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Idle Circuit

Manchu16

Active Member
I have heard competing recommendations on adjusting the idle circuit on an MA4-5 carb. Both are A&Ps on the field and the only text reference I have found is engine warmed.

One says I need to get the engine up to operating temps, best to fly it for a bit. After landing run engine up to 2000 rpm for 30+ sec to get CHTs back up running full rich, reduce to idle, then slowly pull mixture. >50 rpm rise too rich, <25 rpm rise too lean

The other says I just need to get oil over 100*C, like I would for a run up. No need to go much higher. Then 1000 rpm, full rich, slow retard to idle then slow lean. >50 rpm rise too rich, no rpm rise too lean
 
I have not found a need to get the CHTs up to full operating temp. Set it with the engine warmed and then verify after the next flight.

The ">50 rpm rise too rich, <25 rpm rise too lean" is sound. However, you need to account for your field elevation / DA. Since that setting is for sea-level. You don't need to get it perfect, 50-ish will do.
 
Engine should be fully heat soaked at normal temps before setting. The best way to do this is by watching the oil temp. I would want it at 150 or more before setting the idle. Absolutely no need to get CHTs up for this.

Go fly for a bit, land and remove cowl, restart and set.
 
Tempest in a Teapot

It’s not all that critical. As my first boss and mentor waggled his finger and told me “Never try to adjust the carburetor on a cold engine.” Unless the manufacturer tells me otherwise, I look for about a 25 RPM rise when pulling the mixture to idle cutoff.
 
cold engines need a richer mixture for optimum. This is why carbs have chokes and EFI systems have warm up logic. If you set idle when cold, you will be forever rich at idle when hot. If you set it properly it will be correct when hot (typical use scenario) and you just use a higher idle speed to compensate when cold. None of this is an exact science, as ambient air temps also necessitate different mixtures (EFI uses intake air temps to adjust mixture and retard timing). The goal is to set a happy medium that best covers your range of uses.
 
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Stumble

I get a little stumble around 1350 rpm. Does idle adjustment help this?

It might. If it’s stumbling when you’re advancing the throttle, especially in a go-around scenario that typically means your idle is too lean.

Skylor
 
I get a little stumble around 1350 rpm. Does idle adjustment help this?

All 5 of my carbureted Lycoming powered RV’s did the same thing. All of them, including my current RV6, had the idle adjustment set according to Lycoming recommendations - ~50 RPM rise on shutdown. I don’t know why, but I imagine it has something to do with the butterfly opening in the throat of the carb, and fuel entering the intake through a different jet - or maybe not. I don’t find it to be a problem. If I advance the throttle very slowly, it doesn’t happen. If I go quicker, it does, but only lasts about a second. My lawn mower did the same thing, so did my 1966 VW bug. My fuel injected RV8 accelerated smoothly all the time, with no stumble.
 
I get a little stumble around 1350 rpm. Does idle adjustment help this?

Carb or FI? They handle this differently.

In a carb, Generally the idle circuit stops increasing it's flow at around 1200 and continues to always flow at that level. The main jet starts flowing around 1000, so between these two, idle mixture plays a part in transition. By 1350, you are mostly out of the transition zone and I would expect some issues in the accelerator pump or worn linkage arm, as it is the acc pumps job to provide the extra fuel necessary to deal with rapid throttle increase.

FI is similar in concept but more reliable and mostly unadjustable. If the stumble is at 1350, I don't believe that a richer idle mixture will help, but worth trying.

Larry
 
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