Prop Wash pulling fuel.

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Lharsch4

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May 22, 2021
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8
Hey all,
My current project is a Rotax 277 powered MiniMax. I have the plane just about ready to fly with all gauges functional, the engine runs and idles well with no filter, and the wood/fabric is in great shape. I’m excited to say the least.
However, my adventures have led me to the prop wash pulling fuel out of the carburetor as it runs, more at idle then at speed. On top of this, if I try to put the cone filter on the plane came with, it will not accelerate and bogs with throttle.
I’m running stock jetting for no intake silencer (148 main, 2.72 needle jet, 45 idle jet, and needle clip is in the middle slot), proper spark gap and new plug. Air filter is clean enough to see light through it, so I cleaned again anyway and still no change. I’m bouncing back and forth between “the air filter is too restrictive and it’s flooding” and “the air filter is forcing the fuel typically being pulled out to go down the intake and also flooding”
Any insight would be very very helpful.
Thank you.
 

Dana

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I don't think it's prop wash. 2-strokes typically spit fuel out the carburetor at idle.

If by "cone filter"you mean a K&N or similar, try cleaning it. Over-oiling it will make it too restrictive and make the engine bog, as will a too-small filter.
 

Lharsch4

Member
Joined
May 22, 2021
Messages
8
I don't think it's prop wash. 2-strokes typically spit fuel out the carburetor at idle.

If by "cone filter"you mean a K&N or similar, try cleaning it. Over-oiling it will make it too restrictive and make the engine bog, as will a too-small filter.
Will def try a bigger filter and see how it goes!
 

Lharsch4

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May 22, 2021
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I don't think it's prop wash. 2-strokes typically spit fuel out the carburetor at idle.

If by "cone filter"you mean a K&N or similar, try cleaning it. Over-oiling it will make it too restrictive and make the engine bog, as will a too-small filter.
Ah ok I’ve never noticed on any of the dirt bikes we keep around! I assumed it was prop wash because the prop wash because it mists super fine in the air stream.
 

MadProfessor8138

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Ah ok I’ve never noticed on any of the dirt bikes we keep around!
That's because the reed valve is generally stopping the back flow of pressure from the cylinder back into the carb.
The reed opens to intake fuel from the carb to the cylinder and then slams shut until the next revolution creates the suction to open it again to allow more fuel in.

Kevin
 

Lharsch4

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Joined
May 22, 2021
Messages
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That's because the reed valve is generally stopping the back flow of pressure from the cylinder back into the carb.
The reed opens to intake fuel from the carb to the cylinder and then slams shut until the next revolution creates the suction to open it again to allow more fuel in.

Kevin
I feel so dumb, you’d think that I would recall reeds being a thing after spending hundreds on VForce reeds.
 

Lharsch4

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May 22, 2021
Messages
8
You might try cleaning it first, as per Dana’s comment. I use https://www.autozone.com/miscellaneous-cleaners-and-degreasers/air-filter-cleaner-and-air-filter-oil/k-n-recharger-kit-spray/92607_0_0 to clean the K&N air filter on my Lycoming engine.

BJC
I have cleaned it twice now! I appreciate it! Rigged up an extra K+N come filter from my Subaru to go on. Several PVC adapters and some tape later it was good enough to run it and slip it on quick, stayed running but still wouldn’t draw enough to get into the top end.
 

MadProfessor8138

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I've never ran a 277 so my opinion may be way off........take it with a grain of salt.
I've only ran 377,447,503,582,670 engines so I'm not familiar with the intake system of the 277 but.....spitting fuel out of the carb is not uncommon on 2-strokes that don't use reed valves or rotary valves on the intake system.
I'm not sure how the 277 is set-up....

It sounds to me like you've got 1 of 4 things going on with it spitting fuel and not accelerating.
1. Carb or intake issue.......carb jetting or valve problem if they use a type of intake valve.
2. Ignition problem........timing off,coil not putting out enough,plug wire breaking down,plug breaking down,etc.
3. Fuel issue.....it doesn't like the fuel or oil that your using.
4. Prop issue......it doesn't like the load the prop is putting on the engine.

