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PA-18A air filter...

daedgerton

daedgerton

PATRON
Arlington, VA
Still working my way around my new to me PA-18A... Has anyone seen a domed air filter set up like this? Hard to see in the picture, but the tube going from the back of the filter to the intake seems offset. I'm only getting 2400 RPM in flight. (haven't really started to troubleshoot yet...). Just looking for thoughts from the group.

IMG_1227.jpeg
 

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daedgerton said:
I'm only getting 2400 RPM in flight. (haven't really started to troubleshoot yet...). Just looking for thoughts from the group.
Click to expand...
Is this all it ever did?
Did you try it with the filter removed?
What is the diameter and pitch of the prop?
 
skywagon8a said:
Is this all it ever did?
Did you try it with the filter removed?
What is the diameter and pitch of the prop?
Click to expand...

Gonna try with air filter removed... Haven't done that yet.. Ive been going through ADSB install (nightmare...) so haven't had any time to mess with it. Have only flown it 2 hours so not very familiar with it at all. And yes, those 2 hours have only produced 2400 RPM.

Gotta look up pitch but prop is a M74DM...
 
The standard prop as it came from Piper is a 74" diameter and a 56" pitch. If the pitch is considerably more than 56", that could be the cause of a lower top rpm.
 
Witness marks indicate filter is properly seating so don’t see any issue with design, in fact, better design than single post which can be prone to breaking off. Unless filter is really (really) dirty, doubt that’s the cause of your rpm drop.
Be a good idea to confirm accuracy of your tach before anything else. Try to get your hands on a “Tru tach” or similar, accurate and simple to use.
Also be sure throttle linkage is producing WOT at carb.
congratulations on your new plane!
 
daedgerton said:
Gotta look up pitch but prop is a M74DM...
Click to expand...
The only way to be certain what the pitch is supposed to be is to remove the spinner (if you have one) and the front plate. Read the number which is stamped on the hub. If it has been changed there will be some re-stamping marks. The real numbers may not be in any written records.
 
Oliver said:
Witness marks indicate filter is properly seating so don’t see any issue with design, in fact, better design than single post which can be prone to breaking off. Unless filter is really (really) dirty, doubt that’s the cause of your rpm drop.
Be a good idea to confirm accuracy of your tach before anything else. Try to get your hands on a “Tru tach” or similar, accurate and simple to use.
Also be sure throttle linkage is producing WOT at carb.
congratulations on your new plane!
Click to expand...

Thanks sir... All great ideas, going to do each of those one by one. I bought this plane with insurance money from my PA12 that ground looped. So I have flying plane while I can slow roll the repairs on my PA12.
 
It could very well be the prop. Simple things to look at: Check the float bowl on the carb, if the screws get loose it will run real rich. Check the timing. Do a mag check in flight at 2400 RPM, I have had a plug that would not fire above 2200 RPM did fine at 1800 mag check. If you get a RPM drop greater than 150 with cab heat could be the carb is running rich.
DENNY
 
What is static and does it meet the TCDS? I would get the prop pitch first before I did any more troubleshooting and then verify it.
 
daedgerton said:
Gonna try with air filter removed... Haven't done that yet.. Ive been going through ADSB install (nightmare...) so haven't had any time to mess with it. Have only flown it 2 hours so not very familiar with it at all. And yes, those 2 hours have only produced 2400 RPM.

Gotta look up pitch but prop is a M74DM...
Click to expand...



I have that same prop on my pa-18 150 and all I get is 2450 to maybe 2500 on a good day! I have a new prop coming but not hear yet. I have been told all along that is the prop. Hope this helps.
 
I had a PA-18A with the round filter. Memory wants to say there was a different exit to the airbox - big square not a small hole. If so what's in Post #1?

Gary
 
My log book says that its a 7456 I can get 120 mph out of my cub with no tail wind. Which is faster than I have ever heard out of a SC. And I can only get 2500 rpm at full throttle level flight. I wonder if the previous owner got the prop re pitched. Something is up. Got me wondering now.
 
skywagon8a said:
A 7456 prop should give you 2700 rpm at full throttle level flight trimmed hands free.
Click to expand...

My log book says that its a 7456 I can get 120 mph out of my cub with no tail wind. Which is faster than I have ever heard out of a SC. And I can only get 2500 rpm at full throttle level flight. I wonder if the previous owner got the prop re pitched. Something is up. Got me wondering now.
 
daedgerton said:
Yeah, I can't even get close to 120... more like 90... Something is up...
Click to expand...
This could be a leaky connector hose between the wing and fuselage or some other leak in the pitot line. Find a spare airspeed indicator, T it into the pitot tube and compare the instruments.
 
Just for clarity how about taking some pics of your filter to airbox to carb from the sides? It might be an interesting setup. If curious push a small rubber hose over the pitot opening then pinch off to seal and roll up the hose some. It should show a higher airspeed under hose air pressure and then hold it stable if no leaks.

Gary
 
Here are some pictures from the backside of the intake... best I could get with the iPhone.

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No expert here and thanks for the pics. It looks like someone mated an early flat air filter filter airbox to a round filter lower cowl. Others may have a clue about the efficiency of the small hole and connection. However if it were me I'd make sure the hose is double sided material with any metal coil laminated between. If the wire is exposed and gets loose and sucked into the carb, and if the wire can flatten so can the tubing restricting flow.....but like I say others here deal with maintenance like this every day and I'm just a former owner.

Edit: Flat filter airbox on left and round filter airbox on right

Gary
 

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I have a 55 18A (obviously) with 150 hp engine, and the air intake into the carb air box looks nothing like this. I do like the "tripod" arrangement of the filter dome attachment, but question the small hole for air intake. Does anyone know if this is stock?

Jim
 
BC12D-4-85 said:
No expert here and thanks for the pics. It looks like someone mated an early flat air filter filter airbox to a round filter lower cowl. Others may have a clue about the efficiency of the small hole and connection. However if it were me I'd make sure the hose is double sided material with any metal coil laminated between. If the wire is exposed and gets loose and sucked into the carb.....but like I say others here deal with maintenance like this every day and I'm just a former owner.

Edit: Flat filter airbox on left and round filter airbox on right


Gary
Click to expand...


I think that is exactly what is going on here... Interesting....
 
Here's the first pic a little clearer. Looks like no exposed wire wrap (good) but I'd compare that hole to the outlet size of the airbox and not have it any smaller. Sharp hole edges, duct wrinkles, turns, and whatever aren't the best for flow. Have a talk with maintenance about the current setup and options. Is there documentation for what's there? As far as rpms vs airbox that's another matter.

Gary
 

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I just put 19.5 hours on a 160hp cub with tiny tires and Sensenich 74x56. At 2350 I was doing about 112-115 mph. Even got to 137. Never thought a cub could go so fast. It was awesome. It’s now on 31’s with 78x44 doing about 90. GPS speeds. I ran it up to 2500 a couple times, but never tried to go over. I’m 2350-2450 guy
 
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