engine to purrr at idle....

Thanks Again
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