I'm at work ATM so I can't take a look but I just rang the local mower repair shop and spoke to the guy who helped me get the engine going again and he said that the 33s don't have a cork in them. He said he can come and have a look for $20 but said if it's buggered, it can't be fixed. He did tell me to try adjusting the lever a bit so I might have another play with that.

It sounds like I might need to let the screw out. Bit more? Is it a fine adjustment or is there a fairly big range it will still work in? In other words, should I be trying to adjust it a tiny bit at a time?

Thanks