We were recently asked to look at a problem.
Bike did 200 miles fine, then while stationary the note changed and it would’nt pull away.
It would start ok but not run.
Spitting back showed unusual pressure in case.
It had New carb and Rebuilt mag.
We took off silencer, it still would not run properly.
The inner baffle in the Howarth patten silencer was retained by two spring clips. These were not effective and the baffle could slide back and block the outlet.
Pressure built up in the system. The inlet to crankcase seal was blown out, the crankcase was pressurised and did wondrous things to the breathing. so we had now two problems.
We changed carbs, mags etc until we found the original cause.
So, if you have a pattern Howarth, I suggest you dismantle it and make sure the baffle tube is securely anchored.
We keep learning!