How many miles on your Grom ODM?
Removing and or replacing the spark plug should make no difference on the sound
I would take out the spark plug and check it for cracks or damage and check the spark plug gap.
If you have a compression checker I would check compression and is should be around 95 PSI which is not the true compression since according to BrianS there is a decompresser unit on the cam that reduces the compression on start up, READ BELOW
BrianS post on the issue
Don't panic about the compression. It's about 160 PSI stock. The Grom has a ACR,(automatic compression release) on the exhaust rocker. Spinning the engine activates the weight on the cam and opens the exhaust valve just slightly. I don't know why they don't mention it in the shop manual. Using an intake rocker on the exhaust fix's the issue, and you get 160 PSI. It's there for starting.