Can someone please post the procedure for conducting the 'pinch test' please? I know its been discussed many times, but can't seem to find it.

Is it done with the engine running at full throttle or idle? Is it standard for both 2 and 4 strokes?

thanks,

open the throttle all the way and give the fuel line a quick pinch. If the engine speeds up about 300 RPM then the mixture is perfect. If it slows down, open the needle a bit. Also remember to make the pinch short, or the engine will die completely. And as far as I know, it is standard for 2 and 4 strokes.

it's a 2stroke thing. it is done at wide open throttle.

Once the engine warms up, pinch the fuel line going to the carb. Listen to the engine. If it revs up more (higher pitch) you are rich. Adjust the needle. Give it time to settle on the new position, and repeat the proceedure.
Once you get to where the engine does not rev up more, you are at peak rpm. DO NOT FLY at this setting. Richen it up a few clicks You have to account for the uloading in the air. If you fly with this setting you will lean out in the air and possible over heat.
I alway leave it plenty rich (3-5 clicks) just to make sure it gets the oil and stay cool.

Be carefull on doing this test on 4 strokes. You can have the engine backfire adn damage your spinner/propeller or engine if you do it. It can be done, but be carefull.

Ditto on what Giestware posted. 4 strokes will kick off props if too lean. I've seen some ejected props travel well over 100 feet when kicked off of even smaller 4S. I don't suggest that a beginner use the pinch test on a 4S engine. Instead, use a tach to find maximum RPM and richen needle so you are a few hundred below maximum RPM.

Pinch test is safe for a 2S engine.

Well this is where I'd have to disagree a little. The pinch test could certainly be done at full throttle and is a way of letting someone who's unfamiliar with tuning sort of sneak up on the correct full throttle tune. But it's far more useful for setting the idle mix because you have no way of knowing just from revs if it's set correctly or not.

At idle, just pinch the line and listen to what happens. If the engine loses revs immediately then it's too lean. If the revs stay constant for a few seconds and then gradually rise a bit before starting to drop off then you're too rich. What you want is a small rise for just a moment when you pinch the line. This is particularly critical with an inverted engine.

The reason all this happens at idle is if the mixture is too rich then excess fuel collects inside the crankcase. When you pinch the line the engine starts using up all this collected fuel until there's just enough left to make a perfect mixture (the point of max idle revs) then it begins to run out of fuel and the revs die off.

If you're running too rich and you open the throttle, the added airflow through the crankcase picks up all the excess fuel and throws it into the cylinder. Then it's either too rich to run or may cool the plug below the temp it can fire the mixture. Either way, the engine stops

The pinch test (as far as I've seen) is done more to set high speed needle.

I'm not saying it won't work for the low end, apparently it does. I've just never come across anyone doing the low end in this manor. ( most of my flying has been done with the same old farts for 18 years or so. Perhaps my exposure to other methods has been limited.

As far as setting the low end, I have no problem setting it by the way it takes a rapid throttle advance. and the way it idles back down.

1,001 ways to do everything I guess. Find what works for you and run with it.

Amen Chassan their are lots of ways to check engine performance. I am satisfied if it starts and runs so I can fly and enjoy my plane and life becomes no better for me.

While out jogging one day this young sweet thing came up along side me , I looked over at her & she looked a little lean . I gave her the pinch test & I still have the scars! MAX H.

Sorry I couldn't resist, will try to control myself!

Hey Bigmax don't star to pinching your self ok.