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Fuel line pinch test
Hi all
How exctly does the fuel line pinch test work to see if your glow engine's fuel/air mixture is correct. Can you use that for wide open throttle and idle? What do you need to listen for? |
RE: Fuel line pinch test
The pinch test works for both the HS and LS, but it's more noticable on the HS. After you have the engine adjusted, use the pinch test as a last check to make sure you are not running too lean. You should hear an increase in RPM if the engine is correctly adjusted on the slightly rich side. By pinching the fuel line, you are manually causing a lean engine mixture and thus hearing an increase in RPM's. This is what you want to hear. If the engine immediately dies, richen up the HS, start the engine and try again. Again, you want to hear an increase in RPM's when pinching the fuel line.
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RE: Fuel line pinch test
This is all correct, but I will add that the HS needle pinch test is done at full throttle. The sound note changes slightly as you quickly pinch then release the fuel line leading to the carb. It's a mometary pinch. Have someone show you how to do it so you can recognize the subtle difference in the note of the engine. If done right, the sound difference, although being subtle, results in a slight change in the engine RPM.
You can do it with a tach.. have someone on the opposite side of the plane holding the tach in the proper position, then do the pinch test. This will show you how it'd properly done, but nothing works like a little experience in doing this. I would do it, then my old instructor, who was usually there, came over and re-did it all the while explaining what was going on. CGr. |
RE: Fuel line pinch test
It can be used to make sure the LS needle is close. Once the engine is warmed up and cleared out, at idle you can pinch the fuel line closed and start counting. If it speeds up or quits before you get to 3 the LSN is too lean, if it runs more than 4 or 5 seconds w/o speeding up, then its too rich and will burble and possibly stall when given fast throttle.
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RE: Fuel line pinch test
In addition to what Barracudahockey suggested, I also tell novices to observe the behaviour of the engine ( assuming the right glow plug is installed ), when the ignitor is pulled off.
The pinch test will get you close, however if pulling off the ignitor causes a large drop in RPM's the engine is too rich and likely to stall when taxiing. The combination of the pinch test, ignitor and nose up test, done properly will give you a reliable run if there are no plumbing or fuel quality problems. |
RE: Fuel line pinch test
You can properly tune an engine with out pinching the fuel line.
If you wish to go the route of pinching the line, make sure the pinch doesn't damage the line. I had some major damage to a 1/4 scale Suhkoi because the pinch tool I used cut the fuel line, andI didn't find out untilabut 15 ft elevationon take off. Tubing over needle nose plier tips is a good ideal Tune the engine with a Tach.First thehigh end, then the idle mix.Adjust foran optimum transistion on a hammer downpunch of the throttle. This will give you your best test of fuel mix on both the high speed and idle mix. If these settingsare right, you have a good running engine and pinching lines and nose high full throttletest are not going to improve it, or tel youyou have aproblem. Don |
RE: Fuel line pinch test
ORIGINAL: opjose In addition to what Barracudahockey suggested, I also tell novices to observe the behaviour of the engine ( assuming the right glow plug is installed ), when the ignitor is pulled off. The pinch test will get you close, however if pulling off the ignitor causes a large drop in RPM's the engine is too rich and likely to stall when taxiing. The combination of the pinch test, ignitor and nose up test, done properly will give you a reliable run if there are no plumbing or fuel quality problems. How about an explanation of the "nose up test" I've seen it and had it done on my planes, but nobody ever realy told me what to listen for. |
RE: Fuel line pinch test
One small point I would like to make. I see many beginners run their engines much too lean. Remember the tank will burn down during the flight and the tune will lean slightly as the tank gets lower. Try to tune for the second half of the tank. That means that you generally want to take off just a click or two richer than the optimum tune. As the tank burns down, the tune will come to you.
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RE: Fuel line pinch test
The nose up test is simply checking the mix with the nose pointed upwards. Without pumps the fuel pressure drops a little in that attitude so the engines leans out a little. With the nose up peaked is ok and even preferable, but if you're too lean the engine will die with the nose up.
