Worrying power loss on take-off

[J Holland]

Re: Worrying power loss on take-off

Some interesting points. Thanks.

I (with an engineer) have been doing some investigative work on her this afternoon. We tried to replicate the situation with no luck and she runs fine which frustrates me somewhat as I was hoping for the same thing to happen when he was there.

We checked the fuel flow which was fine. The fuel filter is clear. The feed to the carb is therefore ok. We did notice that Carb Heat "cold" was giving about 25% hot air deflection which we have corrected. Unless mistaken, I don't think this would have caused the sudden drop. Tomorrow he is going to take the carb off and give it a good inspection and clean out in case it has muck in it.

My only other thought has been inspired from a post above. Before she went in for the top OH she did have a fair bit of MOGAS in her main tanks and wing tanks. We syphoned the three tanks out after the overhaul as the temps at the moment are 34 degrees here. When she went in for work they were around 16 degrees so MOGAS was ok to fly on then. After syphoning out the tanks we put 7 gallons of AVGAS in the main tank and ran her up on that. I flew her for 20 minutes and she was fine. Whilst flying in the afternoon (shortly before the trouble started) I recall turning the tap to drain the remnants of the MOGAS into the main tank that hadn't come out of the wing tanks when we syphoned them. There must have been about 3 gallons of MOGAS (more than I expected) that would have drained on top of approx 6 gallons of AVGAS that remained in the main tank after my 20 minute flight earlier in the day. It was definitely after I did this that the problems at full power started.

The MOGAS in the wing tanks would have been 7 weeks old and the temps have been high recently. My question is whether there is a chance that diluting AVGAS with 1/3 ratio of 7 week old MOGAS might have caused the problem at high power settings?

Ideas (however unusual) welcome?

[1948TCARTDRIVER]

Re: Worrying power loss on take-off

I had a similar problem with my 1948 Taylorcraft, it happened about two weeks after I bought it. I was doing my run up and the engine would only turn about 1200 rpm. I checked the mags, carb heat, fuel primer, and fuel shut off position, nothing had any affect. I suspected a fuel problem and was somewhat upset with my new purchase. I Pulled the fuel line to the carburetor and got my first clue to the problem as tiny bits of varnish came out with the fuel. I pulled and split the carburetor and found bits of varnish in it also, blocking some of the small passages. I cleaned the carb and flushed the tanks with fresh fuel, we were running car gas at the time. I think a change of fuel my have caused the problem. Anyway it was now time for a test flight and we broke all the rules. A friend and I were partners on the airplane and we had both worked on the carburetor. We both wanted to show our faith in our work so we both decided we would test fly it together along with a tool bag just in case we ended up in a field somewhere. The runnup was fine and the test flight went fine, oh the confidence of youth.

I have been bucked off a horse a time or two and had to make a dead stick landing in a rental airplane, it is not easy, but it is important to get back on the horse.

[Len Petterson]

Re: Worrying power loss on take-off

33 deg C in the air and heat absorbation on the runway maybe 50C + in the air just above the runway, =Vaporlock! Fuel hosefitting at the carb was so hot I could not hold on to it! We instaled a firesleeve over the hose, testflown today, Ok I hope.
We hade similar sitiuation with the Thorp at simmilar heat here in Canada last week, hot and heavy 2 onboard emergency was decleared.
The O-sh-t factor got to high. We landed safly!(;f
I also experiensed a inward swelling "choking"fuel hose earlyer this year, the engine would not take trottel, I found out taxing out, not in the air.
If the hose has been dry for some time, funny things can happen. They may look good on the outside, but? ( So did my old girl friend) Aslo check airflow to the tanks. Wents HAS to be open! Reminds me, on my next walk around I shall stick a wire though the tubes on the wingtanks fuelcaps. Just to make shure! We have a lot of hornets around.
Len

[barnstmr]

Re: Worrying power loss on take-off

I have experienced vapor lock also. But this occurred at low power. Generally speaking vapor lock isn't a problem when fuel flow is high (such as cruise or takeoff power).

[Ragwing nut]

Re: Worrying power loss on take-off

My bad on misreading when the comps were taken. Pop the valve covers and check the valve springs. Not too long ago I had a forced landing due to broken valve springs. Everything was fine except for an eratic rpm at full throttle I could not trace until both valve springs broke on one valve. Run up was ok, flew ok, except I was always fiddling with the throttle. I know you just topped it, but it is something easy to check.

Mike

[dewhitaker]

Re: Worrying power loss on take-off

Hi J - glad you found the carb heat issue and have resolved it. If it was 25% open, that might have been enough to cause the mixture to be too rich for proper combustion. I have seen what happens when a take off is made with the carb heat on, the performance really suffers. It would be interesting to know if you had any dark smoke out the exhaust during that turn around the pattern.

IMHO, the mixture of auto and aviation fuel should not be a factor. Many people routinly mix the two in a 4 to 1 ratio to achieve a lead content similiar to 80 octane. Just make sure your auto fuel does not contain any alcohol, which will attack rubber and coatings on corks.

[jdoran]

Re: Worrying power loss on take-off

If it were me, I'd remove the drain plug at the base of the carb, and watch the flow of fuel into a bucket. Raise the tail to a level (cruise) attitude, then watch the flow to see if it changes any. This would tell you if a fuel blockage or venting problem might be the cause.

If the fuel flows look OK, I'd think a valve may be suspect, especially a tight valve guide that swells when the engine is run at higher RPMs.

