RCU Forums - Pinch test

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Can someone please post the "how to" on the pinch test. Can't remember it exactly. Was tinkering with the low speed needle on my Mag .91RFS today.

thanks,,,,

:D

as the fuel runs in it will "spread out" and if speed picks up its rich, because there is less fuel entering the engine. if it slows down or quits it's lean. Low end controls transition from low to high rpm as well as idle. I don't want to give out any wrong info for low speed needle settings.

D, Trimmer

Pinch test does nothing for the low end.

Easy test on the low end.

Run up the motor, get the high right, first. Use the pinch test for the high.

Idle the motor down, let it idle for 10 seconds, or so, then go to WOT. What does it do????? Burble, cough and choke then slowley come up to max RPM? If so, the bottom is too rich. Only ajust the low in tiny amounts, 1/8 turn at a time or so. If the motor sounds like it is starving for fuel or dies, the bottom is too lean.

Sounds good guys. I ended up leaning it about 1/8-1/4 and it seems to have done the trick. What it was doing was loading up at idle and I'd have to clear it (WOT) after about 8-10 seconds of idle. The transition was nice, quick and responsive, but it just wouldn't hold the idle without loading up. I also have to remember the engine is still very new as it only has about 5 tanks through it. Thanks for the help.

It takes a good ear to really get the pinch test right.

You probably already know this, but let me explain again just in case.

After starting and allowing the engine to come up to temp, (keep the glow driver on while you do this). Add a bit of throttle and remove the glow driver. Slowly go to half throttle, then to full throttle, let it run there for a few seconds, then back to idle. It should run fine.

Go to full throttle, the quickly pinch and release the fuel line at the carb or just behind it. The engine should increase RPM a slight bit then return to normal right away. This will sound like a slight change in the note of the engine. If it does not do this, or if the RPM go up quite a bit then it will need some adjustment. Let the engine go back to idle and try it again. Once the engine has reached full throttle RPM, add about a 1/8 turn more rich and pinch off the fuel line. If it sounds good, return to idle. You don't want it to sputter but to run cleanly at full RPM. If it is to lean, it will immediately cut out so you add more fuel. Once you get the RPM to increase, make slight adjustments to peak out the engine RPM by leaning it out a bit while doing the pinch test.

Once you are done with this, the high end is reliable.

The low end, well, that's a bit different. A good idea of how it is performing is to quickly go to full throttle from idle and see if it hesitates in the transition. Keep in mind that there may be some excessive fuel in there, so go to full throttle to clean things out, then back to idle, then add throttle quickly to see if it hesitates. You may have to change that low end setting (My OS engines have a little adjustment screw located in the spray bar of the carb on the opposite side of the throttle arm. It takes a very small flat blade screwdriver and SHOULD BE DONE WITH THE ENGINE SHUT DOWN!!!). Only make very small adjustments here, then restart and see if you improved that transition from idle to full. If it improves, then you may be set. If you need more, then do it over again.

Good luck.

DS.

I use the pinch test for low end only... and only as a last resort

Start engine, go to full throttle and adjust the high end by sound.

Then go to full idle. Let idle for a few seconds. If the idle slowly decreases, it's usually a sign that the low end is rich and therefore loading up as it sits there idling. If you go to WOT and it hesitates, again, this is a sign of a rich low end. If it quits when brought to WOT, it's probably lean on the low end.

When all else fails, I do the pinch test.

Bring it to idle and let it idle for a few seconds (Providing it will) then pinch the fuel line.

There should be a slight increase in RPM (Which you can hear) then the engine should quit.

If it quits right away without the RPM increase, it's lean. If the RPM slowly increase and and maintain for a few seconds before the engine dies, it's rich on the low end.

The pinch test works just fine on the low end. At idle or just above, pinch and hold.If it runs more than 4or5 seconds it is rich. Less than that ,it is lean.

Thanks guys.

Hey Minn, the engine I was working on is sitting in the skybolt. Flew it again this past Monday, three flights,,,,man I like that plane. ;)

What a surprise... (NOT) :D

cruzomatic
Once I have an engine set, the last check I make before flying, on every flight, is to hold the model nose up at about 60 degrees. Open the throttle fully and wait for 10-15 seconds. The engine should continue to run unchanged. This test ensures that your mixture is not going to go lean on the climbout, worse time for an engine failure.
This test is not required on engines fitted with a pump.

Please excuse my ignorance - what is "WOT"

Wide Open Throttle :):D

What is WAT... sounds like an Abbott and Costello skit.

Some of you youngsters have absolutely no idea what I am talking about. I'm sure the older crowd knows, though. ;)

"What time tomorrow are you gonna tell me the pitcher's name???" :D

Quote:

ORIGINAL: MinnFlyer

"What time tomorrow are you gonna tell me the pitcher's name???" :D

I don't know!

He's on third base!!!!!! [X(] :D:D:D

Ken

We interrupt this program to go back to the original topic for a moment...

Another useful trick for getting the low speed needle close to correct can be done with the glow driver. Start the engine, leave the glow driver connected and run the engine up to full speed for 10-15 seconds. Then idle back. After about 15 seconds, remove the glow driver. If the engine slows a small amount, the low speed needle is about right. If the engine slows considerably, the low speed needle is set too rich.

WOT's on first!!! :D

Sorry, Cruz and Pipe.. the devil made me do it.

Who's on first.

WOT's on second.