I don't know how-long it's been since the PA VMax carburetor specialist did the work, but I bet the issue is a plugged pilot jet in the cylinder which isn't firing at idle or low-speeds. The hole is very-small. The pilot jet is in the left jet block hole when you're looking at an upside-down carburetor, with the float bowl off. The middle hole is the 'main bleed pipe,' which is the longer of the two brass jets; the right hole has a brass plug in it. The two jet block holes with brass jets in them also have rubber plugs in their holes, installed after screwing-in the brass jets.
I doubt it's the CDI box, I doubt it's a COP piece. The easy way to trouble-shoot that, is to switch the poorly-firing cylinder's COP with another cylinder's COP. Then, if the 'miss' moves to the new cylinder, you have suspicion that the COP is bad.
Most of the time, the CDI boxes work, or they don't. It does happen where the CDI box causes an intermittent issue, but that isn't a common occurrence.
Try swapping-in the original CDI box, and see what happens. To just see if the original CDI box allows the bike to start and to run, you don't need to remove the CDI box installed now. Just unplug it and plug-in the other (old) CDI box. You don't need to ride it anywhere, you're just seeing if the bike will run with the original CDI box. If it does start and run, and the miss is still-there, I bet-upon the blocked pilot jet.
This should help you:
http://vmoa.net/VMX12-Service-Manual01.pdf
https://www.ronayers.com/oemparts/a/yam/500456a5f8700209bc79313a/carburetor
I doubt it's the CDI box, I doubt it's a COP piece. The easy way to trouble-shoot that, is to switch the poorly-firing cylinder's COP with another cylinder's COP. Then, if the 'miss' moves to the new cylinder, you have suspicion that the COP is bad.
Most of the time, the CDI boxes work, or they don't. It does happen where the CDI box causes an intermittent issue, but that isn't a common occurrence.
Try swapping-in the original CDI box, and see what happens. To just see if the original CDI box allows the bike to start and to run, you don't need to remove the CDI box installed now. Just unplug it and plug-in the other (old) CDI box. You don't need to ride it anywhere, you're just seeing if the bike will run with the original CDI box. If it does start and run, and the miss is still-there, I bet-upon the blocked pilot jet.
This should help you:
http://vmoa.net/VMX12-Service-Manual01.pdf
https://www.ronayers.com/oemparts/a/yam/500456a5f8700209bc79313a/carburetor
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