Alternative Engines?

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o1moregil
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Location: Midland Texas

Post by o1moregil »

Ive seen the magnum xl 28 with a can and pipe run on an Avenger b2, about 3 years ago Me and Vic (Vic from Colorado)had a lot of one on one combat weekends and it seems like a good engine, tact same as my os fx., also a guy From Livermore Ca. (can't remember his name) used to run the magnum xl 28 with a venturi , I believe he had them on Bandits.
combatgoblin
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Post by combatgoblin »

I have the CS.25 it is extremely tight and I was not able to get it to run this does not mean it will not run but will require considerable break in it is well made and comes with a streamlined muffler.
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boiler
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Post by boiler »

Some of the WI guys have flown the GMS in Limited B. At least they had a gold head. They can beat me up with them.[;)] I order their mufflers for the FX for 1/3 the price that OS wants. A little on the heavy side but you need some extra weight for Limited B and 2548.
sgilkey
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Post by sgilkey »

The CS looks interesting and I like the price! Judging by the photos it looks like the typical Chinese OS25FSR-clone 10mm neck carb, which most recently has been used on the TT PRO25. That carb is a certified POS. IF that's NOT the carb, then there's hope. Or replace it with the carb from a 40FP or LA.

I have never personally run a GMS. Back in the early 00s when a lot of Canadians flew with us, their prices on OS engines we SO HIGH that a lot of them flew the GMS. It was awful. About 1/3 of them ran pretty decent, and a decent one was OK. The rest, they couldn't get a full heat out of. Seemed to have very inconsistent performance, to say the least. Things may have changed in the meantime. The GMS/TOwer 46, 75, etc. seem to have a pretty good reputation so maybe GMS has cleaned up their act.

The Mag28XL, from what I heard, may be going out of production soon??? I have a bunch of those and like them but the FX is superior in every way. The mags vibrate more than the FX, especially at Open B rpm. They like to shake themselves apart if you don't watch it. the carb will come apart unless you carefully prep it and loctite the rotor screw/jam nut, and put a piece of fuel tubing over the LS needle to keep it from shaking OUT or IN (cutting off all fuel flow). The carb retaining bolts, too. If you thoroughly prep the carb, it will be very reliable. THe NVA is very sensitive but once you get a good setting it holds it well and seems reliable. The crankpin is pressed in and I have seen several come out- they start rubbing on the backplate. This seems to only happen at very high rpm. The wrist pin is held in with circlips and I have seen some come out, with catastrophic results- also at high rpm. For limited B they seem to last forever, Open B in the 18-19k range they may come apart at some point, and vibrate a fair amount (watch for fuel foaming issues) in the process. They do make very good power on a MCP. They used to come with a bigger carb, the latest ones I've seen have the smaller .25 carb, a bigger carb really wakes them up. A way to keep them together and reduce vibration for Open B might be to run a 9x5 to keep the r's down a bit. I used to do that with my iron-piston OS 28F engines and it works fine, rpm drops into the 17k range and pulling power is still very good, it just doesn't SOUND like you're going as fast.
spaddawg
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Post by spaddawg »

Ed I have been running the GMS .25's since back in 2002. This little motor is a $59.00 rocket with a KY Can on it. I run them all in open "B" and turning a 9X4 MAS on 15% to 20% I am getting around 18,500 and up. The only down side to the GMS is they tend to have air leaks around the carb and in some cases the from bearing making hard to get them to shut off. Be cause the carb has both a high and low end adjustment I just set the low end a little on the fat side that when I close the carb it will load up and die with in 3 to 5 sec. and not harn the engine. You can ask Terry or any of the west coast guys who have seen me run the GMS and they will tell you how well they run. The big plus of the GMS is that it has a large exhaust port like that of the OSFX so if you go to a bigger carb it can handle the extra airflow with no problem. And with a bladder it should really turn on the power!
sgilkey
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Post by sgilkey »

hmmm sounds like the GMS has a lot of potential, then!
draftman1
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Post by draftman1 »

I saw Jame's GMS at an event. I was impressed with it. a couple of guys here tried GMS and they didnt put out the power. Jame's must know the tricks. Im sure alot of breaking in is one of them
THend
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Post by THend »

We used to call James the "Mad Chinaman" for his GMS engines...

