I have a clark cg25 with a 4 cylinder mitsubishi engine. It backfires through the carburator consistently while under load. At idle, it runs fine. I checked cam timing, and looked at all the valve springs. Compression tested around 100p.s.i. all the way across with 4 puffs a piece. It sounds like an early head gasket failure across cylinders 2 and 3. Are the heads prone to cracking? should they be milled? I hope some of you fleet guys can weigh in. I also replaced the coil and cap.