I bought another one, SK .91, that is. Even though the price went up to $100, the Magnum .91XLS is $150 (I also have one of these) and the OS .91FX is $210 (I did own one of these and sold it), so the SK is still a bargain.

My first one is running perfectly. I have not seen any of the maladies reported in another SK thread here. Today, I flew around at half power for 3-4 minutes, then hit the power and it went instantly to full throttle, no hesitation, no smoking, no stumbling. I have probably over a gallon and a half of fuel through the engine and it is running very strong, pulling a trash can, over 8 pounds Ultra Stick vertical as long as I want. I have not used a started on it. It normally starts in 1 back flip.

I did break it in per the instructions for the 5 tanks of fuel, 1: rich 2-cycle, 2: rich-lean cycling, 3: high low throttle cycling, 4: 4 clicks off peak run, 5: set idle. I also ran several flights at a rich 2-cycle. Maybe it's the castor blend fuel I use (Omega 10%) or maybe it's that I set my engines to leave a slight smoke trail, but I rarely have a lean run or engine trouble.

I am running a 15-6 prop, which Ray Nano from SK Engines Tech Support recommended. He said a 15-6 (minimum size) or a 15-8 so I am running a 15-6 Master Airscrew Scimitar. I know I ought to order an APC 15-6. The prop looks big, it doesn't scream, but it pulls like I don't know what. I had to buy another SK .91 just because this one is running so darn good.

I ran the new one on the bench this afternoon, doing the break-in. 5 tanks. Zinger 15-6 wood prop. I have yet to adjust the low end mixture on this one. I let it idle for 1-minute and 10 seconds, timed on my watch. I popped the power and it went to full throttle, no hesitation. That is hard to beat.

I don't know what rpm it runs because, even though I have a tach, I rarely use it. I can tell how an engine pulls a plane and this one pulls to my satisfaction.

I know some people are having trouble with this engine. Maybe they didn't break it in as stated in the instructions, but used their old standard break-in or what some local expert told them. Maybe they are running a smaller prop. I would normally use a 14-6, but Ray says it won't run well. It will go lean in flight. Why argue with the guy who was in on the design of the engine. I am running the big prop like he said. My 2, and the 50 I also bought, run great.

Thanks for info Ed,I run in my 3rd SK 50 Last weekend.It has aTower muffler on it.With a 12-6 APC it screams.I couldnt tach it I dont have any.But It seemed 12-6 prop is less yet.I think it is Tower muffler s effect.On the second tank an advanced pilot friend of mine flew it in front of a World models Dago Red and flight was great as engine too.Im so happy to discover SK brand engines.I thank you especially with your comments to agree my pioneer support of SK brand engines.I hope to hear your SK 50 comments here too.

Good report, Ed. I should probably buy another while the price is as low as $100 myself. It sounds like a keeper.

Ed Cregger formerly Artisan

I found an old, .91, Ultra Thrust tuned muffler in my muffler box. It happens to be the long chamber one that I bought 4-5 years back to run larger props on an OS .91FX that never ran well so I sold it. I thought I'd give it a try since I still have SK 90 #2 on the test stand.

At this point I am running a big, black, heavy, Master Airscrew 15-6 and turning 9500-9800 rpm on Omega 10%. This is about what my #1 engine turned, too. I installed the Ultra Thrust and cranked it up. I leaned out for a peak of 10,650 and a nice easy 10,400 rpm. That's a good 800-900 rpm. I think that is pretty good on a big prop and at lower rpm.

For this weekend, I'll install it on engine #1 on the Ultra Stick and give it a try in the air. The needle seemed fairly insensitive so I don't think it will lean out in the air or in maneuvers, although you can't tell in advance. I have had engines just scream on a pipe on the ground, but lean out when I pull g's in the air. I'll report later.

I have 2 SK 90 engines, and both run fantastic. I did have one snap a crank, but warranty service repaired it, and it is running just fine. I have yet to have a deadstick with either engine, except for the one time when the crank went. I would not hesitate to buy another one. Good power, good value, and pretty darn good warranty service. Oh...one more thing. I did have a little trouble with one of the engines throttle barrel sticking, but after taking a little emory cloth to it, it seems to be working fine now. I currently have one SK 90 mounted on a Hog Bipe, and the other mounted on my Super Sportster 90/120. Both engines pull both planes around pretty good, and both are propped with 14X6 Zinger props. I may try an APC 15x6 on one of them just to see how it likes it.

