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Engine lost power on takeoff

Patric_L

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Hello to all...
I decided to start this thread in hope that somebody might give me other hints to look at, or convinces me to try it again.πŸ˜†

Airplane Specs: Pitts S-1, O-320 Engine (about 450h TSOH), Ellison TBI, Wooden Fix Prop

Last Friday, the fog lifted and I decided to leave work early and went to the airport to have some fun in the Pitts. It was pretty cold -> 4Β°C (39Β°F) and I preheated the engine for a good hour with my heater that blows warm air into the cowling from the bottom. Nothing new, strange etc. I do this since I fly the airplane (2014) on cold winter days. I jumped in, started the engine and taxied to the Gas station to add 15L (4 Gallons) of fuel, so the tank was about half full. Started the engine again, taxied to the runway, Mag Check at 1700-1800RPM..nothing strange, everything worked as normal.
When I was on the runway I slowly added Power to wide open, the engine stuttered a bit which is not unusual on my airplane on the first take off for the day when everything is still a bit cold. Power came back and I was in the air...the the engine started to run rough and lost power, I reduced the power what helped a bit, tried to add power again and the engine decided to run pretty rough, I think I heared some bangs from the exhaust and it lost so much Power that I think it wouldn't be enough to stay in the air. But this all went pretty fast. At this point I had about 1300ft of our 2600ft Runway behind me. Behind the runway there are about 400ft of Gras downhill before you arrive in some bushes and a 10ft descent on a road. Behind that you have everything from trees, lots of trees, fences, valley etc.
I think I thought at this point that chopping the power now will not make me land and stop successfull on the rest of the runway but the bushes will stop me and may hurt the airplane..but I will be ok. Not chopping the power and try to bring the engine back to live will maybe make me land somewhere with by far not so good options. So I went the first way, put the airplane into a slip to loose energie, put her on the runway one wheel first, still in a light slip to break her down a bit and braked as much as it seemed possible...the moment the tail went fully down I went very hard on the brakes, went over the runway and came to stop about halfway between runway and bushes. The Pitts is totally fine, no scratches and I was able to turn around and taxi back to the hangar.

In front of the Hangar I tried to apply full throttle, engine still was rough and stumbled, so I shut her down, got out and calmed down a bit. 10mins later I was back in and tried again if the engine runs now. Going from idle to WOT the engine makes power, up to 100% but then again starts to stutter. From my feeling, she's too lean and hasn't enough fuel to burn. If I lean a bit and do the same at about 50% throttle the engine quits completely what, in my eays, proofs that it runs too lean. So what now?

In my Job (IT) I'm a big fan of: when a problem occurs...find out what has been changed lately, maybe that caused your problem now. Problem here is, I changed nothing, I built nothing, I added nothing..and the last maintenance work was also some (~30) flights ago. The only difference is the outside temperature since the winter arrived.
Sunday I removed cowling, airbox etc.. What I found so far:
1. The holes for the differential pressure tube were completely clogged with old avgas left overs. I don't know since when this problem exists, and I don't know how much of a problem this is for the TBI. Rotec for example doesn't have such a tube on their TBI and said they tried it, but it made no difference.
2. The bowden cable for the mixer lost a clamp, so it is kind if easy to close the mixture lever on the TBI maybe 25% by hand. If you let it go, it goes to about..let's say 97% full rich again. The lever in the cockpit will not move.

Everything else so far looks good, fueltank vent is open, fuel filters both look great, fuel pump vent tube doesn't show any signs of Avgas. I don't think it's a mechanical problem like a sticky valve or so since the engine runs totally fine until you want serious power, throttle back and it runs normal right away.
Maybe it's a mix, because of the cold weather, the clogged holes it runs a bit on the lean side and a shaking mixture lever will make it too lean? I think that is possible and would be the easiest fix.

Am I on the right way...or are there any other good hints to look at. There is so much knowledge in this forum, and I don't want to be in that situation again.

Thanks to all who have read this to the end and thanks for any tips in advance. And as always, sorry for any typos or using the wrong words...my German is still better than my english.

Patric
 

cwilliamrose

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