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"Morning roughness" C90 - please advise


Well-Known Member
Supporting Member
Sep 9, 2011
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Hello fellow Cub lovers!
Really could need some good advise on an engine problem - so it can be sorted out before the 75 year anniversary for D-day in June where we want to fly from Oslo/Norway to Normandy/France! :)

I restored an L-4 Grasshopper with a C90 engine that had approx. 3000 hours SMOH. Engine had been stored in boxes since a prop strike in 1976 when it was inspected and assembled in 2015. New slick mags was installed. Carburetor is a MA-3SPA. I have a rather coarse prop for a C90, it's a Hercules propeller, 72" with a 45 pitch. It gives a nice 85 MPH at 2200 RPM on cruise - then the consumption is approx. 20 ltr/hour, without leaning. This consumption should be comparable to other C90 Cubs.

First season (2015) it ran flawlessly. Next season I started noticing some occational "cough" on climbout only with cold engine, lasting only a few seconds. It was not frequent... but became gradually more frequent... until now where it happens every takeoff. Here are the symptoms:

Engine starts easily, always on first or second stroke (handpropping). I keep it at 800 RPM's first couple of minutes. Leaning mix on ground. After a minute or so I increase to 1000 RPM waiting for >75 degrees on the oil. Runup at 1700 with normal mag drops approx. 50RPM - runs smoothly on single mag L/R. Due to the coarse prop, I only get approx. 2000+ RPM static on ground. That is why I can never reproduce the problem on ground... frustrating!

On takeoff roll, when I get some airspeed, the RPM's stabilize at 2300 at a climb speed of 60 MPH. Then, just when airborne, I ALWAYS get a rough engine with 2-300 RPM drop. It lasts for 1-2 minutes before the engine BOOM goes back up to 2300 RPM and flies smoothly for the rest of the flight. Even touch and goes with new climbout's will run smooth like a dream.

This is what I have tried so far when the roughness appears:
- Leaning mix, no difference.
- Carb heat (often fly formation with two other L-4's they never have this problem - so it definitively not ice!).
- Mags changed to L/R/BOTH - no difference.
- Reducing throttle back below half will make the engine run smoothly again - but only at approx. 2000 RPM or below. With pax, 2000 RPM barely gives positive climb...

A mechanic suspected fuel type, as I had changed after first season from AVGAS LL100 to a new unleaded aircraft fuel UL91. I therefore changed back to LL100 AVGAS, flew 10+ hours on that - but there is no difference. I'm back with UL91 now - no lead means clean plugs and I understand that the engine manufacturers recommend this fuel.

Leak test is performed annually and I tried it both on cold and warm engine - nice values steady at 80/75 +/- 2 PSI. Nice firm compression. Engine sounds really good (well, sounds "normal").

When the problem occurs, I was filmed once from ground. Forward the following video to time 5:10 where my "43 J" will fly by with coughing engine:

You can go to 5:00 to see another healthy C90 to compare the sound. It almost sound like one cylinder is dead.. could it be valves, lifters ? (I will try to get the video clip without the background music...)

Since this occurs only on first takeoff with "cold" engine, the problem is definitively related to temperature. I can warm the engine enough on ground so this does not occur, but then it takes at least 5-10 mins at 1500 RPM's to be sure it will not happen. All other Cubs at the airport warms to 75 deg, then taxi out and fly off like a dream... I'm getting jealous.

- Could it be an unusual coarse propeller that makes the engine "work hard" ? Am I just on some limit that will be OK with warm engine? I was able to borrow a 44 pitch prop that had been shortened to give higher RPM. It gives only slightly higher RPM's than my Hercules 45 prop, so I cannot conclude based on the test flights with this 44 prop.

So dear Cub friends, I'm running out of ideas. I would really appreciate your opinions and ideas. Please keep in mind that this problem is definitively related to temperature, so please skip far fetched theories...

Thanks folks, eager to hear your opinions.

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