John

[crispy critter]

Re: Worrying power loss on take-off

J, Maybe a silly question but have you checked the compressions? Do it both hot and cold.A couple of broken rings or gaps lined up and stuck on one cylinder would certainly cause this problem,at low engine speeds she'll run great but throttle up and put a load on her and she will let you know it.

[Mark Julicher]

Re: Worrying power loss on take-off

Are you dead certain you don't have an old neoprene needle in your carb? I know the old stuck needle issue usually causes total power loss, but check it out anyway.

[Ragwing nut]

Re: Worrying power loss on take-off

Something else to check that came to mind from Kevin's post, check the spark plugs under pressure in a plug checker. I have seen plugs work fine until they were put under load and quit.

Mike

[Gary Snell]

Re: Worrying power loss on take-off

I would put my money on a sticking exhaust valve. Runs fine on ground when cool but when it heats up on climb the valve sticks open. Losing one cylinder equates to the rpm drop you describe. After landing things cool down and the valve unsticks. Was the valve/guide clearance to spec during the overhaul? Exhaust guides need more clearance than intake. Going from memory exhaust guide clearance is .003 to .005 vs .001 to .003 for intake. If this turns out to be your problem, it is possible to ream the guide without removing the cylinder.

Good luck.

[Ron Coleman]

Re: Worrying power loss on take-off

Hi J.

Depending on what kind of cylinders you have, the effect that you are descsribing can be a result of an incomplete breakin procedure or rings that do not have the proper gap. What happens is, you get a mini-seize as you pull top rpms under load (everything gets really hot). The engine cools a little, speeds up, gets hot slows down, etc. I would pull a couple of top plugs and check for scoring on the cylinder walls. If you have serious scoring you may have broken a ring. You won't see an oil temperature change or oil pressure change but if you put a cylinder head temp gauge on the cylinders, you will be able to tell, in flight, which cylinders are causing the problems.

Ron C
N96995

[J Holland]

Re: Worrying power loss on take-off

Well, the plot thickens….

You will recall my nervousness about flying the bird after my engine power loss after the top overhaul.

I have since had the carb checked over properly and a fuel flow check done. Both showed no problems. The carb was put back and extensive ground runs made. There was no sign of any problem and we “assumed” it was resolved.

After building up some confidence after 20 minutes of further ground runs I took to the skies and she climbed away beautifully. A short 20 minute flight to my favourite strip for lunch and the engine ran as sweet as a nut.

I re-started her after an hour and taxied to the hold. Power checks were fine and I set off home. Once again, 15 seconds into the run (30 feet up) the power just dropped by 400 rpm. Luckily there was a good headwind and a 900m strip so I could put it back down quite safely. My nerves were a bit shot though at this point and frustration building quickly.

I sat on the grass and thought hard about what I am doing flying at all with three kids at home (all under 7). At that point I decided to throw it all in and let someone else buy her and see if they could solve the riddle.

As I waited for my wife to drive the hour long drive to pick me up I decided to carry out some further ground runs. Chocked, full power was fine for up to a minute. This was the case for 10 simulated “idle to full power on take off” accelerations. On one of them it did drop slightly and card heat restored it which intrigued me. Another 6 of these and she had given me a small amount of confidence back.

Knowing my wife was still 30 minutes away I got permission to make a fast taxy down the runway with the intention to get “horizontal” onto the mains. This is when the power loss seems to occur aggressively and I wanted to try and trick this plane into thinking I was going flying which is when she seems to want to bite me.

I held full power for thirty seconds on the brakes facing down the runway. The engine was fine. I applied carb heat for 10 seconds and there was a tidy drop of 150 rpm and the power restored when I selected cold again. I released the brakes and set off. With the wind on my nose the tail was up almost instantly. Within about 5 seconds the power died again down to 1500 rpm.

PLEASE HELP!!!

Someone suggested an object in the fuel tank and I didn’t think much about that until tonight when I have started to wonder if they might have some sense in their thinking. Would the horizontal attitude let something go to the front of the tank and cut of my fuel?

Look forward to some more suggestions.

If the fault is unfindable there is gonna be one cheap T-Cart for sale very shortly. My wife gave me a Kitfox catalogue today and said that I should consider a “more reliable aircraft”!!!!!

Further suggestions welcome

[Garry Crookham]

Re: Worrying power loss on take-off

J,
In an earlier post you stated that the problem occurred after you drained the wing tanks into the main tank. I would suggest that you drain all the fuel from the main tank and carb and refuel. While the main tank is empty, check to see if there is something in the tank that could be restricting fuel flow.
I recently helped a customer with a starting problem with his PT-22. The cause of the problem was that after sitting for a couple of months the Marvel Mystery Oil he used in his fuel for upper cylinder lube had separated and the carb bowl was full of MMO...no fuel. Fresh fuel is a good starting point.
Does your fuel system vent through the caps, or are all the tank vent lines connected to a single vent tube? If the system is using a single vent point, make sure there isn't a blockage in the vent line to the main tank.
As a last resort, secure the airplane in a level flight attitude, tail on a stable surface, airplane tied down and chocked. You can now try to recreate the problem and if it occurs check mags, carb heat etc. without the worries of being in the air.
Good luck.

Garry Crookham
N5112M
Tulsa

[crispy critter]

Re: Worrying power loss on take-off

Sounds like primer lines could be sucking air.There is several possibilities.