912s Engine Problem Help

[brannan78]

We are having a curious issue with our 912s (Speedster IV) and was hoping you might be able to lend an idea.

On a three occasions after takeoff during or soon after initial climb-out the engine acts as though it’s failing (decreasing in power and RPM) with almost a sputtering sound. At first we thought it might be too lean but the plugs look okay. Thought about maybe bad diaphragms in the carbs but would like to know if others have found this.

In two cases powering back to less than 4,000 RPM helped the issue. It does it for about 30 seconds soon after it runs fine for the remainder of the day, and the issue doesn’t arise for weeks and multiple flights after.

On two of the occasions it happened in the first minute of flight and not again. On the third occasion it happened in the first minute on the first three takeoffs and then not again.

Any ideas?