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  #1  
Old 08-06-2020, 09:36 AM
Rick RV-4 Rick RV-4 is offline
 
Join Date: Aug 2006
Posts: 299
Default Finnicky carb at Idle - HELP!

Okay engine gods and goddesses. Just installed a freshly overhauled MA-4-5 carb from Chief Aircraft. Standard O-360 with a Sensenich GA prop, and E-Mag/P-Mag combo. Advanced Flight Systems digital tach so I assume the tach is accurate.

Adjusted the idle RPM to about 740. Seemed to run smoother there than 650-700. Next I worked on the idle mixture screw, working to get it to give the 30-50 RPM rise when going to cutoff at shut down.

I don't know why but I seem to be having a heck of a time getting all of the parameters to come together and work properly. Currently it runs just a little rough at idle (but smooth at or above 1,000 RPMs) until it warms up (5-7 minutes). Takeoff, cruise, and post flight it runs great (including at idle). But at shutdown, I see about 10 RPM gain (too lean?) but going to cutoff will not always shutdown the engine (too rich?). It will just barely run at cutoff, but will keep stumbling until I shut off the ignition switches.

Seems like this should be a simple process (it was with the old carb) but this one is giving me fits. Any recommendations?

I have checked the timing on the E-Mag and P-Mag and they are good. Also checked for any obvious leaks or visual signs of issues. Compressions were all mid 70s.

Last edited by Rick RV-4 : 08-06-2020 at 09:39 AM.
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  #2  
Old 08-06-2020, 09:41 AM
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scard scard is offline
 
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Location: Cedar Park, TX
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Default

Don't always assume "freshly overhauled" is perfect. I just had one overhauled by a highly recommended shop that came back with the mixture completely inop, clearly mis-assembled.
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  #3  
Old 08-06-2020, 09:58 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by scard View Post
Don't always assume "freshly overhauled" is perfect. I just had one overhauled by a highly recommended shop that came back with the mixture completely inop, clearly mis-assembled.
+1

Assuming you have the linkage installed correctly, It is possible that the overhauler did not install and clock the mixture arm properly when reassembling the carb. This could leave the mixture valve slightly open with the arm at the ICO position and give your symptoms. I would first insure that your cable is getting the mixture arm to the stop on the carb when pulled out.

I prefer to adjust idle mixture via the lowest MAP procedure instead of EGT rise. It is much easier to get right. Rough running that gets better as the engine warms up usually points to a lean mixture. Generally speaking, the colder the engine, the richer the mixture needs to be. This is usually accomodated by a choke that our carbs don't have. Therefore, idle mixture is a bit of an exercise in compromise. It can't be expected to be perfect, both cold and hot. SOme go a bit over rich to give better running when cold and others just warm up at 1000 RPM to deal with the the somewhat lean condition when cold.

Larry
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Last edited by lr172 : 08-06-2020 at 10:09 AM.
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  #4  
Old 08-06-2020, 01:46 PM
Ralph Inkster Ralph Inkster is offline
 
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Location: Calgary, Alberta
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Default

Give the intake runner hose clamps a tweek while you are under the engine too.
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  #5  
Old 08-06-2020, 07:36 PM
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rjcthree rjcthree is offline
 
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Default Low MAP procedure

A quick search didn’t find reference to a lowest MAP procedure. Can someone link it for those of us whom are lacking search-fu today?
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  #6  
Old 08-06-2020, 07:53 PM
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Quote:
Originally Posted by rjcthree View Post
A quick search didn’t find reference to a lowest MAP procedure. Can someone link it for those of us whom are lacking search-fu today?
I don’t think there is much more to it than:
1). Warm up engine
2). Set throttle butterfly to desired idle rpm
3). Adjust idle mixture to minimize MAP
Iterate on 2 and 3 until convergence.

At least that’s how I do it.

Basically looking for “best power” mixture at the desired idle speed. If you are at an atypically high density altitude, you may want to “cheat” slightly rich of “lowest MAP” so as to not be too lean in more typical conditions.

Peter
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Last edited by pjc : 08-06-2020 at 07:56 PM.
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  #7  
Old 08-06-2020, 10:22 PM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by pjc View Post
I don’t think there is much more to it than:
1). Warm up engine
2). Set throttle butterfly to desired idle rpm
3). Adjust idle mixture to minimize MAP
Iterate on 2 and 3 until convergence.

At least that’s how I do it.

Basically looking for “best power” mixture at the desired idle speed. If you are at an atypically high density altitude, you may want to “cheat” slightly rich of “lowest MAP” so as to not be too lean in more typical conditions.

Peter
That's it in a nutshell. Good advice also for higher altitudes. Much better to have the ability to idle well at lower altitudes.
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  #8  
Old 08-07-2020, 06:26 AM
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Larry DeCamp Larry DeCamp is offline
 
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Default Low MAP symptoms ?

My engine runs fine winter and summer both on the ground and on landing with throttle chopped. Yet, I only get about 10 to 20 rpm rise at idle cut off. I don’t have a MAP gauge. What is the characteristic of “lowest MAP” if adjusting idle mixture ?
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  #9  
Old 08-07-2020, 07:23 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
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Quote:
Originally Posted by Larry DeCamp View Post
My engine runs fine winter and summer both on the ground and on landing with throttle chopped. Yet, I only get about 10 to 20 rpm rise at idle cut off. I don’t have a MAP gauge. What is the characteristic of “lowest MAP” if adjusting idle mixture ?
An engine will produce the greatest vacuum (lowest MAP) and highest RPM at it's best power mixture. This is an optimal, albeit a tad richer than necessary, setting that will provide good running and also have a good margin for conditions that require a richer mixture, such as a lower altitude or colder ambients. An engine typically runs fine on mixtures leaner than best power at idle. If I was setting the mixture in winter, I would go a tad leaner than lowest MAP, otherwise it would be richer than best power in the summer, where I do much more flying.

The primary benefit is ease of setting. It can be a bit challenging to get a reliable EGT reading with the very fine resolution required when leaning at idle level RPMs. If your mixture setting works well in the conditions you fly in, there is no real benefit to changing it.

Larry
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Last edited by lr172 : 08-07-2020 at 07:31 AM.
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  #10  
Old 08-07-2020, 01:08 PM
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rjcthree rjcthree is offline
 
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Default Speed of ICO

When you pull mixture to find ICO RPM rise, how fast do you pull it during testing of it? Do you search for it by moving slowly, move it slower than normal cutoff by X, or?
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