Not much help.........sorry.

Kevin
 

BJC

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I have cleaned it twice now! I appreciate it! Rigged up an extra K+N come filter from my Subaru to go on. Several PVC adapters and some tape later it was good enough to run it and slip it on quick, stayed running but still wouldn’t draw enough to get into the top end.
I use a RC-3180 K&N Universal Clamp-On Air Filter on my 180 HP Lycoming. You can calculate the area of my filter verses yours, and ratio the HP to see if your’s, in clean condition, is big enough.

BJC
 

Dan Thomas

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Many years ago the owner of a VW-powered Taylor Monoplane spent weeks trying to find out why his engine wouldn't rev up properly. He had two carbs on it, one on each head, with "scoops" made of short pieces of tubing cut at a 45° angle, with the angled opening facing forward so the prop would "ram" air into them. An old guy came along and asked what was up. The Taylor owner told him. The old guy told the owner to turn those scoops around so they weren't getting the pulsating air off the prop. He said that the air could be bouncing out of the carb in between the blade pulses and causing all sort of manifold pressure and fuel flow issues. The Taylor guy turned them around and the engine ran like it was supposed to.

Many years later I was running up a Cessna 182 after an oil change and inspection. Cowling was off for the short run so we could check for oil leaks before cowling it up. It ran rather roughly, yet we could find nothing wrong. Finally I installed the cowling and it behaved itself. The carb inlet airbox was getting that pulsating airflow, messing with the carb's venturi pressures and fuel flow. The cowling carried the air cleaner, which damped those pulses by directing the airflow down and across the filter, not directly at it. Those old Cessna engineers knew what they were doing.
 

Lharsch4

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Joined
May 22, 2021
Messages
8
Thank you all for the help!
As of now, dropping the needle seems to have fixed it!
I also found a misfire from the coil wire to throttle cable. (Seamless plug for the idea of running your plane in the dark to find any arc it might have.) will order a new coil/wire combo and then I think it’s finally ready to fly!
 

MACOWA

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Apr 19, 2021
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39
Hey all,
My current project is a Rotax 277 powered MiniMax. I have the plane just about ready to fly with all gauges functional, the engine runs and idles well with no filter, and the wood/fabric is in great shape. I’m excited to say the least.
However, my adventures have led me to the prop wash pulling fuel out of the carburetor as it runs, more at idle then at speed. On top of this, if I try to put the cone filter on the plane came with, it will not accelerate and bogs with throttle.
I’m running stock jetting for no intake silencer (148 main, 2.72 needle jet, 45 idle jet, and needle clip is in the middle slot), proper spark gap and new plug. Air filter is clean enough to see light through it, so I cleaned again anyway and still no change. I’m bouncing back and forth between “the air filter is too restrictive and it’s flooding” and “the air filter is forcing the fuel typically being pulled out to go down the intake and also flooding”
Any insight would be very very helpful.
Thank you.
Check yer choke
 

Lharsch4

Member
Joined
May 22, 2021
Messages
8
update:
Problem worsened over several days of letting the plane run 30-45 minutes at a time, a few times a day. Eventually wouldn’t run smooth below half throttle. Another carb disassembly revealed the choke plunger had an invisible tear when pressure was applied. replaced with a needle type plunger and problem is solved when reverted back to sea level jetting, idling better than ever and much smoother all around (Who would have thought). thank you all!
 

MACOWA

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Apr 19, 2021
Messages
39
update:
Problem worsened over several days of letting the plane run 30-45 minutes at a time, a few times a day. Eventually wouldn’t run smooth below half throttle. Another carb disassembly revealed the choke plunger had an invisible tear when pressure was applied. replaced with a needle type plunger and problem is solved when reverted back to sea level jetting, idling better than ever and much smoother all around (Who would have thought). thank you all!
Hello, Lharch4 Good shootin' ! My first 277 was on a j-3 kitten years ago, same symptoms, and a real head scratcher. But now we both can field strip and reassemble a Bing-54 blindfolded. Best of luck with your Mini Max.
 
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