I was told that if you add extra shims to the motor it will lower your temps.Is this true and if so how does it affect fuel economy?Thanks

More shims will usually improve mileage a bit and may lower temps as well.

Since I know where you race let me suggest removing one .1mm shim and going to 16% nitro. This should give you a big run time boost with little loss in power.

How many shims do you guys advise for a Axe Rossi X3 using 16% of nitro.

I would think that engine should already be correctly shimmed for that nitro content.

im 37 this is my first car how do you shim an engine

Well since you are 37 I wouldn't worry about it If your maual doesn't tell you what the correct head clearence is for nitro percentage or how many shims to add then I call the manufacturer.And if I can't get ahold of the manufacturer I ask the guys that are running the motor or have had the motor.Most engines come shimmed for 16% and 20% nitro.You have to measure the head button clearence to get the shims exact.Or you can add shims till it doesn't detonate which will leave pitting in the head button and piston.I have not tried this but most people I believe take a piece of solder and stick it in the glow plug hole and turn the crank over to tdc then pull the solder out and measure the compressed peice of solder to determine the exact amount of shims needed.Best bet is to call the manufacturer as they are the ones that built the engine and ask them.