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  1. #1
    Jezmo's Avatar
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    BAX 95AX question

    Several questions.

    One, what is the max Nitro percentage the 95AX will tolerate comfortably?

    Two, what plug have you guys found to be best?

    Three, I have read the comments about the knocking sound the 75AX and 95AX make and my question here is, Are you guys really convinced it's not pre-ignition/detonation?

    To expand on my questions Bax, I've been in this hobby for around 48 years. I've been involved with building race engines of all types for more than 40 years. Having said that, let me say this engine is the most strange I've come across in all my years of glow engine modeling. I have telemetry on the plane so I can see in real time the engine temp and rpm etc. It runs very cool compared to others and no amount of leaning will get it to run even near 200 degrees F. Yes mine makes significant noise at some power settings. In the air it sounds horrible as if "Knocking" or maybe detonating. I can't see any indications, such as the glow plug element being distorted, that it is detonating but it sure sounds that way.

    My reason for this thread is it dies a lot in flight. Most of the time it is on throttle retard from full to around half and it just quits without warning. It's now been in two different planes with two different fuel systems so that's ruled out and plus it doesn't sound like a fuel issue anyway when it quits. In gasoline terms it sounds like you just shut the ignition off inflight. Hence my question about the plug range. It currently has a genuine OS #8 and changing the plug to another #8 makes no difference. I've run it leaner and richer on both the idle and main circuit and either way on either needle makes the problem worse especially if it's rich enough to four stroke. At that point it's guaranteed to die inflight making me think the plug might need to be just a bit hotter. Again, it does not like to be even the tiniest bit rich. I've also thought about blocking some of the cooling fins on the head as this thing will never exceed 170F in flight even so lean it will barely stay running. As far as the quitting in flight issue leaner is better if that helps, but even that is not a sure cure.

    I am not concerned about the knocking noise as the engine is very smooth turning over with no discernible slack in the rod bearings. I added it because the more it is doing it (the knocking sound) the more likely it is that it will die. Problem is that the knocking doesn't seem connected to needle setting but rather the weather conditions, hence my concern about pre-ignition and the max level of nitro tolerance. High pressure very dry days are the worst. It will do it with 10% but seems worse on 15% so do you recommend trying an additional head shim? I hate to tear into this thing as it's quite new, without hearing from you, so if you can, please point me in the directions you see as best.

    I would prefer to stay with 15% fuel as this is what I use in everything else. My Tower 46, all of my SuperTigre's, other OS's, etc, etc all love the 15%. Your advice is appreciated Bill and thanks in advance for any help you can offer me.

    Edited to add: Bill it has a 14X6 prop on it, do you think it might help to put a 13X6 on it? I didn't try that because of how cool it's running and unloading it with the smaller prop would seem like it would make that worse.
    Spektrum DX8i, DA DLE SuperTigre OS FOX Saito Enya Jett TT: John 3:16

  2. #2
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    RE: BAX 95AX question

    You can run up to 15% nitromethane in your fuel and  you won't "hurt" the engine.  Prop the engine to run around 11,000-12,000 RPM for best operation.

    To the best of our knowledge, the knocking sound at part-throttle settings is caused by incomplete fuel burn in the cylinder that completes once the exhaust port opens.  You then hear the remainder of the fuel/air mixture burn off quickly.  We've come across the knocking sound in a large variety of large two-stroke engines from different manufacturers, yet we've found absolutely no signs of metal-to-metal wear or other kinds of damage.

    If an engine tends to quit a few minutes into every flight, you likely have some kind of fuel-delivery problem to the engine.  As a result, it's going lean and quitting.  Common causes are split fuel tubing inside the fuel tank that unports when the fuel level goes down a bit, a slightly-too-lean mixture setting, a too-lean idle mixture setting (it can affect high-speed fuel flow), a too-large propeller that oveloads the engine, bad cooling, and so on.

    The best thing to do is put the engine on a test fixture with the fuel tank in the proper location and run the engine to see what you get.  That puts everything out in the open so that you can see what's happening.


    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com

  3. #3
    Jezmo's Avatar
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    RE: BAX 95AX question

    Sorry Bax, I've been away and just got to read your reply. Yes, I agree the knocking noise could be fuel burning in the muffler. As far as the fuel system is concerned that's NOT the problem. I've done enough testing to eliminate that as a possible cause. I will try some different plugs in it and I suspect that may be the problem since it doesn't tolerate being rich at all. This is the most sensitive engine I've owned when it comes to being a little rich. At the point of just starting to four stroke it will not staying running at all in the air. Guaranteed to die. The most reliable flights are when it is on the verge of being too lean.

    I was hoping for an easy answer but I will pick away at it until it's figured out. Thanks again Bill and when I get this solved I'll post it back. As I mentioned above, I strongly suspect the plug is too cold because it doesn't tolerate being rich at all and I mean absolutely NO four stroking That usually spells too cold of a plug in my past experience or possibly too large of fuel droplets requiring an idle bar to keep them from extinguishing the glow plug element.

    Best Regards,
    Butch
    Spektrum DX8i, DA DLE SuperTigre OS FOX Saito Enya Jett TT: John 3:16

  4. #4

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    RE: BAX 95AX question

    I have to say that the .95AX is the most touchy O.S. engine I have. Loads of power, but excessively sensitive to needle settings. And you're right; to keep it from flooding/quitting at less-than-full throttle, I've had to lean the low-end out more than I'm used to doing.

    I now have another plane in which I'd like to mount a .95AX, but my idea of fun does not include constantly fiddling with an engine's settings and worrying that it's going to die at an inopportune moment..

    I'm using the #8 plug and 15-percent nitro. Field altitude is 7,000 feet.

    Richard

  5. #5
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    RE: BAX 95AX question

    If any engine is very sensitive to needle settngs, and you're not running a pump or a tuned pipe, then you have a fuel-draw difficulty.  The engine should not be particularly sensitive to the high-speed needle.  However, operating at 7,000 feet above sea level can be helping you to have difficulties.

    If an engine is properly broken in, and properly set, you should not be having to worry about tuning and "chasing" the needles.  The engine should be pretty good for the rest of the day, unless you get a lot of temperature change.  If your needle is hard to set, you may have too much propeller, too little propeller, too long a fuel system, a tank that's too low, and so on.  Get it onto a test stand and work it out.  The engine should be able to take a set and keep it fairly well.  If you have to worry about dead-stick's every flight, then you have something 'way off, and it's likely not the engine's fault, but something in the installation or operation.


    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com

  6. #6

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    RE: BAX 95AX question

    Bax,

    Good info, some of which has been tried.

    I should have mentioned that I run either a 14x6 or a 13x8 on that engine. I've also purchased a Perry pump because I think the tank may be too low, and I'll look into getting the next size up fuel line to replace the standard medium(?) size.

    I hope this all does the trick because I've been very happy with my O.S. engines over the years.

    Richard


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