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.

 

[Cubonaut875]

Interesting thought comes to mind while reading this and thinking how I actually fly my plane. I first learned to fly in the early 70's, I was taught to never let the engine idle for more than 15 seconds or so and to always clear the throttle so the carb would not load up and the engine would quit. My Supercub was carbureted as was the J3, I flew Champ and Citabria before. I actually never actually let it idle for any length of time - as it occurs to me as I think about it. I think that this habit has stayed with me with my FX3. I would assume in the current world of flight instruction this emphasis may have changed especially with fuel injected engines being more prevalent. BTW Neal - just watched your excellent YouTube on adjusting idle - very good!

 

[Neal]

I'm rarely in idle as well other than just a test, however, the other time I'm at idle is in some pattern work where I'm doing practice engine out or just trying to keep speed under control. So base and final may see idle throttle position, although windmilling will keep it around 1100-1300 I believe, and then on roll out and other rare scenarios. Good to know that your engine will remain running at any throttle position even if minimally used.

I read something about cold weather starts not to go to 1000 RPM right away. I welcome any feedback from your northerners about this. Should you keep the throttle full aft until you start to see oil temp rise for example? Or is it okay to go straight to 1000 RPM after start in any outside air temperature condition?

 

[Tsquared]

I'm rarely in idle as well other than just a test, however, the other time I'm at idle is in some pattern work where I'm doing practice engine out or just trying to keep speed under control. So base and final may see idle throttle position, although windmilling will keep it around 1100-1300 I believe, and then on roll out and other rare scenarios. Good to know that your engine will remain running at any throttle position even if minimally used.

I read something about cold weather starts not to go to 1000 RPM right away. I welcome any feedback from your northerners about this. Should you keep the throttle full aft until you start to see oil temp rise for example? Or is it okay to go straight to 1000 RPM after start in any outside air temperature condition?

Don't forget that you will be in idle when your practicing power off stalls.... you guys do practice stalls right? My most recent flight had my RPM's at idle around 1300rpm while practicing my engine out landings. 1100-1300 rpm sounds right to me coming in to land at idle.

To the second point/question - Define "cold weather" please

I've heard the same story about keeping RPM's below 1000 on a cold start, just keep in mind that Lycoming recommends a preheat system anytime it's "cold". I've seen cold defined as temps below 10F. In some cases cold is defined as temps below 40F.

With the Tanis preheat (cylinder heads and engine oil) my cylinder heads are typically 80F warmer than ambient temp and oil is 35-40F warmer as well. For me I plug in my Tanis when temps start to hover around 40F or lower consistently. That said I still keep close eye on my oil temp after start and don't taxi or operate above 1000rpm until my oil temp is at least 120F in the winter. I know with some of the radial engined warbirds I've flown in, we don't taxi (ie rpm >1000) until the engine oil is up to temp regardless of the outside temp.

 

[Neal]

From the Lycoming manual:

 

[Waldom]

Just a note to readers that don't own an airplane in the Experimental category; adjusting idle RPM and idle mixture is not classified as preventative maintenance. The posters likely have a repairman certificate issued for their particular aircraft.

I think what is discussed is a fine idea as well as being a very simple procedure, but to comply with FAR part 43 you would need a repairman's or mechanic certificate. Don't forget the engine logbook entry.

 

[Neal]

Yes, I do have my repairman certificate and I recommend anyone that is eligible to have one, get it. Whether you use it or not, or feel capable or not, it's your privilege to have. Very simple to get with local FSDO.

 

[Neal]

Here is my video on this topic which also states this is for knowledge and if needed get your A&P to make adjustments.

 

[Neal]

Just a note to readers that don't own an airplane in the Experimental category; adjusting idle RPM and idle mixture is not classified as preventative maintenance. The posters likely have a repairman certificate issued for their particular aircraft.

I think what is discussed is a fine idea as well as being a very simple procedure, but to comply with FAR part 43 you would need a repairman's or mechanic certificate. Don't forget the engine logbook entry.

Let's take a moment to look at the reg and figure this out as what I'm finding, out of curiosity as to what someone without a repairman certificate can do. A quick search revealed this:

Non-Builder Maintenance | EAA

FAR Part 43 specifically states that the rules of that part do not apply to experimental, amateur-built aircraft. Therefore, any work (not just maintenance) on an experimental aircraft can be performed virtually by anyone regardless of credentials. (This does not apply to the condition inspection). Let common sense be your guide as to what maintenance you conduct yourself.

It seems rules you are referring to may refer to certified aircraft, not experimental or home built, etc.

 

[Waldom]

Yup.

Clearly any reasonable person can complete this task. The legal niceties apply to aircraft in the standard airworthiness category, which require an A&P for the work or a repairman in the case of an airplane in the experimental category.

The only reason I mentioned this is that if you are not rated and make the adjustments, keep your hangar doors closed while you're doing it. If you want a logbook entry, do the job with the knowledge and "under the supervision of" a mechanic and they will make the entry.

 

[Neal]

After tinkering a few times I have my idle dialed in. I had it a little higher than I desired around 760, that was a nice idle though but probably not the lowest stable that I was targeting. As it was quite windy yesterday I decided to tinker with this some more and now I'm down to around 650-660. It's amazing the difference in 100 rpm but this is a stable RPM for my plane and where I'm going to leave it. I think 700-800 is probably a sweet spot for a good idle, what CubCrafters set on mine leaving the factory of 550 was not stable and too low.

As to my idle mixture, while it ran great it was not showing the RPM increase of 10-50 rpm when pulling the mixture to full lean during cutoff at 1000 rpm. I had no rise in RPM. I adjusted the mixture 5 clicks rich and now I'm seeing the rise which is great. So I have that properly dialed in. I always taxi with the mixture about 50% out or showing a RPM rise, which now I'll be able to actually find.

Good getting the new plane dialed in. Now to go fly and have fun after this and coil pack drama.

 

[Cubonaut875]

Checked my idle RPM over the weekend, it appears my factory set is at 650.