Question Idle rpm for Lycoming IO-360?

[Neal]

What is the desired idle RPM for a Lycoming IO-360? I have a modified IO-360 CubCrafters calls a 363i. Lightspeed ignition, fuel injected. My concern is the RPM is set too low. In my recent experience with bad coil packs I did extensive testing at idle, full throttle, and on each ignition side including both. My idle RPM as it came from the factory is around 560 RPM. That seems low to me and in testing I didn't let the engine quit but it sure seemed like it was about to. At each indication of the engine about to quit I pushed the throttle up to keep it running.

Today I decided to make an adjustment to the idle stop. Very easy to do and for those with a FX-3 like mine, it can be done through the left side "gill". I made an adjustment of the stop screw of about 3/4 turn and then ran it and that put it around 830 RPM which was a little too high for what I wanted, my desire is more around 700. So I backed out the screw a little, in the end it would be a 1/2 turn adjustment and I'm near 700 to 730. It idles much more reliably here. I'm going to make one more adjustment as I'd rather be in the 650-700 range and see how that works out.

I'm curious what others are showing for idle RPM and/or what is a stable RPM at the low end that can be expected? If anyone knows of any guidance from Lycoming I'd love to learn what you know.

 

[Neal]

Final tweak this morning, now sitting around 650-700, definitely don't want below 650.

 

[ve6yeq]

My understanding is that you want the lowest stable idle that conditions (e.g. temperature, density altitude, etc.) will permit. This will give you the smallest amount of idle thrust and hence the best ability to slow down went landing. From the “factory”, the idle on my factory EX-3 ranges from 450 to 550 depending on the engine temperature and outside temperature - a bit too low to idle stable when cold but fine once up to temperature.
Mark Keneston recommends 600-700 rpm and from my experience at a almost sea level density altitude and cool outside temperature, 600 rpm is very stable for me at all times and I am thinking of getting a very short handled flat blade screwdriver and adjusting mine up.
Did you also follow the Lycoming directions on how to set the idle mixture as well as the idle RPM?

 

[Neal]

I did my idle tests after flying prior to shutdown, the RPM around 550 was too low for my comfort not trusting it would remain running. I'm now around 650 and pleased with this, it's on the lower end of the comfort zone from listening to it idle but I will continue to monitor and realize mixture is a part of this.

I am aware of adjusting the idle mixture dial, not hard but not something I want to touch unless needed. My last plane needed an adjustment, this one came from CubCrafters almost full forward and seems to have a nice lean idle mixture. During shutdown when pulling the mixture to full lean per the condition inspection checklist in the AMM you should see approximate 10-50 RPM rise. I don't really get that telling me I'm lean, and that's fine, but probably could error on the richer side but I'm not touching it. I still pull the mixture during taxi in any effort to avoid fouling plugs.

I did use a short stubby flat head screw driver, not the easiest but doable. I'd suggest a quarter turn at a time and test. The easiest option but requires disconnecting the bottom two center orange tubes and removing the bottom center panel would be the easiest access but more work. Fun stuff, dialing in your plane...don't trust that it's perfect from the factory.

 

[ve6yeq]

My assumption has been that they set the idle for the conditions at Yakima when they installed the engine brand new and with the break in oil. So a combination of different conditions, oil and the engine break in being completed, has changed the idle conditions.
I definitely see different idle performance after an hour flying than when I start it up even with engine preheat - even just the run up changes the smooth idle RPM and oil pressure on a cool (5 C) morning.

 

[Neal]

Excellent point. In 2021 when doing my fly-off at Hood River I had to ask a fellow cub owner about why the engine ran rough prior to takeoff, not realizing you needed to lean out. And definitely a difference in conditions in the aspects you mentioned between Yakima, then and now. You have to wonder how many don't adjust things after delivery and break-in time. I think as an owner you need to dial your plane in and know how to do it, or have someone do it for you. It's been an amazing learning experience to date for me, not all by my doing

 

[Tsquared]

My experience with idle RPM in my FX3 echo's both of your experiences.

When RPM was set to 550-600rpm idle, my engine would die on its own within a minute or so. Most recently this was down in Alabama in October so it was plenty hot and at 586' MSL. My personal opinion is 550-600rpm is too low.

Prior to my engine failure the RPM had been set at 700-750rpm and the engine seemed to like that. Right now it's set at 880-900rpm per Lycomings recommendation, but similar to ve6yeq experience, idle performance differs significantly at times. When doing pattern work recently, idle RPM dropped to 750 while waiting for traffic on final, at other times holding short, it ran around 850rpm very comfortable. At some point I may dial the idle RPM back a little down (maybe 750-800rpm but TBD) but for now I'll leave it as is. Field elevation at my home field, KMIC is 860'msl and with winter coming the temps will be dropping so I will be keeping a close eye on both idle RPM and idle mixture.

If you're not already leaning the mixture when taxing you really should. On my first conditional inspection, my plugs were rather fouled, they were serviceable, just not optimal.

 

[Neal]

When I had mine initially adjusted which was around 800 I think, and flew, it caught my attention of the idle speed on base which was around 1300. I need to see where it is now with 650 set.

I think in the end having focused on this is to set your idle RPM at the minimum stable RPM for your plane. I also like that my idle mixture is on the lean side, from what I've learned these engines like the drown themselves out at idle such as my recent experience and conversations with Lightspeed Engineering, so keeping your idle mixture on the lean side is probably a good place to be. Of course you can control that with your mixture control, but do you? Or enough? Good lessons learning in aircraft ownership and info share.