Idle Speed

[aerhed]

Steve, it varies a little from engine to engine, but at 900 you're near the top of the idle circuit and the main jet is taking over from idle bleed. Try your cutoff a little slower (not molasses) and listen to the sound as well as watching the tach. Try it down a hunnerd RPM too. The greater the throttle plate opening, the less fuel gets pulled through the idle holes. At some throttle position and airflow the idle circuit is negated.

[gasman]

High idle settings might be part of the flip over problem.................

With the wing of the 9, the rpm must be set low or it will float and float and float etc.

[alpinelakespilot2000]

Originally Posted by aerhed

Try your cutoff a little slower (not molasses) and listen to the sound as well as watching the tach.

Thanks. How long is "a little slower"? 2-3 seconds to pull fully out? Longer? I can get the rise in rpm if I just screw out the mixture, but I'm thinking that's not what is meant by checking for rpm rise on cutoff.

[aerhed]

5 seconds isn't too slow. Screwing it to cutoff isn't bad either. It's just that you get a choppy progression sorta. You don't have to go to quit either. You can sweep for the rise and push back in to re-catch. That way you can check several times without restarting. Your desired rise is whatever RPM rise your book says (like 25) and add about ten RPM per thousand feet up to around 80 or a hundred (Leadville).

[ronoc74]

I set the idle on my fixed-pitch 9A to about 550 and it certainly helped with landing. I don't actually run it at 550 but with a P-mag EI it's happy at 600. Make sure that your idle mixture is correct. My engine stopped twice during stall testing and it turned out the mixture was way too rich. I wouldn't want to get a little slow on final, open the throttle and find the engine quits! According to Precision documentation you lean it at 750 rpm and should see a 10-50 rpm rise with a warm engine.

[az_gila]

These instructions from a Grumman mechanic should work on O-320 and O-360 engines. It notes the interaction between idle rpm and mixture.

Note that the lean rpm rise is altitude dependent.

http://www.aucountry.com/ACA_Folder/...gine_Idle.html

Interestingly he gives the altitude adjustment in the opposite direction from aerhead's previous posting.

[Rupester]

Originally Posted by gasman

With the wing of the 9, the rpm must be set low or it will float and float and float etc.

...and float and float and float ...

With my Silverhawk, I have to check lean RPM by using the Vernier adjuster. When I just pull the mixture back, it quits right now . Not at all like the FI C-172s I used to fly. They would sputter and shudder before shutting down.

[alpinelakespilot2000]

Originally Posted by Rupester

With my Silverhawk, I have to check lean RPM by using the Vernier adjuster. When I just pull the mixture back, it quits right now . Not at all like the FI C-172s I used to fly. They would sputter and shudder before shutting down.

Same is true with my carb.

So, what I'm gathering is that I don't have to pull all the way until it quits necessarily, just slowly screw out the vernier checking to see that I get the rpm rise.

Thanks for the interesting reading Gil. I'll have compare what I have against that.

[aerhed]

Gil, that's backwards then. You want correct mixture at sea level with a little cushion (RPM rise). When you go to altitude your mixture will be richer, whichs gives a larger RPM rise as you sweep towards cutoff.

[jpowell13]

My idle (warmed up) is now at about 590RPM. Seems to run fine at that speed without quiting, and landings are better and shorter. I added Locktite to help secure the throttle linkage jam nut on my carburated 0-320, and noticed that I can get a pretty good look at the linkage through the exhaust opening of my lower cowl with a flashlight. I plan to take a look whenever I add oil. That, along with verifying idle speed prior to takeoff, should cover it for me. Thanks to everyone for sharing experience and knowledge.

John