He did very well with them and I never noticed him lagging at all in the speed arena.
NeilD
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Post by NeilD »

Hey Ed. I use this motor for my 25X48 Corsair used for just sport flying, and it is a really good reliable motor. I don't know if it will have the power you guys are looking for in Open B, but it is a good running motor.

http://www3.towerhobbies.com/cgi-bin/wt ... LXTRY8&P=7

The price is right too, and I like the way it mounts straight to the firewall w/o any additional mount.
sgilkey
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Post by sgilkey »

I found a decent deal on a used GMS25 so I bought it to give it a try. NOt sure if I will get much chance to test it yet this season but I'll try.
sgilkey
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Post by sgilkey »

I received my used GMS 25 yesterday and have been able to do a tear down and inspection. Overall I'm pretty impressed, just looking at it. I've been fond of Magnum XL 28s for years, despite their drawbacks and sloppy machining they can be made into reliable and durable engines for the limited performance classes. Just based on visual inspection, I would say the GMS is superior in virtually every way

The crank is one-piece- no pressed-in crankpin, which on the Mag will work it's way out if you really flog it (ie Open B/pipe rpm, not observed in Limited applications). The crank gas passage is fairly nicely machined with a ball-end cutter, making a nice transition for airflow, similar to the crank on the 32SX and Nelson engines. ON the Magnum there is usually a big burr in the crank window from the sloppy machining, and I usually grind off this burr, which leads me to observe the Magnum crank does not appear to be hardened. THe GMS crank window is clean and burr-free and I see no need to "clean it up, " but I touched it with a grinder just to see if it's hard, out of curiosity. It is not as hard as an OS crank but maybe harder than a Magnum, hard to tell. This is usually only a problem on the little Mag 15XL anyway, because it had a tiny little 5mm crank and it bent relatively easy in a crash. I've never bent a Mag 28 XL crank so maybe I should just shut up

Bearings look like typical Chinese with metal retainer, don't know if I'd trust them at Open rpm but who knows. The front has a one-sided rubber seal which I removed so that I could clean out what appears to be a factory-pack of grease which has stiffened. It may just be congealed fuel, not sure...

The prop driver is tapered-cone. I hate the way the FX driver can fall off and get lost, along with its little shim washer (what a silly design...why don't they just incorporate the thickness of the shim washer into the prop driver hub, as in other designs.....). But one advantage of the FX design is that it can be easily field-stripped to clean out streamer string, dirt, etc., something I had to do several times at my last contest! So this is not really a positive or negative, in my mind

The match of the liner ports to the case bypasses is very good, unlike many Chinese engines that have a big mismatch here. The wrist pin has teflon pads on the ends, which seems to imply it's a full-floating pin. This seems strange since the GMS does not have the "rotated" porting of the FX, and a fully floating pin could catch in the liner ports????? It looks like the teflon pads are oversized, and are pressed into the wrist pin boss of the piston, to keep them from moving. I don't plan to remove them to find out. Not sure I would trust this design at high rpm??? Seems a number of heat and vibration cycles would eventually cause the plastic pads to creep, loose retention, and fall out, with catastrophic results???