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RE: Fuel line pinch test
For the newbies, for the most performance enhanced engines like the Jett, the pinch test is not recommended as it could damage the engine if not done correctly. Just a FYI
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RE: Fuel line pinch test
ORIGINAL: vasek For the newbies, for the most performance enhanced engines like the Jett, the pinch test is not recommended as it could damage the engine if not done correctly. Just a FYI |
RE: Fuel line pinch test
ORIGINAL: Campgems If you wish to go the route of pinching the line, make sure the pinch doesn't damage the line. I had some major damage to a 1/4 scale Suhkoi because the pinch tool I used cut the fuel line, and I didn't find out until abut 15 ft elevation on take off. Tubing over needle nose plier tips is a good ideal Yeah clamps, hemostats, etc. can end up breaking or cutting the line... I still use the World Models fueling fitting that allows me to safely pinch test the line in a cowled engine... and that is done only by hand.... http://images.rcuniverse.com/gallery.../lg-131056.jpg |
RE: Fuel line pinch test
ORIGINAL: jester_s1 The nose up test is simply checking the mix with the nose pointed upwards. Without pumps the fuel pressure drops a little in that attitude so the engines leans out a little. With the nose up peaked is ok and even preferable, but if you're too lean the engine will die with the nose up. So, if I point it up and the engine doesn't lose RPM, I'm good to go, Right? |
RE: Fuel line pinch test
Right.
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RE: Fuel line pinch test
Actually the engine should increase in RPM's when you point it up.
You should also perform this test with about half a tank of fuel or less, since the engine leans out as the fuel is used up... If the RPM's stay the same or drop over time when the engine is pointed up, it is too lean ( unless your engine has a pump in it, which typically only a few of the higher end 4 strokers do... ) |
RE: Fuel line pinch test
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Gentlemen
Tunning any conventional glo engine to a high speed pinch is in fact a very old technique that was developed to a high art by fellows in controlline speed as well as high performance free flight. It is now commonly practiced by many high performance users such as the pylon folks and works dare I say, even better than a tack. I highly reccomend anyone to give it a try and especially if you are gifted with a fellow flyer who does practice this technique. Does it require a little intuition and a sharp ear? Yes of course but the process is so easy and requires no tachometer and the fellows will get consistantly better running engines for it. It taken some years but I have almost everyone in this club using the practice and virtually every one is experiancing far less engine trouble and definately better visits to the field. I teach the technique to every student that will listen. Kudos to oppose as he has done an excellent job in this thread explaning the technique. Those who claim the technique is bad or unreliable just because of they insist on pinching with some tool or pliers is absurd. The pinch is always done with fingers, never a tool it won,t work you cannot acheve the right pinch timing with a tool. For those who claim that a cowl prevents the practice, well hogwash. You can use the fancy world fitting that oppose linked or like I do just a couple of holes like I do. As in the pictures of my Fokker D8 which I just maidened a couple days ago, providing a pinch line is a no brainer and all my two stroke cowled engines have a similar pinch line. This also provides an increditably easy place to fuel from. John Here is my war weary Hellcat (in this case its an arf from CMP) that is also a newer addition with a big glow engine and you can see the pinch/filler line in the same relative relationship: |
RE: Fuel line pinch test
Yes, what John said. In pylon racing, that momentary pinch will bump the engine up onto the pipe, approximating where the rpms will be in the air. Most don't bother with a tach once they are familiar with their engine. This little pinch just prior to launch is better than needling the engine to peak rpm and then backing it off a few hundred rpms - but it does take some time to get the hang of.
Kurt |
RE: Fuel line pinch test
Hi!
