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Rich running engine?

pilotmansam

Active Member
Here is a question I will throw out for those gurus on here smarter than me, which would be all of you:

My RV-6 0320 fixed pitch engine seems to have developed a roughness which starts as soon as I throttle back at cruise - even if I am cruising at low altitude, i.e. 2000'. It smooths right out as I lean the engine.

I am about ready to pull the carb and inspect, but it was just rebuilt a few years ago, and I cannot see anything externally amiss. No fuel leaking with airbox off and fuel pump on. The bottom plugs look a little oily, but they are not terrible.

The only thing I have done lately is rebuild the FAB. I had an earlier version of the FAB which held the filter up to the carb with a separate plate. The new design has the filter captured between the carb plate and the bottom of the fiberglass box. But it's the same filter. I just have a hard time seeing how this could be starving the engine for air.

Anybody ever had such an experience?
 
Counter intuitive

Here is my two cents on this topic. I have been experimenting with jet size/air intake on my 0320. I have had a hot #2 on climb out and my fuel flow has been high 10 to low 11 gph. Should be closer to 13.
What I have learned is the ratio mix of fuel/air is counter intuitive. Because the carb is a Venturi, the more air going in results in sucking in more fuel.

I started by drilling out my jet from 42 to 41. (3k difference). It only slightly created more flow. I contacted the carb company and they suggested a possible air flow restriction. To verify, I removed my air filter and went for a flight. Bingo,
13.5 gph but ran to rich. I had to lean to best power in order to have the engine run well.

My point here is. I think you have had an air flow increase due to your mod on the air box, this has resulted in more fuel being sucked in and running rich.

I?m no expert in this. But it?s what I have learned. Seems now your fix is a smaller jet.
 
OK. Thanks for all of that feedback. Rob: I believe the carb heat door is staying shut, but I will be sure.

I do not have an engine monitor - wish I did.

I have thought about the possibility that the problem could be too much air flow, so I might try a smaller orifice. I guess that would be just trial and error. Lots of work for each trial!
 
Air flow

I tried opening up the snorkel opening thinking it would increase air flow. (Lot of work)
I’m sure it did but is still being restricted by the air filter.
My next fix is to time my Lightspeed ignition with a timing light and I recently purchased the SDS intake flange mod kit. If that doesn’t fix the issue, I will drill out the jet from 41 to 40. Hopefully I will then get the flow on take off that I am looking for.

You could try restricting the air flow with a partial carb heat flap. See if that leans out the Engine.
 
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