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  #1  
Old 12-28-2020, 01:14 PM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 183
Default Poor mixture authority

Iím running an O-320-A3A with an MA-4SPA 10-3678-32 carb. The engine runs smooth (except below 750 rpm; I attribute that to the wood prop). I canít help but feel I am running lean due to the mixture control having very little travel before an RPM drop is observed. I did validate the mixture control is actuating the lever to both stops.

On the ground at approximately 1200 RPM I did not observe any RPM rise before a drop. In the air at 8500í and WOT I observed approximately 50-70 dF EGT rise on the three cylinders for which Iím instrumented; I canít say if this is peak or not, merely thatís when RPM visibly dropped off. RPM rise is either not there or imperceptible on my tach. The one CHT probe I have is on #4 and sees up to 370-380 during a long 100 MPH climb and around 300-320 during cruise.
Iím not instrumented for fuel flow. These temperatures at with ambient temperatures about 55-60 on the ground.

I will be installing a JPI soon so Iíll have an idea of what all four cylinders are doing plus have fuel flow information before making any changes. The lack of mixture authority is what got my attention though. I saw the posts here about RVs running lean and did some research, including calling MSA. They said the 3678-12 is the richer carb for my engine compared to the -32 I have installed. Yet, I saw posts here from the past year from others who called them and were told the opposite.

My theory is that this is one of two things - either my carb is running lean or I have one cylinder running very lean and causing an RPM drop off while the others are still rich.

Any thoughts or other avenues to explore here?
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  #2  
Old 12-28-2020, 03:24 PM
Taltruda Taltruda is offline
 
Join Date: Oct 2017
Location: Las Vegas, NV
Posts: 766
Default

From idle, when you move the mixture to ICO, what rpm rise do you get? You stated that you get none at 1200rpm, but how about at idle?
It does sound like maybe the jetting could use opening up. .I would expect more egt change from full rich to peak egt in cruise. .
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RV-8 empenage finished 10-2020

Wings Started.. 11-2020
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  #3  
Old 12-28-2020, 09:08 PM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 183
Default

Quote:
Originally Posted by Taltruda View Post
From idle, when you move the mixture to ICO, what rpm rise do you get? You stated that you get none at 1200rpm, but how about at idle?
It does sound like maybe the jetting could use opening up. .I would expect more egt change from full rich to peak egt in cruise. .
I'll check the ICO rise next time I fly, but its not my primary concern since its easier to adjust than the jet.
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  #4  
Old 12-28-2020, 10:00 PM
Taltruda Taltruda is offline
 
Join Date: Oct 2017
Location: Las Vegas, NV
Posts: 766
Default

Quote:
Originally Posted by 00Dan View Post
I'll check the ICO rise next time I fly, but its not my primary concern since its easier to adjust than the jet.
Itís just another data point to check, and I figured that I would mention it since you couldnít get a rise at 1200rpm.. thatís probably because you arenít really on the idle circuit then. It does sound like your main jet may be a little lean, as I would expect more of a egt rise in cruise though. Have you read the posts about the ďMooney modĒ on RV carbs?
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RV-8 empenage finished 10-2020

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  #5  
Old 12-28-2020, 10:15 PM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 183
Default

Quote:
Originally Posted by Taltruda View Post
It’s just another data point to check, and I figured that I would mention it since you couldn’t get a rise at 1200rpm.. that’s probably because you aren’t really on the idle circuit then. It does sound like your main jet may be a little lean, as I would expect more of a egt rise in cruise though. Have you read the posts about the “Mooney mod” on RV carbs?
I have indeed, but wasn’t sure if they were applicable since I have a 320 with a -4SPA rather than a 360 and a -4-5.
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  #6  
Old 12-29-2020, 06:20 AM
mahlon_r mahlon_r is offline
 
Join Date: Jan 2005
Posts: 1,053
Default

Quote:
Originally Posted by 00Dan View Post
Iím running an O-320-A3A with an MA-4SPA 10-3678-32 carb. The engine runs smooth (except below 750 rpm; I attribute that to the wood prop). I canít help but feel I am running lean due to the mixture control having very little travel before an RPM drop is observed. I did validate the mixture control is actuating the lever to both stops.

