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Old 11-12-2023, 02:37 PM
mechmike mechmike is offline
Join Date: Feb 2020
Location: Powell River
Posts: 24
Default 0-360 Higher static RPM with air cleaner installed MA4-5

Friend of mine called me up about a recent 75 rpm drop in full throttle static RPM. (poor climb out noticed) Runs perfectly smooth with good mag drops.

To try and remedy this he has:
Had the mags done, timing checked several times by different shops
Full rich mixture going to the stop on the carb MA4-5
Full throttle going to the stop on the carb
Carb heat door wide open.

I told him to pull his air cleaner off and try it. He says it's 20+ rpm lower without the air cleaner on.

I asked if he gets an rpm raise if he leans it during his full throttle static, he does.

Going to pull and look in the carb tomorrow, does anyone have any thoughts?

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Old 11-12-2023, 04:29 PM
rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 10,458

The air filter is known to improve the flow transition from the FAB into the carb.
I know of instances where an engine ran very poorly when the filter was left out.
The actual effect seems to vary from one airplane to another.
Opinions, information, and comments, are my own unless stated otherwise.
You are personally responsible for determining the suitability of any tips, ideas, etc. obtained from any post I have made in this forum.

Scott McDaniels
Hubbard, Oregon
Formerly of Van's Aircraft Engineering Prototype Shop
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Old 11-13-2023, 06:26 AM
vic syracuse vic syracuse is offline
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Join Date: May 2005
Location: Locust Grove, GA
Posts: 2,825

The filter needs to be installed. If you ever try to run the engine with the filter and AirBox not installed, you will see why. The airflow needs to be ‘smoothed” entering the carb or fuel servo.

That being said, don’t forget to check the intake gaskets and hoses for leaks.

Vic Syracuse

Built RV-4, RV-6, 2-RV-10's, RV-7A, RV-8, Prescott Pusher, Kitfox Model II, Kitfox Speedster, Kitfox 7 Super Sport, Just Superstol, Hummingbird helicopter, DAR, A&P/IA, EAA Tech Counselor/Flight Advisor, CFII-ASMEL/ASES/RW
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Old 11-13-2023, 07:37 AM
mahlon_r mahlon_r is offline
Join Date: Jan 2005
Posts: 1,107

It is normal to get an rpm rise when leaning out at full power. Try a full power mag check. If its good at full power that will eliminate the entire ignition system for defects. Full power check will show up defects that don't show up at mag check RPM. Compression test is in order and if engine isn't relatively new check valve travel looking for a worn cam lobe. Check for exhaust restrictions.
Good Luck,
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
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Old 11-13-2023, 08:49 AM
PilotjohnS PilotjohnS is offline
Join Date: Jan 2016
Location: Southwest, USA
Posts: 2,948
Default Hi speed miss

During phase 1 after 10 hours, I had a high speed miss at full throttle that wasn't present at run up RPM. Turned out I had a bad ignition switch..yMMV
John S

WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for their use.

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or builders log here on VAF: Pilotjohns
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Old 11-18-2023, 07:28 PM
mechmike mechmike is offline
Join Date: Feb 2020
Location: Powell River
Posts: 24

Thank you all.

It also had another known good set of mags put on to test it, same low, full throttle static. P leads removed, same thing. leak downs are high 70's and even, cam is 3 years old, muffler was un-clamped and scoped for broken baffles.

I looked at the carb, dropped the bowl, perfectly clean but the venturi was a goopy mess of blue from avgas staining... like wet slime, not dried blue stain.
Above the carb, in the sump shows bright aluminum so it seems to have poor atomization maybe from fuel coming out of where the jet screws on.

It had the old white float and didn't have the updated venturi so the owner wanted to send it in to get rebuilt as it's been more than 10 y (he's only had the plane for 6 months)

Carb is the 10-5193


Last edited by mechmike : 11-18-2023 at 11:58 PM.
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