There are 3 things needed for your engine to run. Air/fuel& spark. With spark it should be blue in color, yellow to orange it is a weak spark. Check the color of your spark, make sure it is a blue color. If so than coil is good. The CDI only tells the coil when to spark so if you have any spark the CDI is good. Assuming the cam is properly timed and valves properly adjusted this leaves the fuel delivery. I would not expect the tank since your problem is not running out of fuel in the carb by what you describe. Reading the plug should be done using an old colored plug. A new plug will take many runs to properly read. Since an old plug is already colored a single run will show change to color immediately and be easier to read. The 2 areas to look at is the base of the threads and insulator. And as stated before there are only 2 tests for checking the plug. Now for your carb, is it a genuine Mikuni? I believe you said it was a VM-26. What series is it 606? Was the carb purchased new or used? If used what was it used on? Unlike a CV carb to where tuning is basically done by changing the main and pilot jets the slide carb is much more tunable. With the slide carb besides the jets you will also consider the slide, metering rod, fuel needle and fuel seat to properly tune the carb. Was this carb used on the old engine and if so how did it run? I know you are frustrated with limited time to work on it but you have to be patient. It's a learning curve.
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