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  1. #1
    ZX1Ninja's Avatar
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    Engine Speed ??????

    I am either missing, not understanding, or just plain don't know something here. I have three different engines on three different planes that will not accelerate past half throttle.

    O.S. 40 FP with 10x7 in raider 2
    Tower Hobbies Pro 46 with 10x7 in another raider 2
    Magnum 61XL with 11x8 in P-51 Mustang 60"

    They all seem to be running good (best as I have been tought or learned), slightly rich with pinch test and light smoke trail. All running recommended recomended props, (or as close to as I can get right now) and 15% fuel. All throttle linkages set correctly and have free operation. Best I can figure is load / RPM (incorrect Prop??) issue, but I don't know because diffent props don't seem to help. Could someone please enlighten me as to what I am missing here and put me on the right track to resolving the problem. Or if the only problem is me.
    Ron

  2. #2
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    RE: Engine Speed ??????

    It sounds as if the high speed needle may be too lean. You say the pinch test but do not elaborate. Ifind the pinch test unrevealing for the high rpm setting but very useful for the idle mixture setting.
    Club Saito # 677-Team Boca Bearings-Star Collectibles Muscatine-Glowhead Brotherhood #19

  3. #3
    1QwkSport2.5r's Avatar
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    RE: Engine Speed ??????


    ORIGINAL: jeffie8696

    It sounds as if the high speed needle may be too lean. You say the pinch test but do not elaborate. IΒ*find the pinch test unrevealing for the high rpm setting but very useful for the idle mixture setting.
    I find it odd you dont see the pinch test as being revealing. Thats how I set nearly all of my engines, and have a tach as backup. If anything, I find a slight rise in RPM on the pinch at WOT to be 500-700rpm rich instead of the typical 300-400rpm rich most people suggest/use. I dont use the pinch test on the idle needle though, as I rely on the transition quality as the determining factor.

    Hmm... Thats just me though.

    As to the OP, if you have a tach, you should tach the engine at WOT and see what rpm its turning. 7" and 8" pitch props are pretty high, and chances are that high pitch is lugging the engines down too much. Try a 5"or 6" pitch instead, and re-tach the engine.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  4. #4
    MJD's Avatar
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    RE: Engine Speed ??????


    ORIGINAL: ZX1Ninja

    I am either missing, not understanding, or just plain don't know something here. I have three different engines on three different planes that will not accelerate past half throttle.

    O.S. 40 FP with 10x7 in raider 2
    Tower Hobbies Pro 46 with 10x7 in another raider 2
    Magnum 61XL with 11x8 in P-51 Mustang 60''

    They all seem to be running good (best as I have been tought or learned), slightly rich with pinch test and light smoke trail. All running recommended recomended props, (or as close to as I can get right now) and 15% fuel. All throttle linkages set correctly and have free operation. Best I can figure is load / RPM (incorrect Prop??) issue, but I don't know because diffent props don't seem to help. Could someone please enlighten me as to what I am missing here and put me on the right track to resolving the problem. Or if the only problem is me.
    There should be no problem with a 10-7 on the TT Pro .46, that combo is known to haul Quickie 500's at a pretty good clip. Should be fine on the .40 too, although I'd lean to a 10-6. Does not explain the problem.

    You say the engines will not accelerate past half throttle, yet you say you do a pinch test. Help me out here - how can you be doing a pinch test at full throttle when they won't accelerate past half throttle...?

    Step #1, open up the main needle 1/2 turn or so and start again. If they throttle up to a rich top end, I'd say you just discovered the problem. I agree with jeffie, my first impression is high speed needle too lean. The fact that all three engines do this is fairly strong evidence that you have a tendency to crank main needles in too far. It would be strange if all three had the same plumbing problem for example.

    If you are having trouble still, then describe:

    - behavior at idle if you pinch test, meaning warm the engine, establish idle, clamp the fuel line off tight right at the carb, and describe what it does
    - what happens when you throttle up (don't slam the throttle, just move it from idle to full in a one second motion)

    I pinch test at idle all the time, I find the standard advice "runs 2-3 seconds before it leans and dies" to be a pretty good baseline for the low speed needle at least.






    Sorry I'm late dear, I had to help my uncle Jack off his horse.

  5. #5
    Moderator blw's Avatar
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    RE: Engine Speed ??????

    If in doubt on the low speed needle on 2 strokes, I'll let the engine idle 25 seconds and see how it handles the throttle.
    The ultimate responsibility of pilots is to fulfill the dreams of the countless millions who can only stare skyward...and wish.

    "It's a new day for Auburn" - Gus Malzahn

  6. #6
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    RE: Engine Speed ??????

    What kind or rpm readings are you getting? If your linkage is *****g out after the carb barrel is fully open it could be deceiving you. Most glow engines seem to give 75% of their power in the first 50% of barrel rotation The only way to know is with a tach.
    Charlie P. (NY) "Gravity is weak but persistant".

    AMA 747089/IMAA 30723

  7. #7
    ZX1Ninja's Avatar
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    RE: Engine Speed ??????

