I had the same problem on my 3w 70. I was told a lot of things, but this is how I fixed it. There is not enough back pressure on the engine. Close off one of your exhaust ports and see if it fixes the problem. It did on mine. To test It, I shoved a paper towel in one side then put a piece of plastic over the exhaust tube to hold it in place. It wont last long, but long enough to see that it works. Once you see that it works, you can either pinch one side closed, or if you want it to look better make a metal cap to go on the other side and use a hose clamp to secure it.