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Do you do any 3D flying, Ed?

I have a Hanger9 (I think) Harrier 90 ARF NIB. While I originally intended to put a four-stroke in the nose, I'm wondering how the SK 90 would pull it around? Any engine that can pull a 15x6 or 8 well and throttles good is running right up there with some of the four-strokes I have.

Was your old long pipe Ultrathrust made for the G90 Tigre? I have one that is made for the Tigre G90 and was wondering how close the bolt pattern is between the SK90 and the G90. If you happen to know. TIA

Oh, which Ultra Stick ARF are you using? A .40, .60 or 1.20? I'll bet the .60. Pardon me if I asked this question before.

Ed Cregger

Ed,

I do some 3D, but not as much as I used to. I did a kit review on the Harrier 3D 46 for R/C Report. Pretty good plane. I have the 60/90 size in the box. I was thinking the same thing, the SK 90 or my OS 1.08 or a Magnum 1.20 4-stroke. I had a Saito .91 in a Mayhem, but it really wasn't enough.

I flew the Ultra Thrust on the SK 90 on the Ultra Stick 60 (the transparent yellow one). I didn't see much improvement over the muffler, so I landed and switched back. The much repaired U Stick ( I got it just before it went into the trash can and my buddy, Flaps, repaired it) weighs over 8 lbs and I get unlimited vertical with a 15-6.

My second SK 90, which I have not flown, only bench run, seems to be better than the first one. At least it got to idling good faster. I can idle pretty low for a timed 1 minute and a half (90 seconds), then pop the power and go right to full power. I probably have 10 small tanks (6 ounce) through it and it is running sweet. I guess it's time to yank the other one and replace it in the U Stick break-in bird and give it some air time. I also have to get some APC 15-6s.

The Ultra Thrust was originally bought for an OS .91FX, but fits the Magnum and SK. I think the holes are too close for a Super Tigre muffler. Due to the low rpm and large prop, I don't think you'll need it, especially after you get a gallon through it.

I guess I need to get another Ultra Stick, but I also need some retracts. Flaps and I are building a seaplane with a retractable center float. Retract tip floats, too, but we may hold on those until we get the center one working. I have an old Heckler 3D plane we may use as a test bed.

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Ed
I was just checking on this very thing today. I was wanting to try one of my SK 90 engines in my Great Planes Shoestring Racer, and was checking out various pitts mufflers that would work. Kangke, and ATS do sell a pitts exaust to fit this engine, but I was checking to see if there were any others that were compatible with this engine. I came up with after some checking that the SK engine exaust mounting holes are 52 or53mm from center to center, or 1.660 inches from center to center. I did not see any pitts exaust while browsing tower that falls within these numbers. Maybe there are some that would work by extending the mounting holes on the exaust, but I am not sure. So I am also curious as to what exaust pipes are compatible with the SK 90 engine, and are interachangable between what other manufacturer?

I tried the Ultra Thrust on the SK .91 and it didn't seem to do much with the 15-6. Could be it is tuned for rpm not in the range of this engine. I have switched back to the stock muffler.

I finally got around to swapping out my engines and have now flown the second SK .91. Same results. Runs great. It's had about a half gallon through it on the bench and I can tell it's a little tighter. It seems to run better with a slightly richer setting. This will probably go away like it did on the first engine.

I am trying a few different props, all 15-6, but different brands. I have an APC, Master Airscrew, Master Airscrew Scimitar and Zinger. I had been flying the Scimitar on the first engine, but had run the black MA on the second on the bench. When I got the APC in, I put it on to fly the newer engine. I did not see anythin startling about the APC. I then tried the MA. It is noisier, but my buddy, Flaps, says it looks to him like it performs better. I need to do some more testing with all of them as I fly in engine #2. I just love testing out new engines, but we went seaplane flying today and I didn't get to fly the Ultra Stick.

Thanks Ed,by your comments here,I am now able to talk more in confidence here to my friends convincing them to have SK 90.