This site maintained by: Curtis L. Olson
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Engine Tuning Tips - Ted Sander Taken from the SPMRC message forum. |
2-stroke tuning Author: Jon Bidinger Date: 06-19-00 10:15 I'm having some trouble with my OS .46FX. It idles just fine, but when I move to full throttle it sometimes chokes for a bit at about 1/2 to 3/4 of full RPM. I've even had stalls. When I test it on the ground by going to full throttle and holding the plane verticaly, I don't hear any change in engine pitch. Could this be an idle mixture problem instead of a needle valve adjustment? |
RE: 2-stroke tuning Author: Ted Sander Date: 06-25-00 05:50 You've got the answer, Jon! On a normal 2 or 4 cycle, coughing when you go to full throttle fast, after a prolonged idle, means your idle mixture is too rich. Fuel builds up, and the engine is trying to clear it out. Dieing clean, with no hesitation is a sign of being too lean. If you are way too rich, it can act like it's too lean, so the problem gets harder to diagnose. Look for puffs of smoke, raw fuel out of the muffler as other idnicators of being too rich at idle. Here's the pro way to set your engine: At idle, use your fingers, a forceps, a needle nose or whatever is safest to pinch the fuel line to the carb (the line going to the needle valve, in cases where the NV is remote from the carb) for 2-3 seconds. If it dies really fast, you're too lean. If it increases RPM, you're too rich. The goal - do the pinch, and get little or no change in RPM, yet the engine stays running for a few seconds. If you are rich, and fix it, you may find that you have to back off on trim to reduce the idle speed. Redo the pinch test, again. The goal - lowest possible idle speed, with the perfect mixture. Do the fast open from idle test as a final check. Response should be immediate. The "point it up test" is only good for checking the high speed needle valve setting. Nose up, it should slightly pick up in RPM, nose down, it should loose a little as it richens up. If in doubt, ALWAYS err on the side of too rich on the high end. Your FX will be perfect after the above. Some other engines (cheaper) have poorly built carbs with excessive amounts of interplay in the mid-range (where the high speed needle takes over from the low speed needle or air bleed setting). These can be major pains to get the right setting. There usually has to be some kind of compromise, which hurts the low or high end setting. On the low side, it may idle poorly, on the high side, it may be too lean and ruin the engine. Like I said, with an OS, there will be no problem like this. A final word - I don't know about the larger FX's, but the .25's for racing DEMAND to be on the rich side on the high end. Set it to where it just barely begins to "2-cycle" as you screw in the needle. Even if you have a tach, and can see that going in further gives you lots more RPM, fight the urge. In the air, they "unload" and what seems to be perfect on the ground becomes too lean in the air. I have a one where the cylinder liner changed from shiny silver to blue, because the lean run in the air "cooked" it. Used to turn 16,300 RPM, now it will only do 14,000! No racing for that engine! |
4 Stroke engine break-in Author: William Randall Date: 07-09-00 23:14 I've just completed building a Piper J-3 Cub with a OS 70 Surpass engine in it. Unfortunately, I've lost the manual for the engine somewhere in the process of building the plane. Can anyone tell me how to break-in the engine? Also, I started the engine (left it in it's factory settings) and had problems getting it to rev up high. As soon as I give it some throttle, it dies. Any suggestions? |
RE: 4 Stroke engine break-in Author: William Randall Date: 07-13-00 00:08 I've now located a copy of the manual for the engine, however I think I have bigger problems. On the advice of the kit, I mounted the engine inverted. Seemed like a good idea at the time, but it quits whenever I try to increase the RPMs of the engine. One of the people at Hub Hobby said that I might need to get an on-board glow ingition system. Is this necessary? Should I bite the bullet and re-mount the engine sideways? |
RE: 4 Stroke engine break-in Author: Ted Sander Date: 07-25-00 23:55 Hmmm - could be lots of things, so start out by simplifying. First, make sure you are using an OS "F" glow plug. Expensive, but nothing beats it for performance. Save the experimenting with other brands until the engine is working and broken in. Second, mount it upright for initial testing and running. This can be done out of the plane - in another plane, or on a test stand, or fabricate your own stand using an engine mount and whatever scrap lumber you have. Just make sure it's strong, and it can be clamped, screwed, or otherwise firmly attached to a strong base. Rig up a way you can operate your throttle. If nothing else, figure out how to firmly hold your plane upside down (the setup stands built into flight boxes, or for assembly at the field, are NOT strong and solid enough!) Use the prop recommended by OS for break-in, or if not specified, the smallest in pitch and diameter they say is recommended. Make sure you are using the fuel they recommend, with the correct nitro. Buy a tach. They are almost mandatory for 4-cycles. Turn the high speed needle valve all the way in, then turn out two turns. Start the engine. Move the throttle up slowly (don't bang it open) to full open. If it still dies while mid way to full, with no sputtering or coughing, your low needle is way too lean. Open it a full turn, and try again. If it coughs and dies mid way to full, you're too rich - screw it in a full turn. If it gets to all the way open, runs a little, and sputters and dies - turn your high speed needle in 1/2 turn. If it dies very quickly, maybe with some smoke, open your needle 1/2 turn. Once it will run at full, it should be running poorly - sloppy, sputtering, etc. Use your tach, and start turning the high speed needle valve in, slowly. You'll reach a point with no change, or it will start to drop in rpm again. Open the needle valve 1/4 turn from that point. Run the whole tank out, running mostly at mid-throttle, with short runs at full speed. Repeat with another tank, with longer runs at full speed. Repeat with another tank, with the runs mostly at full speed. Now, adjust your high speed so that it is about 300 rpm down from the peak setting. Finally, start adjusting your low speed. See my previous post about how to do that. Same deal for four cycles as two cycles. Recheck your high end after you get the idle set. Whew! NOW put the engine in your plane, in it's inverted position. You may not be able to get as low an idle with it inverted - fuel tends to go to the lowest point and drown the plug, but you will be able to get it to run, although with a faster idle than you may like. You may have to readjust the low speed mixture slightly. Watch it when starting - ALWAYS turn the prop over by hand several times before hitting it with the starter. Fuel can accumulate in the head, cause hydrolic lock, and you can literally bend parts by trying to force it. Fly your plane a few flights. It's not broken in yet, but is getting there. Treat it nice, with no lean runs, and you will see it getting stronger and stronger over the next few gallons. If the idle speed is just too fast for your liking - wants to taxi too fast, too hot on landings, etc. THEN think about an on-board glow. They only fix one thing, the ability to run at a lower idle, for longer. They will not fix a problem with engine acceleration, or mid to high range running. Finally - expect to go through the whole adjustment routine again, if you change props, fuel, or brands of glow plugs. You should only have to check it the first flight of the day, and on days with wide temperature swings (60 in the morning, 95 at noon), and usually only slight tweaks to the high speed. All your stated issues, so far, are due to the needle valve settings being not right, and the engine not being broken in. My guess - high speed needle not open far enough, followed by improper setting on the low speed. Need help? Ask almost anyone at the field next time you're out! |