So... if this were put into stasis, would the effect still take place? How would Adrenaline affect this (assuming that adrenaline still takes place- I'm a bit confused on the "Using anything to add ATK to this creature is utterly pointless" part)?
I like the idea, but it also seems a bit UP. at 8 or 10 you could play a shrieker or dragon, both of which seem to be superior- no waiting period as the machine charges up, and no fluctuations in attack. While this does have a good synergy with Overdrive (unless "Using anything to add ATK to this creature is utterly pointless." negates that, too), what niche would Coal-Burning Machine fill in ?
Increasing this creature's ATK stat is pretty useless - once it reaches 10/13 ATK, it starts to decrease again. Overall, it will probably deal about two or three more damage, but, eventually, it will even out again. I didn't mean that as some sort of hidden clause; it was more of a note that I wanted to point out. If I had said "Using Adrenaline on a poisoned creature will make it die faster", it is true, but it doesn't mean that using Adrenaline on a poisoned creature will make the poison go any faster in addition to the poison + adrenaline combo. I am not sure if I am making this clear, but while it is able to gain or lose ATK, it will eventually be negated naturally.
Using Overdrive on this would just turn it into a creature with Overdrive. Fuel or Cool, depending on which is active at the time, would be removed. However, since it gains 1 HP per turn, it wouldn't be a terrible idea to use Overdrive on it later (although it wouldn't be practical when there are already other options in Gravity with more health).
Adrenaline would take into account its ATK at the beginning of the turn, then it will attack the number of times that would correspond to that. If a 4-ATK creature with Adrenaline attacks for 4, then 3 (75% of 4, the original ATK), then 2 (50% of 4, the original ATK) damage (I am not 100% sure, but I think that that is correct), then a CMB with Adrenaline that starts the attack phase with 4 ATK would attack one time for 4 damage, then one time for 5 damage (7*.75=5.25), then one more time for 5 damage (10*.5=5). Its ATK is now 8. As for the other four values, feel free to calculate them on your own.
Do a creature with Acceleration's stats change if it is delayed? Whatever the answer is, that is the answer to your question.
I am considering adding a clause saying that any outside change made to the creature's ATK will also change the boundary values for Fuel and Cool (4/7 and 10/13).
CBM constantly gains health, making it a rather stable creature, and it has an average damage of 7/10. It is a more stable creature than Shrieker with only one less ATK for the unupgraded version, but the cost and the unreliability make up for it.