I have a Cline Modified Tarten Twin (glow / 0 nitro) which has never really given too much trouble.......I have until an unfortunate landing run a Bolly Carbon Fiber 19x10 prop which the engine would turn about 84-8600......Which is really the sweet spot..............

Now that I'm waiting on a replacement prop..I've tried a few others...
A Zinger 20/8 wood which only turns about 7300 ?????
I also tried a Zinger 20 6/10 and cut it down to 19 and could only get 7300 rpm..Both really load the engine and it runs hot...

.Why would a light wood prop turn less rpm ??? I'm going to try a zinger 18/12 to see if it will go 8000 ....

I've gone so far as to add more oil to the fuel (alcohol) to see if it will help disipate the heat..No help ...........

The Bollys are a little hard to get for me....Any suggestions on any other props or combos to get the RPM up......This engine is on a 30% extra.

BillJ, sometimes I think that people put too much emphasis on how fast an engine can turn a prop. You can lightly load an engine enough and get extraordinarily high rpm figures but this does not necessarily equate to performance. I used to fly a Byron Pitts with a Tartan twin on glow with no nitro turning a 20x10 Zinger. I don't remember what the max RPM was but it was a terrific performer. I never had any over heating with this combo because it was running on glow fuel, which runs cooler than gasoline, and I had very good air flow through the cowl. Most of my running was in the air and not on the ground. Of course the engine picked up quiet a bit of RPM while flying. What is your cooling air flow situation when you are experiencing the over heating problem?

Actually adding more oil to the fuel probably adds to the problem by creating more friction thus more heat. The vaporization of the alcohol does most of the cooling. As I recall back in the days (1985-87) when I was running the Tartan, I would buy a gallon of FAI fuel containing 21% oil and cut it with 2 gallons of methanol making 3 gallons of 7% oil mixture. How much oil are you using? Before you go try 7% oil lets here from some other Tartan twin users to verify or correct that figure.

Ken

Try a Zinger Pro or a MA wood Simitar.
In the same size they will turn better than a square Zinger and make better thrust .