hi
i seen guys pinch the fuel line to tune there engines what are they looking for? how do i know if i need to lean or rich the low end ,thanks

If the engine speeds up instantly when the line is pinched it's rich.

If the engine drops off instantly it's lean.

What you want is a slight RPM increase when the line is pinched.

The pinch on the line is very quick.

Tim

You only touch the low end if you have throttle transition issues. That is from idle to full power if there is a slow transition you lean the low end, if it quits you have to open it up. You are looking for smooth crisp transition from idle to full power. The pinching is to see if the engine is lean at full power. This is usually done with the engine running at full power. I just open the hi speed needle until the engine start to slow down then turn in about 3 clicks, due to my engine is cowl in. You also look for a smoke trail when you fly, it keep a healthy happy engine. No smoke means you are running lean. Now it depends on which engine you have that determined if turning in is lean or rich for the low end. Some turning in is rich and out is lean. Look in the carb opening if you see a bar going across the opening that`s the one that is opposite.

By pinching the line momentarily, they are cutting the flow of fuel to the carburetor.

As every engine should be run on the reach side at full rpm's, when they pinch the line, the mix tends to go to lean.
Leaner mix induces an increment in rpm's.
Too lean mix induces the engine to stop.

They try to hear a little increment in rpm's just before the engine starts dying.
That indicates that the mix is a little rich at full rpm's, as it should be to protect the lubrication of the engine.

Some good reading here:

http://www.hooked-on-rc-airplanes.co...ning-tips.html

http://www.*********.org/cars_eng-tuning.htm

http://www.rcuniverse.com/forum/m_88...tm.htm#8841395

Another test used to determine if the HS needle is a bit rich is to hold the plane vertical with the throttle wide open - a bit less fuel flows up hill. If the rpm increases then the engine is a bit rich - a good thing. If not, add 1/8 turn to richen the HS needle. Yea, yea - some engines pressurize the fuel tank and this isn't a viable test but if you were using one of these you wouldn't have posted the question.

Once the HS needle is set work on the low speed (LS) needle to obtain a nice smooth idle without any glow plug ignition attached (20 - 30 seconds). After messin' with the LS needle go back and ensure that the HS needle is on.

Once you get it right - QUIT MESSIN' WITH IT.

BTW - lnewqban is a superb source for just a whole lot of stuff - big thumbs up!!!

thanks guys for all the information, lots to read,rye

I've been blasted for this on RCU before but it still works for me. For the low end anyway. I let it idle for 10-15 seconds and then attach the hot shot. If it picks up RPM it is too rich. This won't help you if you are too lean. But I always start with the needle on the rich side.

David

Thanks SeamusG, kind and undeserved words.

One more thing: stop the engine each time you adjust the low end needle or air bleed.
This is specially dangerous because it is normally done with a small screwdriver.
The engine vibrations may make it slip into the prop, and into your body from there.[]

Increments or reductions to that low end should be done in very small increments (no more than 1/8 turn); otherwise you will loose perspective of the changes you are producing.

When tuning car engines, the pinch test is the most common way to test the low-end. Let it idle for ten seconds, pinch it halfway between carb and tank, count to three. If it starts to rev between three and five seconds, the low-end is good. If it revs instantly or shuts off entirely, it's way too lean, and if it doesn't react at all until five seconds have gone by, it's too fat. One generally releases the line when it reacts so as to keep it running, as it's slightly harder to restart car engines.

Another common way for us to tell is to let it idle for thirty seconds, then nail it. Smoke, sound and acceleration tell us what's going on with the needle, if the low end is good you'll get a fair bit of smoke, it will accelerate smoothly, and it would sound like it's running properly. If it bogs, it's too fat, and if it zings up to redline like it's been prodded with a branding iron it's too lean.


As for aircraft engines, I imagine they're similar, but I haven't had an aircraft engine long enough to determine if any different procedures are required to tune them. I do have to reset my carb and retune it though, so I'll know more in a few days.

On a plane the engine is dead in 5 seconds of pinch. As opposed to a car the plane engine is spinning a prop and that places a load on the engine requiring more throttle opening to maintain idle speed. Also, each engine is different when it comes to low speed setting. If you set my SuperTigre engines for any rpm rise at all doing the pinch test at idle, the mid range will be so rich it will most likely die after a prolonged idle when advancing the throttle quickly. Turning the spray bar will not be enough to lean the mid range either. My OS engines and most others will tolerate a slightly richer idle than the tigers. Here in the Houston, TX area, I like to get them lean enough so there is no rise in rpm from a pinch at idle, and if after tuning the high speed needle, there is any bog when opening the throttle from idle, I open the low needle one sixteenth at a time until the bog goes away and that's the spot where it stays. On the high end, I tune for a 300 to 500 rpm drop on pinch and if raising the nose 90 deg vertical causes any slowdown in rpm then I richen until there is no drop when doing so. This has worked for me for 40 years now.

Car engines don't tend to be that picky across brands. I've never heard of guys with Traxxas engines having to tune them differently from how one would tune a Picco, again differently from a CEN, etc etc. They're all tuned the same. Maybe it's because the carbs in use on the surface are so similar? There's two, slide and rotary, and they both have the same needles in the same location doing the same task. Conversely, there's a huge variance in the types of carbs used in aircraft compared to the types used on the ground, that may account for certain brands preferring certain methods.

Dave,

In theory the hot shot trick works for a mixture test but it runs into a problem. If a glow plug is in real good shape an engine will maintain the same RPM it had when a hot shot was in place on the glow plug when used to start the engine. Unfortunately a weak glow plug will always cause a drop in RPM when the hot shot is removed from contact. So the hot shot test may just be illustrating the condition of the glow plug when it is reattached.

Just something to consider.

I totally agree with you. It's not a perfect test by any means. I like it because most of my planes have no way to pinch the line. I also think many guys have no idea how long to pinch the line. I go months at a time without a dead stick and see guys that have flown for 20 plus years have several a day. So if you find something that works for you, stick with it.

David