M20C Engine Stumbles

[Jerry 5TJ]

Today departing KSAC the engine in the M20C 'stumbled' twice for a fraction of a second, the two events about 1 second apart at about 100 AGL. I aborted the takeoff and landed on the runway ahead with no problem.

Two full run-ups went fine.

Departed and flew home with no further issues.

Conditons

• second flight of the day -- had been sitting 1 hour between flights
  
• ground temp 80F
  
• Density altitude ~1100'
  
• Engine CHT after a moderate wait for departure was between 350 and 400 (original little gauge hard to read)
  
• had been idling with lean mixture
  
• went to full rich mixture for take off
  
• fuel boost pump was on
  
• checked fuel selector = fully in detent on L tank, which had 17 gallons in it
  
• fuel drained before first flight of day
  

Any theories as to what makes the engine go "ahem" twice and then be fine?

[gregwatts]

My guess is that there was still enough water in the tank to make your engine burp. If you had made a steep enough turn during the first flight, there may have been a "water bubble" that made its way to the engine.

The same happened to me in the 201 recently.....but I was out of runway so my choices....had I needed them.....were few.

[N33GG]

I would be curious what your engine TSOH is, and how long ago it was OHed. Possible sticking valve, and maybe time for a wobble test?

[Jerry 5TJ]

OK re the water bubble theory, will look for evidence of any water.

Engine is 700 TT since new 16 years ago. What's a wobble test?

[Seth]

Scary situation and good job in aborting the takeoff and landing on the remaining runway.

I too am curious as to the wobble test? What is the wobble test?

-Seth

[carusoam]

Jerry,

Consider having your mechanic wash everything between the carb and intake valves. There is a fare amount of blue goo that builds up in there. The intake tubes of my 65C had so much, I was surprised.

It is a good opportunity to change seals on the valve covers, intake tubes, and tighten oil return lines from the valves.

After my intakes were cleaned, the idle was noticebly smoother.

16 years without a cleaning could be a good time to replace seals anyway. They may be made with newer materials now than what were used back then?

Best regards,

-a-

[N33GG]

I wasn't going to use the term, but yes, it is referred to as Morning Sickness, or Morning Cough. The problem is the valves get caked up and have too little clearance and begin to stick. The wobble test is where they remove the springs, and manually wobble the valve to insure enough clearance in the guides. If too tight, they remove the valve and clean out the guide, and reinstall the valve. Not a big deal. I had Maxwell check my engine with a wobble test recently.

[N33GG]

BTW, my engine also has relatively low hours, but OHed a LONG time ago, around 1978 or so. So I watch it very carefully.

[Jerry 5TJ]

Nice 70F day here in N CA, two more flights in the "C" today with no cough or stumble.