912ULS rough rpm drop

[Woodymuler]

I have been quietly lurking here for several years and have benefited greatly from the combined knowledge base. Thank you all for that!

I Had my DAR inspection last week and received the A/W Cert. for my KF5/7FWF.

The engine, a 912ULS, has several hours on it from ground runs and slow taxi tests. But I’ve never run it much over 4000 rpm (Ignition check). Carbs are in cinc and It runs smoothly from idle up through 4000 with a good ignition check.

However, at 4600-4800 it stumbles and rpm’s drop to the 3k range with rough running until you throttle back.

This was two days ago and I have not had time to look into it beyond pulling the upper cowling.

My thoughts- mixture leaning out due to something in carb?
Spark plugs?
Ignition wires?
Excessive oil K&N air filters?
Intake air leak?

Any suggestions before I start taking things apart?

Thanks,
Todd

[Dave S]

Hey Todd,

Congratulations on getting your A/W cert!

It's pretty hard to estimate what the problem might be over e-mail; however, the best luck is normally on the side of using an organized approach.

Normally that means first being sure the engine is healthy internally (should be if it is new), then being sure the entire ignition system is 100% and then checking on the induction and fuel system after that.

On an ignition system I like to start at the spark plugs and work my way back through the entire system. The same when I get to the fuel system working backwards from the carbs through the system including a fuel flow test @ the carb inlets and fuel pump inlets.

I would doubt the air filters or an intake leak would be the source of the problem as the filters flow very well anyway; and, intake leaks normally express themselves at low or idle RPM.

Occasionally a person can spot something out of sequence; but, I have always had the best luck sticking with an organized approach. Others mileage may vary.

Good Luck & Keep us apprised of your progress

[AvDES LLC]

One thing I’ve noticed that did that while I was tinkering with my 912 is an excessively low float height. If you’re able to get good rpm for a bit and as you increase throttle is begins to hang and finally big down there are two likely things to focus on first -
1) verify float height in carbs - I’ve gotten away from the 10.5 mm measurement and just turn off fuel , pop bowls, measure the height of the fuel in the bowl - should be about 12.5 mm from bowl top

2) possibly an over rich condition - when you do a run up and immediately **** down, turn off fuel, and pull a carb from the intake - wipe your finger around inside. If it’s wet with fuel, you’re getting excessive rich when you’re transitioning from the needle valve circuit into the main jet circuit within the carbs.

In all of my tinkering, I’ve noted it’s more likely to be the overflowing carb and less likely to be the main jet as the 158 main keeps it relatively well fueled at wide open throttle and the needle jet comes into minimal play at that point