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Running lean with new expansion pipe?

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A while back I bought a FMF Gnarly for my KTM 200 off Ebay, I have a feeling it may not be the correct pipe but managed to get it to fit regardless.

With the stock expansion chamber my bike would run a little rich. Once I installed the new pipe, I would get a nice brown coloured spark plug if I rode it at partial throttle opening and crisp throttle response. However if I ran it at full throttle, hit the kill switch and pulled the clutch in while rolling to a stop, the spark plug would be completely clean of unburnt fuel.

After a few jet changes, I went from a 160 main jet to a 170 with no obvious sign of improvement. If I put the stock pipe back on the bike would almost foul the plug from being rich.

I am sure there was no leak in the exhaust as I used fresh o-rings and ample exhaust sealing silicone to ensure a tight fit.

Is it normal for a new pipe to require such a drastic change in main jet size?

and would it be safe to run such a pipe should I find a main jet large enough for it to run properly?

Any advice would be appreciated

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