FD620D

[Paul219]

Do these engines run rich? I noticed it smelled very rich, I adjusted the fuel mixture, leaned it out, but now it starts harder. Before I made the adjustment, i didn't have to use the choke to start the moter, now it seems I do.

[JohnF]

Hi Paul (I am assuming),

The carburetor on the FD620 has two* fuel circuits: a high idle (main) and low idle (idle) circuit. By the way, the names in the parenthesis are what I would call them and the other names are from the manual. The main fuel mixture ratio is fixed by air jets that are never changed and a fuel jet (inside the fuel bowl) that is changed when conditions require it - usually big changes in elevation. If you search I posted a table of elevation/jet.

I assume you are asking about the idle circuit. It shares some of the main circuit (so some of the fixed air jets) but also has an air screw on the back for fine adjustment. [Edit: I have an error that I greyed out here; see post below for explaination - I can't figure out how to strike/del in this editor] I would caution folks that this adjustment works opposite a fuel screw - in (tighter or closed) is rich and out (open) is lean. (Note: If you tweek the idle mixture you probably have to tweek the idle setting.)

Anyway, the idle mixture is actually pretty sensitive to ambient change. I'm obsessive enough to twiddle with it (plus I change elevation a lot). Not so sensitive that it annoyingly causes running issues at every change of weather but sensitive enough that "ideal" idle moves day to day.

If you don't change elevation then I would set the idle mixture to run just a tiny bit rich and forget about it.

To answer your first question during idle - I've had the same experience.

* - there is a dip tube with metering holes in a reservoir that serves the purpose of an accelerator pump that is sort of a pseudo-circuit built into the main circuit. If we were to talk about circuits by function then we could call this a third circuit.

[Paul219]

Thanks John, and yes, my ame is Paul. The operators manual says to turn the mixture screw in all the way, then back out 1 3/4 turns, this is approximately the correct setting, then adjust no more than 1/4 turn in either direction to fine tune. In is lean, out is rich. Is this correct in your mind?

[plott hound]

you are correct paul.

[JohnF]

Yes, I had that backwards. (marked up my original post) See the engine service manual here.

The pilot screw screw controls the amount of fuel/air mixture allowed through the pilot outlet,...

Not technically a fuel screw but acts just like one.