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Finnicky carb at Idle - HELP!

Rick RV-4

Well Known Member
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.
 
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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.
 
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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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?
 
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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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.
 
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 ?
 
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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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?
 
ICO speed

Thanks to all for advice. I do pull very slowly and got 10rpm, so I richened a little and got it to 20. Concerned about too rich since it works fine so left it alone. Will tinker with max rpm next time the cowl is off 😊
 
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 ?

Max rpm (throttle fixed at idle) is a reasonable proxy for best power if you don’t have a MAP indication (but may be tough to discern if you have an unsteady idle).

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?

I have always pulled relatively quickly (2 sec transit?) from full rich to ICO.
 
Thanks guys. Good advice so far. I will recheck the mixture arm to make sure it is going all the way to full lean when I go to ICO. That might explain why one thing is telling me I need to go a tad bit more rich and the other might explain why it won’t shut down like it used to
 
Figured it out

So as a previous poster recommended, I checked the mixture control arm. It was just shy of being able to go back all the way to cutoff, so a small adjustment made it possible to go to full cutoff. I also adjusted the idle mixture screw just a tad more rich so that solved the slightly rough running after a cold start.

Thanks guys!
 
Rick,
Make sure the mixture lever is also going to the full rich stop when you push the red knob full forward. That is the most important limit to reach if you can’t hit the stop on both ends - that’s according to Vic Syracuse in one of his articles. My RV4 doesn’t have enough throw to hit both stops completely, so I adjusted it to hit the full rich stop. It’s now just barely shy of the ICO stop, but will still kill the engine if I set about 1000 RPM before selecting ICO. That’s where you are supposed to set it to see the slight increase in RPM before shut down. If I select ICO at my slow idle speed (700 RPM), it will usually just stumble as it’s trying to quit, at which time I just turn both mags off (one SureFly) - which is also mentioned in Vic’s article. With a light weight carbon prop (I have one too), there isn’t as much flywheel affect, and a slightly higher idle RPM may be required. Mine also runs a little rough at slow idle when cold, and smooth when warmed up. I think this is totally normal with a carbed engine, especially with a very light weight prop.
 
Will check it!

Thanks Scott. I’ll check it out just to make sure. Is there a link to Vic’s article (or where can you find it)?

Cheers,

Rick
 
Vic writes articles for EAA and maybe Kitplanes, not sure, and he also has many webinars that I’ve watched. All very good information, but I don’t know which of those it was, relative to the mixture control. I’m pretty sure it was mentioned as a result of one of the many pre-buy or condition inspections he’s done. I take notes on some of these when I hear/read something that I didn’t know and want to check on my plane. I don’t remember where this one came from. I’ve met Vic once, and he’s a very approachable guy, and active on this website. PM him and ask about your situation with mixture control.
 
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