I put the exhaust tube on my magnum and started tuning it. I had the high end perfect, then fixed the bottom end and got it right. I double checked the top end by holding it up and made a small adjustment and it started surging. I put the needle back but it continued to surge. I noticed that it ran fine through all the rpm range except for wide open. I happened to pinch the exhaust tube a little and it picked up rpm. Am I losing muffler pressure from a leak somewhere? What else could make it do that? I will try another glow plug first to see if that helps.