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io360 stumble under acceleration

mmcelrea

Member
Hi.
I've been having an issue with my IO360 since day one. Long story but it started out as an io370 from Titan with dual mags, its now an IO360 with slicks and an FM150 FI. The crank and mags were swapped out after 16hrs due to overheating and a crack in the leading edge of the prop. There are now around 45 hrs on the engine since rebuild but every time after taking off the chts climb towards 420. I've carried out the fuel flow check according to Don at AFP and it is slightly rich but ok. The engine seems to idle fine when cold but not when hot which makes the idle mixture hard to set although I don't think its far away. It also stumbles when accelerating from idle when hot.
Id appreciate any advice as I'm getting to the point of throwing away the fm150 and replacing with a carb as every the rv7 on the field has absolutely no issues with that setup.
 
The engine seems to idle fine when cold but not when hot which makes the idle mixture hard to set although I don't think its far away. It also stumbles when accelerating from idle when hot.
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Need to spend more time on the idle mixture setting. Good cold, bad hot idle points to a rich idle mixture. If the stumble occurs when accelerating from idle, but not from higher RPMs, that also points to a bad idle mixture setting. The fact that the stumble from Idle only occurs when hot, also points to a rich idle mixture. Stumbles usually occur due to lean mixtures but can also happen with over rich. Different feeling/sound, but tough for the novice to tell the difference.

I would start by adjusting the idle mixture on a cooler day using the lowest MAP method and see if things improve. Cold engines and cold air need a richer mixture than hot engines and hot air. The key is to set your mixture using the lowest MAP method on a day that sits in the middle of the temp swings that your engine normally sees. FOr now, just get the engine warm and have someone watch the RPM and tach. Set for 800 RPM and observe MAP. Lean the mixture, reset throttle to 800 RPM and observe MAP. Keep doing this until you get the lowest MAP possible at the constant 800 RPM. The further the MAP ends up vs where you started indicates how far off you were.

Larry
 
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Thanks for the reply.
Should reducing the mixture manually have the same effect? It’s just that I can’t get it to idle smoothly using the mixture knob.
 
In my experience the AFP systems run/flow great, but hot idles are not as good as Bendix or a carb. You may also check your nozzle size, they used to supply them with .028 nozzles I believe, changing to .025 nozzles helps a little bit. Stumble off idle is usually lean idle mixture
 
Thanks for the reply.
Should reducing the mixture manually have the same effect? It’s just that I can’t get it to idle smoothly using the mixture knob.

No. Using the red knob to adjust idle mixture is insanely difficult due to the very fine resolution required to do so. You need to do it with the prescribed idle mixture procedure. I am not even sure that the FI servo's idle circuit is impacted by the red knob. My guess is that it is independant of it, but don't know for certain. Carb is a different animal and the red knob will choke flow at idle, but within a 1/4 turn resolution, so very difficult to do.

Larry
 
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