    Pinch test is done at WOT, engine speeds up as it leans and if I continued would die. Thought this was standard procedure. Idle is fine, slight stumble from idle to WOT when the stick is slammed. Other wise smooth transition. Planes all fly fine, problem is noted when moving the throttle up during fligh, no noticeable difference in speed/climb/our performance. No decrease though either. I don't have a tach, so this is the way I was tuaght.
    Ron

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    RE: Engine Speed ??????

    Although I run 10x6 on the FP the props all seem fine. How does the engine tune with a 1/2 tank of fuel? Usually if I can tune an engine on the ground but have problems in the air, it is usually a tank or tank position problem.

  9. #9
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    RE: Engine Speed ??????

    Thats more clear. You are not seeing additional rpms after half throttle while flying. This is a common problem when using high load props , if you dropped down in pitch or diameter you may see better throttle response but the plane may or may not fly any faster.
    Club Saito # 677-Team Boca Bearings-Star Collectibles Muscatine-Glowhead Brotherhood #19

  10. #10
    ZX1Ninja's Avatar
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    RE: Engine Speed ??????

    I don't necessarily care about the speed of the plane as long as it's not slower. I was thinking it was a load issue. But if the engine responds correctly that would be the goal.
    Ron

  11. #11
    1QwkSport2.5r's Avatar
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    RE: Engine Speed ??????

    I have found, at least with the engines I have, that a 1" diameter change doesn't affect rpm too much, whereas the pitch can drastically affect the rpm of the engine. 7" pitch to 6" pitch can be as high as 1500rpm depending on the engine, but from a 9" dia. to a 10" dia. only affects rpm by maybe 200-300rpm.

    I suspected the load was too high for the engines, so dropping some pitch while maintaining the diameter should give more throttle response, and perhaps give more speed because of the higher rpm. Unless you drop from a 7" pitch to a 4" pitch that is..
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  12. #12

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    RE: Engine Speed ??????

    I guess I do not understand what is going on.  A Tower Hobbies Pro .46 is a lot more engine than an OS 40 FP.  Generally, the TH is a screamer especially when used with the Tower Hobbies muffler.  It seems to me that a 10 X 7 on a TH 46 is under proped to me.  The standard for that engine ought to be 11 X 6.  So I am thinking the engines are showing their age and the Tower Hobbies probably needs new bearings, there is no bearing cure for the FP as it is a sleeve bearing engine.

    And I think any stumble on acceleration indicates the low end is not acceptable.  It indicates to me that the low end is not set correctly.  If acceleration test is your measure, then you have failed. 

    I do not use a pinch test.  I use a TACH, and acceleration test.  If it has a smoke trail that is okay.  But I do not worry too much about smoke trails.  Smoke trails indicate to me that there is some Castor in the fuel mix and that is a good thing.

    But then I do not shoot for the max, I try to get a smooth running engine and take the performance given.

  13. #13

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    RE: Engine Speed ??????

    TRY SOME NEW FUEL!

    MAX

  14. #14
    Moderator blw's Avatar
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    RE: Engine Speed ??????

    Same here with using acceleration/performance tests and a good tach. I never regretted the money spent on a TNC.

    I also use the ear test. You have to listen to what your engine is telling you.
    The ultimate responsibility of pilots is to fulfill the dreams of the countless millions who can only stare skyward...and wish.

    "It's a new day for Auburn" - Gus Malzahn

  15. #15
    ZX1Ninja's Avatar
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    RE: Engine Speed ??????

    I do appreciate the input.

    Well, I don't have a tach, but perhaps I may consider getting one. Engine performance is the goal.

    I have always been told no smoke trail is bad, no smoke equals little to no oil, casuing engine damage and failure. Since this make perfect sense for a two stroke engine I am willling to accpt a slight performance loss if it means longer lasting engines. It would however seem appropriate that a slight performance lose should not result in no change in flight after half throttle.

    If these small two strokes do deliver 75% of their power in the first half of the throttle, I would still expect some response in the last half, 25% should be at least noticable. So I will be rechecking my adjustments to make sure they are correct.

    The TH 46 manuel shows several props, all in the 10x? range. It only shows one other prop of 11x6.

    Based on the way this thread is leaning, I purcahsed a couple props to try and see if there was a change. They were not as big a change as I had wanted due tot he local supply store let me down in their stock, but I will advise if you are interested, in the results after I have a chance to see what happens.
    Ron

  16. #16
    1QwkSport2.5r's Avatar
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    RE: Engine Speed ??????

    ORIGINAL: wcmorrison

    If it has a smoke trail that is okay.Β* But I do not worry too much about smoke trails.Β* Smoke trails indicate to me that there is some Castor in the fuelΒ*mix and that is a good thing.
    If it has a smoke trail, that indicates there's oil in the fuel, period. Castor or Synthetic, Olive oil, or peanut oil.. Doesn't matter. If it smokes, there's oil present. Using the smoke trail for anything other than reference is redundant.


    ORIGINAL: blw

    Same here with using acceleration/performance tests and a good tach. I never regretted the money spent on a TNC.

    I also use the ear test. You have to listen to what your engine is telling you.
    Some of us can't afford the luxury of owning a TNC tach. For us, a Hobbico works fine.

