Hey guys, I'm having a problem with a Fuji/Walbro carb. I ran it today and the problem suddenly appeared. I had to go to work, so I will have to try it again tomorrow.

The engine ran fine at start, gets about 18 lbs. of thrust (with a Top Flight 20x6-10 prop @ 6500rpm) just after start up, then, gets progressively weak, with power dropping off to about 13 pounds of thrust.

I've rebuilt the carb in the last couple of weeks, the engine is moderately tightly cowled, I don't think it's tight enough to cause a problem. It doesn't get tight...in other words, the prop spins nicely...will "bounce" back off of compression a few times.

Pulse line is tight. Atmospheric pick up vented to interior of fuselage.

It acted like it got lean, bogging but eventually getting to full throttle, but if I do a lean check (putting finger into v-stack slowly, it dies without any rpm change which is correct.) I can run it up to full throttle, but after a while it just doesn't get the thrust, and it will eventually start running bad enough to make me concerned about flying it...in other words, I won't fly till I get the problem sorted out.

Timing is 28* CH Ignition

I flew it the day before and it ran great. So now I'm stumped. I also don't think it's the fuel filter, as I completely cleaned my tank... (aircraft and bulk tank, that is) just a couple of weeks ago.

I'm wondering if anyone else has had this problem?

Thanks in advance,
Darrell

Check the fuel screen in the carburetor. It's a prime candidate for clogging and causing reduced power due to restricted fuel flow. Also make sure the engine's getting a good flow of cooling air. Cowled engines need to have the air flow between their fins. Many times, cooling air doesn't go through the fins, but goes from the air inlet directly to the air outlet. If you can see the air outlet when you look into the air inlet, and your view isn't obstructed much by the engine, it's likely that the air is bypassing the engine.

If all else fails, take the engine out of the model, put it on a test fixture and see if it will run there. If it will go "all day" with no problems, then it's not an engine problem, but something in the entire airframe/engine/fuel system combination.

I found the problem and a couple of other interesting discoveries and posted them in the gas engine forum.

Thanks Bax!