N8087A Finnaly home at Provincetown

johneeb · Post by **johneeb** » Thu Nov 03, 2005 2:21 am

gahorn wrote:Leaning on the ground has no effect in our carbureted engines, actually. At least not at typical operating rpms. The idle jet is not affected by the mixture control at all, and the engine operates completely on the idle jet below 1000 rpms.

George, back when I had a carburator, every time I shut down the engine by pulling out the mixture controll to full lean I got a 50 RPM rise. I therefore regularly pulled the mixture controll (while on the ground) to the sweet spot where the engine idle speed rose only 49.75 RPM and kept running, I thought that it must be leaner. What do you think

JTS · Post by **JTS** » Thu Nov 03, 2005 2:23 am

I'm with Miles in respect to ground leaning. I believe that you can effectively lean with positive results while around idle rpm's by simply pulling out the mixture knob smoothly until you get that slight rise in rpm.
I think it's been mentioned here before.

Jody

JTS · Post by **JTS** » Thu Nov 03, 2005 2:27 am

lol........That's just too funny John.

Jody

cessna170bdriver · Post by **cessna170bdriver** » Thu Nov 03, 2005 2:51 am

gahorn wrote:Leaning on the ground has no effect in our carbureted engines, actually. At least not at typical operating rpms. The idle jet is not affected by the mixture control at all, and the engine operates completely on the idle jet below 1000 rpms.

George,

It's been a very long time since I've looked at the Marvel Schebler cutaway drawing, but if the idle jet isn't affect by the mixture control, how does idle cutoff work?

Even at idle, if you pull the mixture slowly, there is a point where the engine begins to roughen, then you can enrichen slightly for smooth running, just like in cruise. However, if the throttle is advanced much at all, the engine will die. My theory has alway been that opening the throttle introduces enough air that it leans the mixture past what the engine can run on. Maybe opening the throttle is really just cutting off the idle jet???

Educate me, George

Miles

Post by **GAHorn** » Thu Nov 03, 2005 10:41 pm

The operation of the carburetor is described in the carb ovhl manual, and also in the C145/O300 ovhl manual. There it states the idle jet is not affected by leaning and all operations at/below approx. 1000 rpm are on the idle jet. I figure they'd know.
The idle cutoff completely stops the fuel flow. As the shutoff operates there is a very narrow band in which the fuel is being reduced to "off". That is what is experienced during shutdown (and is a method of determining that the idle mixture (the knurled knob on the carb body) is properly set to the slightly rich position required for smooth idling at low density altitudes (and low temps.) It is not intended as a method of leaning the idle circuit. The idle circuit mixture is controlled by the idle mixture knob on the carb body.

cessna170bdriver · Post by **cessna170bdriver** » Fri Nov 04, 2005 12:01 am

gahorn wrote:The idle cutoff completely stops the fuel flow. As the shutoff operates there is a very narrow band in which the fuel is being reduced to "off". That is what is experienced during shutdown (and is a method of determining that the idle mixture (the knurled knob on the carb body) is properly set to the slightly rich position required for smooth idling at low density altitudes (and low temps.) It is not intended as a method of leaning the idle circuit. The idle circuit mixture is controlled by the idle mixture knob on the carb body.

That's pretty much how I thought it worked. Looks to me like "intended" is the operative word. They may not have "intended" the mixture control to affect the idle mixture, but in "reality" the idle cutoff is not an instantaneous phenomenon (it comes into effect over a range of travel, however short ), so it can, regardless of "intention", be used to adjust fuel/air ratio below 1000 rpm. This is handy for me in that I can have the idle mixture screw set so it's not overly lean at Bakersfield (near sea level), and also have a way of obtaining a sufficiently lean idle mixture to prevent plug fouling 20 minutes away at 4000 feet on the ground in Tehachapi.

For any throttle setting from idle to just short of WOT, I can adjust the mixture precisely enough such that any further opening of the throttle will cause a LOSS of RPM. That's where I like to set the mixture for taxi, regardless of altitude. I may be all wet (no pun intended), but I'm convinced that that is a major reason I run 100LL with no TCP, MMO, or any other alphabet soup and still don't have to clean plugs between annuals, or ream valve guides at all.

Miles

Miles