Hello,
You are probably getting your left turn from what is called the "p factor". Depending how much right thrust you have on your engine will determine how much left turn, yaw, or bank your model will produce. If you have it set-up with little or no right thrust, it will pull to the left regardless of what holes you have in the cowl. Being a pilot I've been fortunate to see the effects of the "p factor" from the cockpit. Even at low power setting while taxing there must be steady right rudder pressure to keep the airplane straight. Upon take-off and climb-out the right rudder pressure is at it's greatest, because of the high power, high angle of attack, and slow airspeed. R/C models are no different and share the same traits as thier full size counterpart. If you were to see the cowling taken off any single engine fullsize airplane you will see they to have right thrust and down thrust on the engine to aid the effects of the "p factor". Even if your model has some right thrust already it may need to be increased, but remember you may always get some left turn tendencies, it's the nature of the beast.