The combustion chamber appears to be cleanly machined. I have not yet checked head spacing. The engine came with three .004 shims installed. I plan to lower the head and run 5% nitro, which has been my standard brew for Limited, and see what i get

The carb looks decent. This is the real problem area on the Mag 28, will have to wait till I run it, to judge the GMS. Nice thing about the GMS is that the carb neck is 13mm and it's secured with a pinch bolt, so if the carb sucks (well, that's what it's supposed to do, but I mean in a bad way) it's an easy swap to install an FX or SX carb. The Mag carb is 12mm and the only decent replacement carb I have found is the ST 25/29/34. But the Mag uses bolts to secure the carb, so the ST carb needs to be drilled and tapped, a major hassle. On the other hand, the ST carb is superb and really cleans up the Mag's running. (A properly prepped Mag carb will work fine, but its needle is very sensitive, and it is a bit restrictive). Anyway, the biggest problem on the Mag is the soft and sloppy threads for the carb rotor screw- on the GMS carb, they appear to be tighter fit. I'll still apply blue loctite, like I do with the Magnum carbs, to be sure. The O-rings on the low speed needle seem to give much better retention of the setting, avoiding the vibration-related needle creep of the Mag carb. Finally, the high speed needle has a double o-ring seal, which is hopefully better than the leaky one on the Mag (which, even though it is sensitive and leaks a bit, once it's set seems to perform quite consistently!). Oh, one other observation- the spray bar appears to be glued into the carb body- I could not remove mine to flush it out. On the rotor (interior) side, there was a thin ring of what looks like adhesive that had come loose and was being held in place by the rotor spring. Not sure it would have caused much trouble, but I removed it to be sure....

The muffler looks MUCH better built than the Mag 28 muffler. There is some casting flash on the header portion, but that can be cleaned up easily. There is no internal baffle. The muffler is a bit heavy, the castings are very robust and it has a nice thick thru-bolt. Strangest thing is the pressure nipple- a hole is cast, and then a pop-rivet is inserted in the hole. then the pressure nipple is threaded into the soft aluminum pop rivet. Looks neither durable, inexpensive, or leak-free, but time will tell. Strange design.......

Maybe James Shell can comment on some of these concerns, it seems like his have been running well so maybe my concers are unfounded, especially about the teflon pads....

I'm looking forward to checking head spacing and getting some test runs. Overall the engine looks decent and I'm excited to test run it, I hope the weather holds out for me!
sgilkey
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Post by sgilkey »

Got the engine buttoned back up and checked head spacing with no shims at .015". I was wrong on factory pack of shims- there are three, two are .008" plus one at .004" I installed 1x.004" for total spacing of .019", maybe a little high for 5% nitro but we'll see.

Hurried home from work at 6 (thank goodness for my 5 minute commute) and put the motor on the test stand. Fueled it up and ran it for 30 sec and shut it down, then started the lawnmower and did a couple of laps. Gotta multi-task to get the lawn done before dark and the predicted wet weekend. Repeat six times. By this time it's nearly dark and the temp is below 60 degrees so the air is dense. I record 13.7k on the MA 10x4 and 5%, with the head shim still in and a junky break in plug that I did not replace. Removing the shim, adding nitro, and a decent plug might help. It's a little weak, but the air was cold. So I would say it has potential to move well into the low 14k range.

Overall running was good- easy to start, good throttling and idle, smooth, no leaks. Broad needle range, esp. considering the 5%. Only issue was that the low end was a bit rich, so I turned in the lo speed needle and WOW did the top end get lean, I had to open the needle over two turns. I think the lo speed needle was restricting the top end. So i readjusted to the rich lo end setting. We'll have to see how that works in actual flighht testing.

Overall seems decent and has potential, I'm looking forward to more testing with the shim removed, and actual flight testing. Hope we get some decent weather before weather hits us hard...
ptsullivan
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Location: USA

Post by ptsullivan »

I wonder if at least there should be at least one shim to maintain the
seal between the head and the sleeve over the temperature range?
Perhaps that is a myth, I am definitely curious.
Please keep posting results. How do you evaluate vibration/balance?
I am especially concerned with how well it maintains a tune. I have
often found the make or break from one engine type to another was not
the outright performance, but the repeatability.


-Pete
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Big Dog
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Post by Big Dog »

Great Review Scott. Brian and Eric are lucky to have such a great pit man on team Gilkey. Thanks for sharing the info.
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