I have been in the hobby for a short while. I am 66 and a retired aerospace engineer. I have put stuff on the moon, but I can't get my engines to transition from idle, w/o shutting off! I am starting at an idle bar setting with the largest internal cylinder even with the venturi. Moving the screw in 1/4 turns, and still can't find a sweet spot. I have several books that don't address the low-speed adjustment. My OS 46 and 61 instructions don't either. Anybody got an on line reference or Clarence Lee's number. What's the fuel line pinch test? I'm on a test bench right now.
Any suggestions would be helpful..I have just ordered a doz of "Hots" plugs to try.

Pinch test: pinch off the fuel. See how long it takes until the idle speeds up then dies. About four seconds until it speeds up is a good starting point. This is only meant to get you in the ball park.

Does it die when you increase the throttle slowly?
Or only when you do it quickly.

If it happens when you do it slowly, your main needle could be too lean, and your idle setting is compensating for this. If the main is set according to the book, then maybe there is a blockage somewhere.

The usual procedure is to get the main needle setting right at high speed, then adjust the idle. The pinch test is a good place to start, then adjust for good transition without being so lean it pops and crackles when you run through the lower speed range. At any speed, pinching the fuel line should result in a slight speed up before it dies.

the engine(OS61 ABC) seems to idle well, rpm increases when I remove the GPstarter. But when incrcg quickly it falters, and will die if the throttle stays open, if shut back, the engine will continue to idle. I can't seem to get it to top speed. I'm using a 5% fuel. I have been adjusting the low idle-it's now 7 3/4 turns out. I had been running it with the fuel tank 2in below the carb inlet!! I have rebuilt my test stand to where the tank outlet is about a1/4in below. Am going to try that.

If it won't keep running at full throttle, then you are way too lean. Having the tank so low is consistent with that.

The carbs are calibrated to run with the fuel tank level and with muffler pressure.

thx, that was probably my problem all along. I will go try the new set up, shortly..

Double check all your fuel lines for pinholes(even the line inside the tank). To get your needles back to a starting position, Turn the high end out( not necessary if it's a remote needle). Turn the idle needle all the way in, then turn it back out 2 turns. Now turn the high needle all the way in, then back it out 3 turns. After getting the engine running, adjust the high end needle to a slightly rich mixture at full throttle, then bring it down to an idle and keep it there for thirty seconds, then quickly back to full throttle. If it sputters or sags before coming to full throttle(or dies), adjust the low needle by 1/8 turn, leaner if it sputters, richer if it sags. Then repeat the above until it immediately picks up when throttled. Always run full throttle for 10-15 seconds to burn excess fuel off, then let it idle for 15 seconds or longer before throttling up. You will have to readjust the high end needle after getting the low end right. After getting it adjusted right, you should never have to adjust the needle more than a click or two for the varying conditions in your area. I've had good luck with OS and Fox Miracle plugs in my OS .46's, the centerline of the fuel tank should be even with the centerline of the carb. Hope this helps.

One time I had an ST behaving like that. Finally, took the tank and lines out, submerged them in water and blew though the lines, presurizing the tank. Guess what, the tank stopper leaked around the neck. Now I always remember to check the tightness of the stopper screw before final installation. I'm always afraid I'll tighten them to much and split the tank.

Thanks Dave, for the procedure. I'll try it after I get my stand fixed . I had raised the tank, strted the engine and the engine mount broke(fixture). So I,ve got a new one to build tomorrow-be an engine setup tester, again. I'll also check for pin hole in the tank and lines.

I might be able to provide you with the factory setting info for your low end mixture if I knew precisely which model OS 46 and 61
you are having trouble with. Are they FSR's, SF's or FX's?

BTW - Backing out any of OS's current model
low end needles is way too much as most are set 1-1 1/4 turns out at the factory. The high speed needle need only be open 2 full turns at the start as well, not 3 turns open. Also changes to the low end should only be made in tiny 1/8th turn increments not 1/4 turn increments.

So which model engines do you have?

I have : OSMax 46 SF, OSMax 40SR and OSMax 60SR.
thx Shemp.

Shemp, I thought you were going to provide the factory settings for these engines. I have been successful in finding the settings on the two SR's. I did not have any leakage probs. but did raise the tank on my test setup to level behingd the engine. Also. I'm not sure about the "pinch test". I pinch for the count of two, with the engine at ~8Krpm, it speeds up a little then coughs and catches up. soes that mean it's too rich? or too lean.

