Pinch Test
 

OK I know this has probably covered a gazillion times but please.............once more. When you have the engine wide open and you pinch the fuel line and the engine picks up speed, it is too rich? And if it doesn't it's right? We tested an engine at the field and pinched the fuel line and not change occurred. I flew the plane and it started flying slower and slower so I brought it in and it was way hot. Someone commented that you want for the engine to pick up speed so it is running slightly rich.............which is correct?

Thanks guys..............Andy

RE: Pinch Test
 

Hi Andy,

I don't have the time to find it for you (gotta get the kid to scouts), but I know there are several write-ups on the pinch test.  The interpretation of the results can be subtle. 

I'll see if I can find a link later tonight if someone does not clarify for you before then.

RE: Pinch Test
 

I set my needle with the tank half full, starting rich and pinch the fuel line and lean the needle until I get no rise in rpm. This way when the tank is full, the engine will run a little rich and as the tank runs down it will lean out. If you pinch and the engine cuts out its too lean, if it rises it's rich. No change is peak or very close to it.

RE: Pinch Test
 

This is correct.

RE: Pinch Test
 

Running boats, you can see the fuel tank and can readily fill it half full to set up as you indicated. And then you get to fill it some more. The pinch test is helpfull in setting up a plane engine where the tank may not be visible. The pinch test is a very very simple and effective way to check that the engine is not set too lean.

RE: Pinch Test
 

Regardless of what an engine is mounted to, the operator should know how much fuel is in the tank, especially with an airplane. If you set up an engine and fly a plane without knowing how much fuel is in the tank, well...[:@]

RE: Pinch Test
 

I set up to get just a tiny rise in RPM when I pinch. You have to make sure you do a quick pinch and release and don't pinch and hold waiting to hear a rise....because if you keep pinched it will riseor flameout sooner or later. When you fly a plane and make a hard turn you should hear a ever so slight raise in rpm when pulling a lot of "g"s. Or at least that's how I feel. The point as i see it is to have just enough richness to overcome a high-g caused lean condition. Otherwise if you tune to no increase at all on the pinch one has to assume it goes lean when pulling a lot of g~force?

RE: Pinch Test
 

You proposed setting the needle with the tank half full. You run boats. I fly airplanes, I don't set the needle with the tank half full. I fill the tank full.

Do whatever "floats your boat". I do.

RE: Pinch Test
 

I guess I do what I feel is in the best interest of the engine. I've found "300 rpm off peak" and " a click or two rich" to be not near rich enough at a full tank. Many of my engines would be too lean by half tank requiring an adjustment, and my engines arent unloading even.. I guess I prefer to set the carb once and run the day out with maybe a click or two adjustment from morning to afternoon.

I do agree one should see decent rise of rpm with a quick pinch at WOT with a full tank is "about" where you want it. "nose up" of course.

RE: Pinch Test
 

Tank placement and unloading due to airspeed are of coarse important. I always try the nose up position, I has to be able to run straight up or you'll be in trouble (on an airplane at least). When the engine can run fine with the nose straight up it is usually rich enough for flight, and there is no need for any pinch test at all.