cuwaert

I have a 3W 55i as everybody will know by now.
When I open throttle it runs say 6400 and after 3 seconds or so it backs off slowly to say 6100 (take a few seconds).
A colleque has a ZDZ50 and does the same.
Anybody gas engine specialists knows what this could be?

CK1

My Feedback: (60)

Check for proper tune . Check for excessive heating due to a lean condition . Check the internal screen in the carb for build up and debis . I recomend replacing it rather than cleaning if it is dirty because : A) you may damage it when cleaning . B) the fine mesh may look clean but can in fact have a film build up that is very restrictive and very difficult to remove without damaging the mesh screen .

Truckracer

My Feedback: (19)

ahicks

My Feedback: (2)

I'm with Truckracer. Very likely a non event. Also very likely it's just happening on the ground. With the additional airflow available once airborne, you may have a different story.

r ward

pump diaphragm in carb ?.

kmeyers

I'm going to say your over propped and poorly tuned.

Biela 22x10 is over propped for the sweet spot. You can use it, but, it is not the ideal.

A 22x8 is good place to start.

Start experimenting with the tuning. Make changes and record your results, then make other changes in tuning and prop selection. You should come closer to ideal by experimenting.

Try some of these changes in the air as well. An airplane is not a static device.

Remember that you do not use the same prop for a WWI biplane that you would use in a WWII fighter, even on the same motor (at least I wouldn't).

cuwaert

Well a flat prop such as 8" makes the plane scream, I'm not a 3D flyer, so it's not hanging on the prop alll the time.
We fly at 10m elevation with 22x10 on 50-55cc planes that do normal flying and 23x8 for 3D. All with canisters and 20 to 25 cm manifold lengths, that's where these engines are at best with. Rpm's between 6800 and 7400 static.
Ofcourse depending on elevation and muffler (pitts and alike) can change the characteristics of the engine (for the worse) with different prop size as a consequence.

kmeyers

cuwaert
I guess I am having trouble understanding your lack of data.
Your not providing more good data.
Question "Why do I get 6400 rpm then it lowers to 6100"?
The most common reason would be tuning. Example : High speed needle 1.5 turns out when it should be 1.6.
This is why I have suggested you experiment a little with tuning.
Another common reason is over propping an engine. Merely saying that others are using 22x10 on their engines is not specific enough. Example: a Biela prop 22x10 is more load than a Xoar 22x10. All props even with the same nomenclature are not the same.
Again you must experiment with different props if you want a different outcome.

I have also thrown out the idea of engine plane type to what prop you should select.
We have at my field a guy who puts a 12 x 4 on a pylon racer and tries to hover a 3d plane with a 10 x 6. He then tells us how the owners manual for the engine says that those props are OK for the engine. He does not get the performance he wants.

Without more data, I see no remedy in your future.

Something to consider.

Truckracer

My Feedback: (19)

Many gasoline fueled internal combustion engines lose a bit of power as they reach full operating temperature. Diesels are an exception. I can't think of any engine I have ever owned that doesn't exhibit this trait and this includes our model engines on through car racing engines, etc. Many race engines produce their max performance at temperatures well under what we would call normal for an everyday, daily driver. Drag racers typically hate to take to the track w/o a proper cool down period between runs. Not because they worry about hurting the engine but because they will have less than maximum performance. The same holds true for our gassers and this is one reason it is important to tune them at operating temperature. A higher or lower temperature will give false readings making them out of tune at normal temperatures. A 400 - 500 RPM decrease at the prop may be excessive but tuning as others have suggested may reduce that number. Again, tune after full warm up. This is why I suggested before that the reduction in power after warm up may be a non-problem where the mechanical condition of the engine is concerned.

kmeyers

This is of course true.
My point is more of a general one, give as much data as possible. After I wrote that last post I read one of his other posts on this engine and he is talking about header length changes. Where and when is that mentioned?

cuwaert

Yes I apologyse for me being confusing.
I had a lot of problems that seemed to be related with one single faulty item, and that is the header length of my canister.
( read also "3w55i low power") In fact I myself was confused because I thought to reach 6400 with the faulty canister header lenght at some particular time. Did so many tests with different exhaust set-up's and without carefully writing down the particular results corresponding the particular set-up's, something one should do when you're 65 year old.
Ofcourse I apreciate all comments and think-work concerning this matter and I promise to be a bit more accurate and thorough in the future.

CK1

My Feedback: (60)

Cuwaert, no need to apologize . You were looking for answers and didn't know what info needed to be provided to get the answers you seek . It is usually up to the Guru's to ask pertinent questions of you before they give out their disertations err. recommendations .