I have been active in pylon racing for more than 30 years and never seen or heard of any anyone using the pinch test! Racers set their engines by slowly needle their engines and listen to how the engine react! The pinch test is just to crude and inexact! |
RE: Fuel line pinch test
ORIGINAL: jaka and listen to how the engine react! Duh! Jan and that is exactly how its done only diff you are Pinching, Listening and 'then' Twisting! Jan I will not get into a pointless and silly battle over years beyond just the fact I have been twisting needles both high and low performance without ever really stopping for something over fifty years! Now simply because you have never seen it done just means perhaps the folks around you are just ignorant of the procedure That does not make the procedure Crude and Inexact. It is a very effective method and you are simply wrong Sir! But hey now since this is after all the beginners forum, it has proven to be a far better method than what is commonly taught such as just twisting to the limit and backing off some arbitrary amount with or without a tach. I would urge any new flyers to make the effort to learn the technique. Your engine operations with most any two stroke glow engine will be far more enjoyable. John |
RE: Fuel line pinch test
I was constantly having problems with my low speed settings, they were always to rich and caused many dead engines on landing when I dropped the throttle. One of our members showed me the pinch test and I have not had a problem since, at least not caused by carb adj.
Gary |
RE: Fuel line pinch test
ORIGINAL: jaka Hi! I have been active in pylon racing for more than 30 years and never seen or heard of any anyone using the pinch test! Racers set their engines by slowly needle their engines and listen to how the engine react! The pinch test is just to crude and inexact! You need to attend a few of our races in the states! |
RE: Fuel line pinch test
Hi John,
Have you posted directions on the pinch test? If so, could you please send me the link? If not, would you mind posting your directions on this? I would appreciate it. jc |
RE: Fuel line pinch test
ORIGINAL: BarracudaHockey It can be used to make sure the LS needle is close. Once the engine is warmed up and cleared out, at idle you can pinch the fuel line closed and start counting. If it speeds up or quits before you get to 3 the LSN is too lean, if it runs more than 4 or 5 seconds w/o speeding up, then its too rich and will burble and possibly stall when given fast throttle. |
RE: Fuel line pinch test
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Prelude what I teach all my students is the following: for the first start of a new engine or a used engine with an unknown history is to assure that the main needle is in a very rich position (this generally is 2 turns and up, the larger the engine the more turns open).
As the engine fires it will be stumbling and reluctant to run and opening the throttle will cause it to die. as it stumbles the main needle is screwed in untill it will go to full throttle. Now with it running at WOT simply pinch the carb fuel line with a very rapid pinch (completely closing the line). This is rapid in fact its done as fast a you can. The engine will either stumble a bit and then increase RPM (Rich) or it will instantly try to die cleanly (lean). remember the pinch will need to be as fast as you possibly do it. In the first case it is still rich and you screw the main needle in slightly then pinch agine and keep doing this untill there is just barely a perceptable increase in RPM before it wants to die. This will be very close to a ideal main needle setting. Now we go to the the mid range and for this I simply use the idle up from low idle and lean if it stumbles or enrichen if it dies quickly from opening the throttle. After doing this go back and pinch the main needle agine at high throttle as the two are interrelated. Thats as simply put as I can do sorry if that seems too intuitive but really if you think about it is less intuitive then using a tach, which requires one to lean at high speed to peak and then back off an arbitrary amount and thats where the guess work comes in and not every engine is going to require anywhere near the same degree of backing off thats a fact. I normally unless dealing with a recalcitrant engine do not normally find it neccessary to do a nose up wot test. The nose up test can serve a purpose but it so often is so poorly done that its meaningless, I see this a lot. Folks will go to wot and stick the nose up wildly move the throttle up and down. This does nothing if you do a nose up then its neccessary to hole the nose up for at least thirty second without moving the throttle. Otherwise you will never hear the sag if it too lean. IF doing a nose up you have waited long enough to confirm there is no sag then a slow reduction to full idle and waiting for another 30 seconds then back to wot. That is a litmus test. The vast majority of time only a few quick pinchs is all that needed and you are very close to ideal. I have done a fair amount of multi engine training and I teach only the pinch, no tacks and it has served very well. Here is one of my students with his copy of my Quad Kaydet That he is totally in love with and having much success. John |
RE: Fuel line pinch test
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Here is my latest, a Corsair Arf that I am converting for arrested carrier landings. Its a two stroke OS 1.08 and for this using the accessable pinch line is imperitive. Because of the constant low speed go arounds. The photos also show how simple the fueling is and how simple the pinch is.
I will add a link also showing what I am talkin about with the arrested landings, this with my war weary Hellcat. John http://www.youtube.com/watch?v=M2WIVHZaSHA&lr=1 |
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