Some people are keen to hear about progress on my GMS 76 adventure and now that it is bearing fruit, so to speak... Here goes!

So, on a whim I bargained with the dude at the local GMS stockist and amazingly he sold it to me for $NZ99 (about US$70, 40% off) sans muffler and carb. At that stage I didn't really know what to do with it, but it was worth a try. Did some measuring and reading and came up with a list of possible mods to try out:

1. Small venturi
2. Lower compression
3. Block boost port
4. Small muffler stinger
5. Decrease blowdown - raise/angle inlet ports
6. Sleeve crank hole
7. Drop liner 1mm
8. Get lower timed liners cut
9. Deeper piston to prevent SPI
10. Stroke by 2mm to get .82 cuin and lower timing

Some of those tricks are easy and some ridiculous. So I started with easy:
7.0mm venturi with adjustable spigot (6.0mm equivalent), 2 extra head shims, wooden plug glued into boost passage, stock muffler with small diameter rubber deflector, 8.0mm delrin sleeve pressed into 12mm crank bore.

I also hit it hard with the dremel, the lathe, the belt sander, the files... anything abrasive and knocked 3 ounces off the weight to bring it to 16.25 oz sans muffler. About the same as a Saito 72. Its a ringed engine with a thick steel liner and a huge crank, so there's no more weight coming off this puppy.

Running on 5/22 fuel, it was typical schneurle RC misbehaviour. Flew the model ok in a 2-stroke, but was quite unrefined and has no power in the 4 stroke. In order to help the model balance I took the centre section out of the muffler and used the end of a mousse can to close off the muffler. Nice and quiet, but suddenly began cutting out in hard inside turns. Went through the whole deal of trying plugs, fuel, props, rev range, tank etc. Only after another muffler change and head mods did it resolve. Now it runs the stock muffler with the centre extension removed and the two cast ends welded up. The stinger is bored out to 9.0mm.

Flights at this stage were in wind and showed some serious wind-up and runaway. Very scary and hard work at 76 Oz and leaning out to 4.3 sec laps!

Next trick was a head reshaping. The stock head is a fairly aggressive looking, wide squish unit. I replicated Bob Zambelli's FP40 shape, scaled up to suit the 26mm bore and measured the geometric compression ratio at about 8.4:1.
Flights after this mod showed a big improvement in manageability. Still low on revs, not much in the way of classic stunt manners and tending to lose power in the climb, but really easy starting. On the advice of Tom Hampshire, I re-cut the head and got it in the 9-9.3 compression range. Seems about right so far. Maybe one less shim or more nitro will do it.

And the final sensible mod on my list; timing changes. Stock timing was about 118deg transfer duration, 150deg exhaust. I wasn't confident of much benefit, but with a new liner costing the same as two decent glowplugs, I thought I'd give it a go. Fifteen minutes with a diamond burr, and the timing became 132deg transfer and 150deg exhaust, about halving the blowdown to 18deg. The ports were also angled upwards by around 15 degrees.

First flight today was an absolute joy. Set at 7500rpm on the ground with a narrow blade 14x4 and off we go. Deep 4 stroke in level, gradually warming up to a steady 5.5/lap. A little rich for my tastes, just a beep at the tops of loops and a weak 2 stroke in overheads. Could use some more overhead urge. Tension is tenuous up high, despite forward leadouts. Maybe some more pitch too. This was exactly the engine run I wanted (aside from tension problems), but never honestly expected to get. It needs some fine tuning and tank work, but its right there in the ballpark. Not so easy to start now. I understand that is typical of reduced blowdown mods. Uses 150cc for a full schedule and some spare laps. It does a little 4-2-4 boogie during the last 6 level laps or so, then a solid two stroke in the final half a lap before cutting. Sweetness.

And now some final specs:

GMS76 mod-o-rama.
Weight: 16.25 oz
Prop: Wood 14x4 narrow blade
Model: Zone 2 (76 oz) on 69ft lines nose to handle
Venturi: 7.0mm delrin with OS40FP RC needle, 6.0mm equivalent area
Muffler: Shortened stock muffler, 9.0mm stinger, small rubber deflector
Crank bore: Sleeved to 9.0mm
Head: wide hemi shape, 9.3:1 comp, fins turned down alot.
Liner timing: Boost blocked, transfer 132deg, exhaust 150deg
Plug: OS F
Fuel: 5% Nitro, 22% Coolpower Pink
Tank: 6oz DuBro clunk, sintered clunk, muffler pressure venting inboard
Run: 7500-7800 rpm launch, 5.5-5.3s/lap, mostly 4-stoke with beeps at 45deg elevation.

So far so good. Its a continuing experiment, so time will tell if it works out in the long run. Looks good so far!
Despite the apparent success at this stage, I wouldn't bother doing this again. It may have been cheap, but it took alot of hours and brain mashing to get it going properly. Next time I'd just get a Stalker or PA or Saito. Its alot of fun to have people say you can't do it, but do it anyway. I'm goofy like that If you try this for yourself, my specs should only be a guide. I can't guarantee it will work the same on another motor, even the same version. QC ain't what it used to be and the factory variation on these cheap motors makes them a lottery. Not to mention my loosey-goosey timing measurements! I rate luck as a significant factor in this engine's development.

Now to see how many flights it gets before the ring dies...