Continental A75 power loss, BIG Mystery

[Tasca]

I run in my J3 replica one Continental A75 -9, feed by a Stromberg carburetor and 2 electronic ignition.
With the wood propeller it fly leveled fill open throttle at 2450 rpm, maximum for this version is 2600 rpm. It use to has a great performance compared to the 8 others J3 there is in our airpark.
220 hours after tbo, I notice a power loss and maximum leveled rpm is now 2200. The takeoff roll is bigger, climb rate is poor and maximum ais lower, sound of the engine has changed a lot. I have 4x1 with no muffler exhaust, it is louder than regular J3 muffler, but is different even from the ground, to silent !
On break maximum rpm is 2150 and on takeoff roll drops to 2100 !!
I look every little thing possible; at least what I could image…


Air induction filter, flown without filter
Carburetor, checked twice, changed one
Induction pipes and connections
Gas feed, gascolator, roses
Changed avgas
Changed ignition system, new plugs, cables, coils
Compression check 80/78 all cylinders
Open the engine and check camshaft, tappets, push rods, all clearances
Test 3 different propellers
Checked ignition time with time light
Engine run smooth as ever, mag check drop is less than 25 rpm as ever with electronic ignition
Muffler is clean

However, no results…. Any clue? I appreciate any help
Thanks all

[rwanttaja]

Is it possible the carburetor heat is jammed on? The full-power RPM change is consistent with the carb heat being on all the time.

Do you still get an RPM drop during runup when you test the carb heat control?

Have you run an analysis of the oil to see if the engine is making metal?

Ron Wanttaja

[Tasca]

I swept carburetors and run without heat box, just to check if there was any restriction on the induction, no results …
I remove the rear section and open the engine to inspect camshaft and tappets, the is no metal on screen and oil tank
Boroscoped carburetor to be sure it was wide open … nothing

[rwanttaja]

I just sent you a PM with the email address for the Fly Baby Continental Engine Guru, Harry Fenton.

Ron Wanttaja

[Tasca]

thanks Ron

I'll try !

[Eric Brown]

Would you happen to a Mitchell Instruments brand tachometer installed in your plane? I also have had the odd RPM drop on the take off roll. Try to borrow a shoot thru the prop tachometer to check tach accuracy.

[Tasca]

Hi Eric I've already made that. I check with electronic tachometer and the indication is ok. General performance has droped a lot, takeoff, climb rate, every thing... it is like i did note open full throtle !
last weekend I put new ignition on it, no results. I'm preparing one magneto to install and try.
tks

[gmatejcek]

This is a real long shot, but is the fuel tank vent obstructed?

[Tasca]

Hi, I've checked every little detail, including tank vent and it's interior for debris, including gascolator and rose to carb. nothing.
I had even changed for another strombergh carb of a friend of mine just rebuilt.
This last weekend, I changed the carburetor for a Marvel and bingo, it works again how supposed.
In strombergh manuals there is 3 different arrangement of venti + main jet gicleux and bleed air.
What may had happen to stop working? On both strombergh, they are very simple and reliable !
Hi, I've checked every little detail, including tank vent and it's interior for debris, including gascolator and rose to carb. nothing.
I had even changed for another strombergh carb of a friend of mine just rebuilt.
This last weekend, I changed the carburetor for a Marvel and bingo, it works again how supposed.
In strombergh manuals there is 3 different arrangement of venti + main jet gicleux and bleed air.
What may had happen to stop working? On both strombergh, they are very simple and reliable !

[FabricGATOR]

I have seen many Stromberg carbs with the mixture control disconnected and wired full rich. I believe that folks think they're inop or defective because they don't work to 'shut off' the engine like a Marvel Schreiber.
As it was explained to me:
The Stromberg mixture control works by creating a vacuum on the float bowl that limits how much fuel is vaporized, thus creating a leaner operating condition for higher altitude (thinner air). This contraption doesn't work like a fuel valve at idle (ground rpm) speeds, so many assume it is not working at all.

Let me ask you this:
Q What fuel are you using? Are you using only 100LL provided by FBO?
Q Are you the sole operator, pilot, aircraft fuel filler?

I had a similar circumstance one time on a 6 cylinder Continental O-200 on a Skyhawk
I think it is the carb, it hasn't been cleaned or worked on in 34 years.
Marvel carb was rebuilt by a local CRS carb shop and when reinstalled, it ran the same way. Ugh
I jiffy rigged an alternate fuel supply for testing with known 100LL and it ran great!

Determined that while Mogas was approved, one of the community student operators (we all know who) must have put some 'bad fuel' (maybe E-85?) from the local filling station (not the one we told him to go to, that we all were using without incident)
Drain, purged fuel system, refilled, all worked as Operation-Normal

That guy while he did eventually get his private, many of us now refer to him as "Crash"

[Tasca]

This weekend I've made more test...
adjust idle mixture and the engine runs smooth now and solve carbon on plugs that had on previous flight and got 2480 rpm ! great !!
The Strombergh I've has no mixture control, it is NA-S3-B
I'm using 100ll avgas with very good quality control, I’m in flying in a air park and something like 65 airplanes get same fuel and nobody is having problem.
I'm the only operator of the Cub and I fill the plane. My PA14-ET use the same fuel and there is no problem.
In this 8 months trouble the tank has been suppled at least 8 times.
There is 2 different arrangement for carbs:
1. venturi 1-1/4 main #49 bleed air #65 (already test) this combination was flying
2. venturi 1-1/4 main #50 bleed air#65
3. venturi 1-5/16 main #46 bleed air #60 (already test)

what may had happen to the engine reduce the gas eating ?? it has only 225 flying hours

This weekend I've made more test...
adjust idle mixture and the engine runs smooth now and solve carbon on plugs that had on previous flight.
The Strombergh I've has no mixture control, it is NA-S3-B
I'm using 100ll avgas with very good quality control, I’m in flying park and something like 65 airplanes get same fuel and nobody is having problem.
I'm the only operator of the Cub and I fill the plane. My PA14-ET use the same fuel and there is no problem.
In this 8 months trouble the tank has been suppled at least 8 times.
There is 2 different arrangement for carbs:
1. venturi 1-1/4 main #49 bleed air #65 (already test) this combination was flying
2. venturi 1-1/4 main #50 bleed air#65
3. venturi 1-5/16 main #46 bleed air #60 (already test)

what may had happen to the engine reduce the gas eating ?? it has only 225 flying hours