You have two choices.  Seal your plug into the aluminum manifold (which is what I would do) don't have that insulator plate bolted on.. 

Or make a solid plate to bolt on in place of the carb over the insulator plate and don't use the plug at all.  If you do that you'll need a gasket on both sides of insulator plate..

Making a block off plate is a waste of time if you ask me because on that model if you have an air leak there it's most likely a warped carb flange. 

Sounds like I’ll be taking off the manifold then! Thanks for clarifying! When I look at the part diagram I cannot find the manifold or gasket on there so I’ll just have to find one on eBay to replace when I re-install. Should I apply yamabond to that gasket as well? Hopefully I can get it done tonight before heading off for the weekend. 

Look under the "Air Cleaner" section for the insulator spacer (aka "manifold") and gasket.  O-ring is shown on carb diagram.

You can test for leaks there at those joints with engine running using various "vacuum leak" techniques. 

This has been covered extensively in another leak down test thread in the last week!! Try first checking you have not over-torqued the carb mounting bolts so that you have bent or bowed the reed mount surface. Is you have, flatten that on some 600 wet or dry paper, and check your big fat O-ring in the throat of the mount, there, too. May not need new parts! And find the recent thread!

I’ll do some investigating tonight 

Finally going to get to taking off manifold/spacer tonight. It seems to be on there pretty good when I tried to pull it off quickly another night. I am planning on just adding some heat and pulling it off and then off to the races on hopefully not finding any leaks! 

Well, this spacer really does not seem to want to come off. Does anyone have any tips for removal?

Removed the manifold and tested again.. my system had leaks that I didn’t spot before so I used some more thread tape and retightened. It is also leaking where it threads in at the spark plug so I have to figure that out l. But with all these leaks it still took almost 8 minutes to go to 0 psi. So it is looking hopeful! 

Fixed my leaks and tried again. Currently have only lost half a psi after almost an hour! So I would call that a success. I am going to leave it on and see how long it hold. After I am going to do a vacuum test. Do I put the carb spacer back on before doing a vacuum. I'm thinking it's a good idea to drain oil before the vacuum test as well? 

Pressure held up for over four hours before I unhooked it and did a vacuum test. The vacuum test slowly leaked down to 1 psi after 3 hours or so and I called it a success. So I guess I’m done with this adventure. Time to put carb spacer back on.