The pinch test is to get an indication of whether the idle mix is rich or lean so ~8K is far too many revs, you should be down around 2.5 or 3K.
Adjust the idle speed down to where it seems close to stopping then pinch the fuel line. If it stops almost immediately then the idle mixture is too lean. If it begins to increase revs noticeably after a short time (maybe a few seconds) then it's too rich. The reason is that excess fuel tends to collect inside the crankcase at idle so this has to be used up before the mixture begins to lean out. What you're looking for with the pinch test is a slight increase in revs before the engine starves for fuel...in other words, just a little on the rich side.

Nitehawk,

I just recently had an engine that did the same thing. It wasn't an O.S. but a GMS. I did everything. I adjusted things slowly etc. I spent many tanks trying to get it to work. I finally gave up and sent the carb into GMS. Apparently they had put in the wrong idle needle. After they returned it, the engine ran great. It took me about 2 minutes to tune and I was flying. I was just relieved to know that I wasn't as big of an idiot as I thought. You may want to look into sending the carb in or trying another one on the engine.

Good luck!

Thx down under--dumb-I shouda known it should have been at least near IDLE thx. I'll get more familiar with it tomorrow!

Nitehawk: Sorry I could not reply immediately
as a few things came up. I'm glad to hear you have sorted out the problems with your FSR's. The factory setting on the low end of the 46SF is one full turn out from fully closed while the carb barrel is also fully closed. Keep in mind though that this setting might need some leaning out as OS often sets their engines on the rich side from the factory.

thx, I think I'll have to spend more time on engines.

this is an old way of doing it(idle mixture)crack the barrel open the size of a pin thn blow thrue a the fuel line to the carb and start opening the idle needle from the fully closed position and when the air starts coming thrue keep going 1/8 turn.hi speed should be out at least 3 turns when doing this to make it easier to blw air into the idle bar.It is a good starting point for idle.OS makes a pretty good carb so if you can,t get it running theres probably a defect.good luck

You're showing your age That's my favorite starting point too_Bob

THat sounds like a GOOD place to begin. At one time my OS 40SF was running "8" turns out. slobbering maybe? But it would transition.

thanks

hey bob i,m only 39 it was a trick a super tiger dealer showed me because i like working on old engines i have no specs fore.

Thanks for the help here, I now feel comfortable with my engines-the OS's are screaming and the transition is good. So good that three days ago I lost it in the sun and had to climb a cedar tree to retrieve it.

Hi guys, maybe someone has an answer, i have 3 vintage 1973 OS40sr engines with the rear rotary valve and the rear carb, i am getting back into the sport some 32 years later.
I have replaced the bearings and cleaned up the engines, but they all have the same problem they had 30 years ago.
If you peak out the engine, and even go 1 -2 clicks rich they will run through a tank of gas on the ground, even if held up verticle, but if you take off, just as you climb out about 75-100 feet they will intermittanlty suddenly stop.

I am running an A3 OS plug with 5% nitro, i nearly totalled a new model after a dead stick argument with a fence, the fence one, so some repairs needed.
The engines will not transition well and almost refuse to increase speed from idle, once you get past 1/2 throttle transistion seems ok.
They do not have any idle mixture adjustment just one needle for all the range, and they do not idle well at all.
I seem to remember running a 9x 6 prop back then, as i still have a heap of them, i fitted a 10x 6 and the slow idle improved hugely but the engine does not pull as well with the bigger prop, any thoughts, its as if the motor leans out with flight speed, i suspected the carb was not getting enough air at the back so i enlarged the hole on the firewall, but it made no difference the rear of the carb is about 5mm from the firewall.
I have fitted a OS46ax rear half of the muffler to the engines to reduce the noise, i would not be able to fly the engine as it was built 30 years ago, but even with the original muffler it still cuts out at about 75-100ft.

I am running pressure from the exhaust, but the engine did also have a crank case nipple on one of the carb attachment holes, would crankcase pressure be a better way to run.
Does anyone have a factory spec sheet for this motor ie prop size etc. Sorry about the long post but i really am running out of ideas and i like the performance these old engines give when they run right.

Best Regards
Lambo1