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Old 10-27-2004, 03:15 AM
Join Date: Mar 2003
Location: Tokoroa, , NEW ZEALAND
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Default RE: GMS Engine Tuning Problem

I just spent the afternoon trouble-shooting a GMS76 (ringed) engine that was behaving very strangely.

After about 2 minutes in the air it would behave as if the throttle were cut back to about 1/2-2/3.

My first thoughts were that it was overheating, but it had done over an hour's running and richening up the needle until it was burbling wouldn't stop it from losing almost 50% of its power. What's more, it didn't have that usual "strained" sound that an overheating engine tends to produce -- but it did have an erratic metalic knocking noise.

We checked the fuel-lines, played around with the mixture, propellor sizes, tried two new plugs, and just about everything else we could think of -- all to no effect.

The guy who owns the engine was using straight 80/20 fuel with Klotz Super Techniplate oil and he tried mixing up a new batch with methanol from a freshly opened brand-new drum -- no different.

Then, when we tried some of *my* fuel, the thing ran like a dream with no signs of any power loss or other problems.

I'm using CoolPower blue (synthetic) that I've blended in a 2:1 mixture with castor oil (making 66% synth, 33% castor) and 10% nitro.

Since he didn't want to toss out the gallon of Klotz-oil fuel he'd just made up, I suggested that he add a few ounces of straight castor and a squirt of nitromethane.

This did the trick. The little bit of additional castor (about 5%) and the small amount of nitro (about 5%) completely changed that engine's performance.

I know that the Klotz SuperTechniplate has a small amount of castor in it -- but obviously it's not enough (for this GMS engine anyway).

The original poster doesn't say how much castor he's running in his fuel but if the engine is relatively new and seems to be sagging, try throwing a few more ounces of straight castor in -- you might be surprised -- I was!

I don't bother buying CoolPower Pink or Klotz SuperTechniplate because I think the levels of castor in them is too low. It's better (IMHO) to buy straight synthetic and add your own castor. This way you can vary the ratio to suit the engine. On my TT42GP I use half castor, half synth, on my TT46GP I use 2/3 synth, 1/3 castor, and on my little norvel I use all castor.