Need help tuning the Aerocarb

[kmacht]

I need some advice on tuning the aerocarb. I tried last night and ran into a few problems. The engine starts right up and runs very nicely at 1500 to 3000 rpm. When I go full throttle it gets up to about 3250 but stumbles at that setting. If I bring it back below about 1200 rpm it starts running rough. I can get it down to 1000 if it drops accidentally much below that the engine cuts out. The issue at high rpm seems to go away when I lean it out with mixture but the egts start climbing pretty high at that point. There is no black smoke and the plugs look clean. Do I keep going leaner with the mixture and ignore egts or is there something else I should be doing?

Keith
#554

[radfordc]

Which prop are you using...the recommended Sensenich 54x46? If so I'm amazed that you can turn it up to 3250 on the ground...you did mean static rpm, right? My engine will only turn about 3050-3100 static.

It sounds to me like your settings aren't too far off. Don't expect the engine to be tuned perfectly at full throttle and at low idle. You must be able to run the engine rich enough at full throttle to prevent overheating. You may be a little too rich at full throttle, but that's what the mixture control is for. Sonex says you want to be able to vary the EGTs by about 100 degrees F by using the mixture control. If you can get the engine to run smooth at full throttle by leaning slightly I would go with that. The worst thing would be to get in the air and find that your engine is running way too lean and not be able to richen it.

A 1000 rpm idle is OK for a new engine. You probably will need to lean with the mixture control to get a smooth idle. After several hundred hours my engine will idle down to 800 but only if I lean quite a bit.

[MichaelFarley56]

What kind of engine monitoring do you have Keith? Do you have EGT monitoring on all cylinders? If you are all full power and start to get a miss that tells me you're (temporarily) losing a cylinder, and if you can stay in that situation for a few seconds, you might be able to look and see which cylinder you're losing. If you can find out, I'd start by re gapping the spark plugs. I've also had a situation where one of my front cylinders was getting way too much fuel causing the same issues, but a quick twist on the AeroInjector (maybe 10 degrees or so) helped equalize the fuel delivery.

I also wouldn't hesitate to take your needle valve and lean it 1/8 of a turn or so. What kind of EGT readings are you getting at full power? If leaning the needle slightly helps, it may fix your problem.

[kmacht]

I'm running the stock sensenich that Sonex sells. I was surprised at 3200 rpm static as well but am not going to complain.

Maybe if I describe the issue a little better it will help. The engine advances right up to 3200 rpm with no hesitation or stumbling. The throttle is probably at about 7/8" all the way forward at that point. If I push it forward the last 1/8" that is when it starts to stumble. It still stays right around 3200 rpm but runs rough. If I pull the mixture about 1/3 of the way out it still goes up to 3200 rpm but no longer stumbles when at full throttle. I remember reading somethign along time ago about having a way to keep the slide on the aerocarb go full open and that if it was completely wide open you could get stumbles like I am experiencing but I haven't been able to find a reverence to that again. Is there a way to limit the slide from completely opening up?

My engine monitoring is pretty basic. I have a very old stratomaster Ultra X (not even the XL). I am monitoring the two rear cylinder head temps and the two rear EGT's. At full throttle with the mixture leaned the EGT's climb right to about 1400 before setteling out. They drop right back down when I pull the throttle back. The CHTS's are all well within limits staying under 300 degrees.

It seems like I need to lean the mixture out some more but am concerned about getting too lean and damaging the motor. On the other hand I don't want to have to worry about giving it full throttle on a go around and forgetting to lean it out with the mixture and having the engine stumble.

The only other issue I am having with the motor is a semi-high oil pressure. It is within limits at around 85 on startup and settles down to around 70 when running but this is alot higher than others have reported. I need to check the pressure with a mechanical gage to make sure what I am seeing is real and not a sender issue.

Keith
#554

[mike.smith]

I'm running the stock sensenich that Sonex sells. I was surprised at 3200 rpm static as well but am not going to complain.

Maybe if I describe the issue a little better it will help. The engine advances right up to 3200 rpm with no hesitation or stumbling. The throttle is probably at about 7/8" all the way forward at that point. If I push it forward the last 1/8" that is when it starts to stumble. It still stays right around 3200 rpm but runs rough. If I pull the mixture about 1/3 of the way out it still goes up to 3200 rpm but no longer stumbles when at full throttle. I remember reading somethign along time ago about having a way to keep the slide on the aerocarb go full open and that if it was completely wide open you could get stumbles like I am experiencing but I haven't been able to find a reverence to that again. Is there a way to limit the slide from completely opening up?

It seems like I need to lean the mixture out some more but am concerned about getting too lean and damaging the motor.

The only other issue I am having with the motor is a semi-high oil pressure. It is within limits at around 85 on startup and settles down to around 70 when running but this is alot higher than others have reported. I need to check the pressure with a mechanical gage to make sure what I am seeing is real and not a sender issue.

Keith
#554

Hi, Keith:

I think I've already given you what worked for me (I really think I just got lucky!). But it sounds like you're pretty close as you are, so it sounds like just some tweaks needed. I don't think you want to limit the carb slide travel. WOT is WOT. There isn't all that much travel in this carb, so I would want all I could get. Unless someone else disagrees ('cause I'm no engine guy), I think you need lean it a bit. Per Sonex and others, 1/8" turn can make a noticeable difference, but I can't see that doing any damage; not when the mixture arm itself will travel a whole 2".

As for the OP, that is high. On my MGL Xtreme Mini I had to set a setting in the device because it was reading like 2psi. A mechanical gauge showed it at 45 or so. So your older device might still have something that has to be set. My instructions weren't very clear about which setting I needed, so a little trial and error got me to the right setting.

Mike Smith

[radfordc]

I remember reading somethign along time ago about having a way to keep the slide on the aerocarb go full open and that if it was completely wide open you could get stumbles like I am experiencing but I haven't been able to find a reverence to that again. Is there a way to limit the slide from completely opening up?

You are right that the Aerocarb works better if you don't open it all the way. I don't know why...it just does. I adjusted my throttle linkage so that the slide doesn't completely close.

[kmacht]

Well I found the reference I remebered. It was actaully in the aerocarb manual right before the tuning section. It states:

"Some installations may encounter rough engine operation or "balking" near full throttle. If this occurs, the movement of the throttle slide needs to be limited to prevent it from opening and exposing the entire intake air passage. On some installations this may be as much as 1/4"".

I am going to double check with the factory but since I am getting a good throttle up to 3200 rpm I think I'm going to try limiting the ammount of travel and re-tuning for the EGT spread.

Keith
#554

[MichaelFarley56]

At full throttle with the mixture leaned the EGT's climb right to about 1400 before setteling out. They drop right back down when I pull the throttle back. The CHTS's are all well within limits staying under 300 degrees.

That seems awfully lean Keith. I know you're leaning with the mixture but I have a bad feeling that, if you're performing takeoffs and climbs with EGT's of near 1400 degrees, you're going to have a pretty hot running engine. I try to adjust my carb needle so I can be in a sustained climb, full power and full rich mixture, and see somewhere around 1150-1200 degrees on my hottest EGT. The reward is a fairly cool running engine. Just my two centns on that one...

[billmaxmcw]

My aerocarb gets leaner when at full throttle. I initially also had the problem of stuttering at full throttle which went away at lower throttle settings. So I richened by adjusting the needle, and got good running at full throttle, and mid-1200 max EGTs on climbout. I lean at cruise power settings. I don't know if this is peculiar to my #2 needle or if they all are cut this way.
Bill, Oregon