On the ground at approximately 1200 RPM I did not observe any RPM rise before a drop. In the air at 8500í and WOT I observed approximately 50-70 dF EGT rise on the three cylinders for which Iím instrumented; I canít say if this is peak or not, merely thatís when RPM visibly dropped off. RPM rise is either not there or imperceptible on my tach. The one CHT probe I have is on #4 and sees up to 370-380 during a long 100 MPH climb and around 300-320 during cruise.
Iím not instrumented for fuel flow. These temperatures at with ambient temperatures about 55-60 on the ground.

I will be installing a JPI soon so Iíll have an idea of what all four cylinders are doing plus have fuel flow information before making any changes. The lack of mixture authority is what got my attention though. I saw the posts here about RVs running lean and did some research, including calling MSA. They said the 3678-12 is the richer carb for my engine compared to the -32 I have installed. Yet, I saw posts here from the past year from others who called them and were told the opposite.

My theory is that this is one of two things - either my carb is running lean or I have one cylinder running very lean and causing an RPM drop off while the others are still rich.

Any thoughts or other avenues to explore here?
Does the engine get rough as your tach indicates the rpm loss? I would ignore the rpm loss and lean until you see peak on the first of the three instrumented cylinders to hit peak EGT. What is the Egt spread from full rich to peak on that cylinder? Does the engine get rough or start stumbling at this point? At idle, below 1000rpm, you want to see a rise in RPM, when leaned out to quitting, to see what the idle mixture is doing.... No rise too lean, 10-25 rise just right. At power, is is difficult to discern mixture using the TACH. It can be done but you need really accurate, sensitive instruments. The egt will give you way more accurate data. If the engine stumbles or gets rough way before you are leaned to peak on the instrumented cylinders that means the one that isn't instrumented is much leaner than the others. If the engine loses power evenly when you are going past peak then that means all of the cylinders are close to the same mixture. Ideally, you would want to have the engine lose power evenly without roughness, as you go through peak and at lower altitudes of say 3000 feet and under, you would want to see at least a 50-75 degree spread from full rich to peak on the first cylinder to peak. On a carbureted engine that is a bit difficult to attain. But if your engine gets rough or starts stumbling when the first cylinder hits peak then you know that cylinder is the leanest and as long as you have the 50-75 degree spread at lower altitudes, on that cylinder, the engine is rich enough and you are assured that the other cylinders are at least as rich as that one or richer.
Happy New Year!
Mahlon
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  #7  
Old 12-29-2020, 10:33 AM
JDeanda JDeanda is offline
 
Join Date: Apr 2013
Location: Ventura, CA
Posts: 244
Default Intake Leak?

Dan, did you do the usual first things in checking out a lean engine? I tangled with a newly installed O-290 some time ago that ran like that. Pilot gripe was high mag drops, and rough on either mag by itself. Weird, right? Long story short, after checking the mag timing, (it was fine) I ran it myself and noticed that a very short pull on the mixture made the engine quit. I had the carb apart looking for obstructions. Very ashamed to admit, after all that, (all that!) I noticed that all 4 primer holes in the heads were not plugged. Well, it's SoCal and it's not unusual to not have a primer system. Whoever built the engine apparently figured the installer would add a primer. So. A trip to the local hardware store for 4@ 1/8" NPT brass plugs and that engine was ALL better. I was surprised that it would run even that well or at all with such a huge intake leak. So, anyway, make sure the carb is tight on the oil sump, and check all around the gasket to see if there's a gap. Make sure the intake hoses are tight and sealing, and check the intake flanges at the heads. Those things can distort and leak if they're overtightened. If you see blue fuel stain by the flange(s) it's a dead giveaway. Once, I even ran across an engine where all four of the paper flange gaskets were missing altogether. You might pull the sparking plugs and see what color they are. If they're light grey or white on the business ends, something is definitely lean. I'd do all that easy, free stuff before rejetting the carb. Safe flying!

