First, the fact that is a gray market has nothing at all to do with the problem. It could be pure orange, red or blue and still be doing the same thing. As Ive known for over 12 years, and this forum has alot to do with, Gray market tractors are nothing more than a title/stigma.
2nd, are you checking the VOLTAGE that the battery is holding and the VOLTAGE that the Alternator is putting out, or are you doing an amperage load test of the electrical draw of your starter? I am guessing you are probably doing voltage checks and just have the verbage a bit off.
On a good battery, sitting with no electrical load, you should expect to see between 12.4- 13.2 VOLTS DC. A good charging alternator will put out in the 13.2- 14.5 VOLTS DC. Alternator test can be done either on wire coming out of Alternator on on the actual battery post.
When you preheat your plugs you are putting one hell of an AMPERAGE draw on the battery, and this is normal. Even though a regular car battery will handle this just fine for quite some time, in time as a battery gets older the preheat draw will cause a battery to maybe not have enough strong turn over power after a preheat cycle. Deep cycle batteries, Tractor batteries or the like are more suited for all for this. But I am not trying to stray you away from a car Batt if that is what you have. Its what i use. But I do know how it will start to act over time.
IF, you have proven that the Battery is good, and the alternator is good, I would tend to look next at the fact that maybe the starter is pulling much more amperage than it should be. I have never been inside a Kubota starter, But many import starters have a set of contacts, that when start to go bad will act exaclty as you as discussing. IF the Kubota starter falls into such a category, it is possible you may be able to just replace its internal contacts.
This is a source I commonly use for replacing import contacts. And yes they even have a nice write up on how to do this job.
http://www.nationsautoelectric.com/index.html
I hope this maybe points you in the right direction.
Mike in Alabama.
