The throttle return spring is on the butterfly shaft and works similar to the rear brake return spring so unfortunately is non-adjustable.
The manual does say to lube the brake lever and throttle cables but gives no indication of what lube to use. A graphic of an oil can is shown and so I wonder if your PJ1 lube goes at all tacky?
Perhaps try some thinner 3 in 1 type oil, or maybe think about a throttle lock, like a Kaoko.
Look at this thread for suggestions. Throttle lock
The manual does say to lube the brake lever and throttle cables but gives no indication of what lube to use. A graphic of an oil can is shown and so I wonder if your PJ1 lube goes at all tacky?
Perhaps try some thinner 3 in 1 type oil, or maybe think about a throttle lock, like a Kaoko.
Look at this thread for suggestions. Throttle lock