Tuning my SJ50

Tuning my SJ50 (Full Version)

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still4given -> Tuning my SJ50 (5/8/2007 8:58:53 AM)
Hi Guys, I had a little trouble getting my SJ50 to start the other day. It would start at idle after plugging the exhaust while turning the engine over with an electric starter but I couldn't advance the throttle without it dying. I would have to edge the throttle up slightly and keep tapping the exhaust outlet until I could finally get it to rev up. Once I got it to go full throttle then it would seemingly run fine. I could bring it back to idle and it would advance properly. It seemed to run fine in the air and was very fast. However, when I would bring it back to idle to land, It would keep running but if it idled too long it would not advance to high throttle again. No chance of a go around if I ran out of runway. :( I don't remember it being hard to start or run before this. I did check the alignment of the sleeve this time and it looks good. Any help would be greatly appreciated. Thanks Terry

bob27s -> RE: Tuning my SJ50 (5/8/2007 1:43:26 PM)
Quick rule of thumb..... if an engine is running well, then suddently misbehaves..... change the glow plug. That would be the first thing I would do. Next, richen up the high-end mixture just a little bit from where it is running now. (yes, it can affect mid/low speed fuel flow) If that does not immediately solve the issue.... ... from what you decribed, the low end mixture may be just a tiny bit lean. Back out (richen) the low end mixture screw a bit less than 1/8 turn and see if that helps. (that condition may be caused by some dirt in the carb that caused the low/mid range to go just a bit lean too... so do not adjust the low needle more than an 1/8 turn) Also worth visual inspection - ensure the muffler pressure line does not have a hole in it somewhere, or is kinked.

still4given -> RE: Tuning my SJ50 (5/9/2007 5:11:46 AM)
Thanks Bob, I'll check all of those things. I do live in the desert and it is likely that some dirt has found it's way into the carb. Maybe I should take it apart and clean it real good. This motor was a sweetheart when I I first ran it. Weird that it is behaving the way it is. First thing I did was install a new plug. There was no change. By the way, I'm using Fox Miracle plugs. These OK with this motor? Thanks, Terry

bob27s -> RE: Tuning my SJ50 (5/9/2007 7:28:50 PM)
You should be ok. I'd avoid the mirical plugs. Although it is technically a hot plug, my experience is they do not run well in the Jett engines for some reason. Midrange was terrible. Also, the OS #8 does not work well either (too cold). You want a hot plug. McCoy 59, K&B1-L, Rossi #2 or #3 , Enya #3, OS#3, Fireball hot, and if you have one on hand try an OS-F plug (yes, the 4-c plug).

still4given -> RE: Tuning my SJ50 (5/10/2007 12:46:08 AM)
Hi Bob, I tried the McCoy 59 just now and it is running the same RPM as the Miracle Plug did. I will leave it in just to try it. I also bought some McCoy 9's which are colder but haven't tried them. I was able to get the starting problem solved I believe. I richened up the high end a couple of turns and then snuck up on it by pinching the fuel line and leaning until it didn't rise in rpm with the pinch. I did basically the same thing with the low end needle and now it idles and transitions well. I have a 1/2" extension between the motor and the pipe. This may attribute to the lower RPM, I don't know. Here is what I am getting APC 9x7 > 16,900 APC 10x6 > 15,900 APC 10x7 > 14,900 I suppose my tach could be suspect although it reads 3600 when pointed at an indoor light bulb. I would imagine the motor will hit in the 17,000's in the air once the prop unloads on the 9x7, but not likely with the 10x6. Blessings, Terry

bob27s -> RE: Tuning my SJ50 (5/10/2007 2:14:47 PM)
It will run on the Mc9 but wont transition - plug will go out. I forgot about the extension. With the 1/2" extension, we would expect to see 500-800 rpm drop. So what you have there is not to far off. But it is a trade of rpm for torque, so the 10x7 should be doing a little bit better than that... my guess is it is unloading closer to 16K in the air. Next time out, fly it with the 9x7. Let it scream in the air. Do a few shallow dives to allow the engine to unload. Find peak rpm, back off 400-500. Only fly for half a tank, then land - let the engine cool off for 5-10 minutes. Then fly it again that way one more time. Then try the 10x6 again. See if it picked up a few rpm. In general, from what you have there.... Id stick with the 10x6 or perhaps 9x7 at the next race (see how the plane performs with the 9x7).

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