My WR400F has been popping ever since I bought it when I rev it up and let off the gas the rpms slow down and the muffler makes a popping sound. It must be the jetting but I don't know what jets to use or how to put them in the carburetor. What size jets and should I use? Will that fix the problem? Also my rear tire is pretty worn in the middle section so I will probably be buying a new rear tire. My tire do you recommend for trail riding and good traction or good all-around tire. I found a dunlop D739 for $58.20 + $12.50 shipping on EBAY. Is that a good tire? It's suggested retail is $97.00!