Lins

I can't remember, do engines typically run richer or leaner when the cowl goes on if tuning with no cowl on? I know, I shouldn't do that, but I don't want to drill holes in my cowl and engine box just so I can tune my ZDZ. I'm just trying to get it 'close' again before flying. It's a AW 75cc Extra with a ZDZ 80, cheers.

Truckracer

My Feedback: (19)

If cooling ducting in the cowl is set up correctly, there should be very little difference in mixtures between cowl on / cowl off. Its all about pressure in the cowl and how it affects the metering diaphram in the carb. More often than not, I see engines go rich with the cowl on especially with big round cowls with huge openings in the front. Sure there are sometimes airflow issues near carbs, etc. but I'll let someone else deal with that.

Tired Old Man

My Feedback: (1)

Should be the same unless you've increased heat retention with the cowl. I would not bother trying to tune them differently if it was mine.

pe reivers

It all depends on your carb location and how the airflow influences the pressure at that little hole in the carb regulating membrane cover. More pressure gives richer mixtures, less pressure leans the mixture.
Example:
fitting a tube on the orifice, and slightly blowing in it with running engine will act as if pulling the choke, making the engine die rich! That regulating device inside the Walbro is extremely sensitive.

Lins

It ended up being lean with the cowl on. I'm guessing I have lots of airflow out the bottom of the cowl then, lowing pressure inside the engine box/fuselage. Provides for great cooling so far! Cheers guys.

captinjohn

Place a static air flow tube on it. Then set & forget! Maybe Pe can give you the details.............

scooterinvegas

My Feedback: (1)

+1