Last edited by JDeanda : 12-29-2020 at 10:40 AM.
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  #8  
Old 12-29-2020, 11:07 AM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 183
Default

Quote:
Originally Posted by mahlon_r View Post
Does the engine get rough as your tach indicates the rpm loss? I would ignore the rpm loss and lean until you see peak on the first of the three instrumented cylinders to hit peak EGT. What is the Egt spread from full rich to peak on that cylinder? Does the engine get rough or start stumbling at this point? At idle, below 1000rpm, you want to see a rise in RPM, when leaned out to quitting, to see what the idle mixture is doing.... No rise too lean, 10-25 rise just right. At power, is is difficult to discern mixture using the TACH. It can be done but you need really accurate, sensitive instruments. The egt will give you way more accurate data. If the engine stumbles or gets rough way before you are leaned to peak on the instrumented cylinders that means the one that isn't instrumented is much leaner than the others. If the engine loses power evenly when you are going past peak then that means all of the cylinders are close to the same mixture. Ideally, you would want to have the engine lose power evenly without roughness, as you go through peak and at lower altitudes of say 3000 feet and under, you would want to see at least a 50-75 degree spread from full rich to peak on the first cylinder to peak. On a carbureted engine that is a bit difficult to attain. But if your engine gets rough or starts stumbling when the first cylinder hits peak then you know that cylinder is the leanest and as long as you have the 50-75 degree spread at lower altitudes, on that cylinder, the engine is rich enough and you are assured that the other cylinders are at least as rich as that one or richer.
Happy New Year!
Mahlon
Mahlon, good points on the RPM drop. I'll admit I didn't particularly consider letting the engine go further lean. I'll try this.

Quote:
Originally Posted by JDeanda View Post
Dan, did you do the usual first things in checking out a lean engine? I tangled with a newly installed O-290 some time ago that ran like that. Pilot gripe was high mag drops, and rough on either mag by itself. Weird, right? Long story short, after checking the mag timing, (it was fine) I ran it myself and noticed that a very short pull on the mixture made the engine quit. I had the carb apart looking for obstructions. Very ashamed to admit, after all that, (all that!) I noticed that all 4 primer holes in the heads were not plugged. Well, it's SoCal and it's not unusual to not have a primer system. Whoever built the engine apparently figured the installer would add a primer. So. A trip to the local hardware store for 4@ 1/8" NPT brass plugs and that engine was ALL better. I was surprised that it would run even that well or at all with such a huge intake leak. So, anyway, make sure the carb is tight on the oil sump, and check all around the gasket to see if there's a gap. Make sure the intake hoses are tight and sealing, and check the intake flanges at the heads. Those things can distort and leak if they're overtightened. If you see blue fuel stain by the flange(s) it's a dead giveaway. Once, I even ran across an engine where all four of the paper flange gaskets were missing altogether. You might pull the sparking plugs and see what color they are. If they're light grey or white on the business ends, something is definitely lean. I'd do all that easy, free stuff before rejetting the carb. Safe flying!
My ~1800 RPM runup mag drops are normal, with just less than 100 RPM smooth drop on each. At cruise power in the air mag drop is about the same, but with a slight roughness associated at that power level. I haven't done a formal intake leak search, but when the plane was in prebuy about 5 weeks ago the engine got a full visual inspection and a trained ear listening for leaks during the compression test. Plugs were pulled and nothing out of sort was noted at the time.

When I install the JPI next month I'll be giving everything a good visual inspection and it will be interesting to see if the different instrumentation sheds some light on it. I'm certainly avoiding any sort of exploratory surgery.
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  #9  
Old 12-29-2020, 05:20 PM
BoydBirchler BoydBirchler is offline
 
Join Date: Jul 2020
Location: Indianapolis
Posts: 91
Default Here is a thread about drilling the jet with the info you need

https://vansairforce.net/community/s...light=carb+jet
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  #10  
Old 12-29-2020, 05:35 PM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 183
Default

I did some testing today. At 20" MAP and approximately 2250 RPM at 2200', I got the following results by leaning until roughness (I didn't pay attention to RPM).

#1: 53 dF EGT rise
#3: 21 dF EGT rise
#4: 58 dF EGT rise

CHT on #4 was stable at approximately 310 dF the whole time.
I tested ICO rise at full idle, which is set to approximately 650 RPM right now. There may have been a small (~20 RPM) rise but it was hard to tell.


Quote:
Originally Posted by BoydBirchler View Post
Thanks for the link.
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