    For a few of my engines, setting the needle 300rpm rich of peak using the tach is nowhere near enough. I have to set the engine closer to 800rpm rich of peak for it to not sag at 2/3 tank and be scary lean by 1/3 tank. Half the problems I encountered setting up my SuperTigre S90K involved using the tach to set the needle only, and being way too lean and having the engine quit. (Setting 300rpm rich of peak per the suggestions I received here) Going back to the "old school method" of doing a quick pinch of the fuel line gave me FAR better results than using the tach to set the needle. Maybe its the 800' elevation, or maybe the sun and moon weren't aligned... I better consult the Mayan calendar.

    I only use the tach for reference - finding peak. The pinch test is a far more tell-tale sign of where the mixture is set than anything else.

    Of course, this is all in my humble and modest opinion.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


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    RE: Engine Speed ??????

    i set mine by ear lol except when trying to get a max rpm reading. to get a max rpm reading you need to creep on it or will not be as high a reading as it could be. also i agree the TT likes to rev some so a 10x7 is the biggest i would go if not a 10x6 and let her rev to 15,000 or so.
    AMA # 126183
    Fly light, fly fast and fly low.

  18. #18
    Sport_Pilot's Avatar
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    RE: Engine Speed ??????

    There is oil in the fuel no matter if there is a smoke trail or not. The so called smoke trail is actually unburned fuel, not oil. If the glow plug is too hot you will have a considerable smoke trail even when the engine is set too lean. This is because you have to richen up the mixture to quench the plug so that the timing is retarded. If the glow plug heat range is right you will only have a slight smoke trail at a rich setting. So a smoke trail is a poor way of determining if the mixture is correct.

    Get a tach. And wear ear plugs. Even with today's mufflers they can do damage to your hearing.
    Glow Head Brotherhood #15

  19. #19
    Moderator blw's Avatar
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    RE: Engine Speed ??????

    Much of any smoke trail on a Saito means you're too rich.

    Yes, of course the TNC costs more. If someone has the money, buy it. It's accurate where the Hobbyco ones aren't. And, they are far safer. No more holding tachs half an inch from a prop running at 10,000 rpm, and no more using flashlights just to get bouncy readings. I don't miss that and now I just hold the plane with my ankles on the stab and point the TNC towards the prop. I could easily live without a tach, but I've never regretted spending the money when I really didn't have it at the time.

    I would rely more on listening to an engine than pinching fuel lines. Just my preference. I get it set on the ground with 300-400 rpm rich on the hs needle, and go fly around the field. If it sounds good, it probably is. However, you can hear problems if you are slightly lean. When I hear engines start going lean, I land and tweak the hs and go around the field again. Later in the flight, I'm listening for half fuel tank performance sometimes and on the vertical lines.

    The low speed need to be set where it will accelerate quickly without much stumbling or hesitation. Get it the best you can. Then, let the engine idle for 20-30 seconds and give it full throttle. Listen for performance as it burns off the fuel. If you have problems, it probably needs leaning. Repeat this as often as it takes. It's a drag to crank, check, shutdown, tweak the needle, crank, etc. But, it's worth it when you find the spot just rich of lean. You are looking for those long idles on final approach where you have confidence in the engine when you have one of those full throttle seconds before the crash moments.
    The ultimate responsibility of pilots is to fulfill the dreams of the countless millions who can only stare skyward...and wish.

    "It's a new day for Auburn" - Gus Malzahn

  20. #20

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    RE: Engine Speed ??????


    For a few of my engines, setting the needle 300rpm rich of peak using the tach is nowhere near enough. I have to set the engine closer to 800rpm rich of peak for it to not sag at 2/3 tank and be scary lean by 1/3 tank.
    i have never heard that an engine will lean out as the tank empties. could you explain this please? what causes it?
    thanks sb


  21. #21
    1QwkSport2.5r's Avatar
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    RE: Engine Speed ??????


    ORIGINAL: sweatybetty


    For a few of my engines, setting the needle 300rpm rich of peak using the tach is nowhere near enough. I have to set the engine closer to 800rpm rich of peak for it to not sag at 2/3 tank and be scary lean by 1/3 tank.
    i have never heard that an engine will lean out as the tank empties. could you explain this please? what causes it?
    thanksΒ*Β*Β* sb

    Have you ever ran a model engine before? Its a matter is physics.. The exhaust pressure is more effective at pushing fuel to the carburetor when the tank is full, as the level drops, the airspace inside the tank increases and pressure acting on the fuel drops thus leaning the fuel mixture. A 3-line Uniflow tank setup averts this to a point, but you end up with fuel getting pushed out the exhaust through the pressure line if you drop the throttle abruptly.

    If the fuel system was regulated by something other than the carburetor, the low tank lean-out wouldn't be as prevalent.

    I'm sure others will have a different explanation than I gave, but the principle is still the same. Full tank = rich mixture, Empty tank = lean mixture. Its the reason engines have to be set rich at a full tank. By the bottom of the tank the engine is running at peak or slightly richer than peak rpm.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.



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