Argess

What you're experiencing is typical of the Force 26. Very touchy to get a tune where for power without getting too hot or too lean. That high idle after WOT points to the HSN being a bit too lean. But as you found out, if you richen it, the engine is a dog. One of the reasons the carb is so touchy is the stock cyl head is inadequate to get rid of extra heat. Compared to teh Force 26, there is way less surface area of the fins on the 26, yet the displacement difference is only minor. Temps can rise without warning when tuned close to optimum. This enigne will detonate at about 330F, but even below that, it seems touchy to get the tune right.

I used an aftermarket head to keep the temps down, and it seemed to help a lot. Using an OS A5 glo-plug also helped as it reduces the compression ratio slightly.

Eventually I tried an extra head shim (all the time using 20% nitro fuel), and the temps dropped a lot and the carb seemed esasier to tune. However it was an old enigne by that point, and I didn't think to re-break it in with the new head shim, so running WOT at much colder than normal temps, the crank pin broke. Interestingly, that failure is common of Force engines. Oh, and with the extra shim, I didn't notice any lack of power, so I think I was doing the right thing with the extra shim.

I wouldn't hesitate to try it again with the stock head and an extra shim (and an MC8 plug as you don't need the short plug to reduce CR with the extra shim), but do another break-in before doing any WOT runs.

I also have heard that the enigne will run cooler and make tuning easier with 30% nitro, but havn't tried that. The MC8 plug is recommended, but as I said, I've had better luck with the OS A5.

ps: For interest:

I did some comparison of the Force 26 with the Force 28. Rough approximations, but close enough to get the picture:

1/ Force 28 has 7.7% larger dispacement than the Force 26

2/ Force 28 has 12.5% more power than the Force 26

3/ The Force 28 head has "approximately" 32.3% more surface area than the Force 26.

4/ I have had no troubles with overheating from the Force 28.

Additionally, as I have had incredible success with the Force 26 after changing the Cooling Head to an after-market unit. Being curious, I invetigated and did a small amount of testing which I thought might be of interest here.

Naturally, the improvement can be considered to be due to the increased surface area (increased diameter and more fins). However, the results were so dramatically improved, I had to wonder if more is in play here.

The after-market head is made of machined billet aluminum which has a thermal conductivity of approx 200 W/mK. Pure aluminum is about 230 W/mK (higher is better). The original head appears to be cast, which has a much lower thermal conductivity. Exact specs are difficult to find as there are significant factors when casting metal, however 80 - 100 W/mK is typical. (Extruded aluminum is anywhere from 120 - 150 W/mK, but the engine head shape doesn't allow for extruding.) All in all, the improvement of adding a machined head over the cast head approx doubles the heat transfer yielding a much cooler engine without any consideration to the increase in surface area. The inceased surface area just makes it that much better.

The addition of a thermal conductive compound between the glowplug head and the heatsink should give a 20 - 30 times improvement in heat transfer to the fins. I havn't tested the Force 26, but i did try it on a Nitrostar 25 (in a Savage 25), which has the same mecahnical two-piece head design. Results were dissapointing. Somewhere between a 10 to 20 degF decrease. However the Nirtostar 25 has a machined head, so the test results may show better improvement with a Force 26 cast head, as heat transfer to a cast material is not as good due to difficulty in machining cast material and exposed air pockets. However, it is without a doubt that the addition of a heat transfer compound does help to some extent with a 2-piece head, and probably more so when the head is cast.