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.

When you say you pinched the exaust tube, do you mean you pinched the silicon pressure line to the tank or do you mean a plasitc diverter tube on the exaust itself?

I have about a 14 inch piece of tubing on the end of the muffler to direct the exhaust out below the floats of my plane. I pinched on it closing about half of the diameter off. This made it rev to a normal rpm. I did take the baffle out, but it was running fine even after I did that.

It really sounds like with that tube the intake/exhaust flow is not getting the little flow stop shockwave at the right time. It is like changing the timing on an exhaust valve, these little engines use a shockwave of backpressure to stop the exhaust an instant before the piston closes the port and aids in combustion. Try a smaller diameter tube for about the last 3 or 4 inches, still get the great baffle out sound and maybe better WOT.If this timing is "out" it would be worse at WOT.

I wonder if I ziptie the end of the tube that it will make it work like when I was pinching it? I guess its worth a shot.

With a smaller diameter tube you could adjust the length of it to the maximum rpm and presto one home made tuned pipe

Now I need a tach, and a good excuse for the wife!

IS there any chance the shear length of that extension, if it's a 2-stroke, is causing the issue?

Jack

yuppers with no baffle its just spreading the pressure out over a large area lowering compression and not allowing the glow plug to hold as much heat as there is not enough hot exhaust being trapped in the combustion chamber.

When he pinched that tube he raised the pressure stopping the exhaust sooner allowing the glow plug to heat better, and the fuel to burn better. adjusting the length of the restricting tube will allow him to time the exhaust flow for the best heat vs flow ratio, which by the way is only useful at WOT

the reason a standard muffler works so reliably is the design guarantees enough pressure to always trap exhaust, a tuned pipe cant do that they can easily restrict to much slowing intake or allow to much exhaust flow causing a colder plug and lower compression with the setup he is using it wont work as well as a true tuned pipe, but since he is using this setup he may as well tune it the best he can

Another remote possibility that you have probably already checked is a pinched silicon line, which might cause a loss of pressure to the tank, but you would think this would show up as more than just a surge at full throttle. Just a thought. If the exaust is the problem, like Phoenixangle said, you could take an ink pen or small marker top and shove up into the tube and ziptie it in place, then drill a hole in the end of the pen top to adjust the diameter of the opening till you get what you want.

I threw the baffle back in to try that. I don't care if this plane is loud, but I do want to try to keep all the oil from getting all over it.