1981 1000J Engine Dealer Show room Poster

I have worked on several J motors from various model types and they all appear the same. The only difference is slight tuning differences for the carbs that I have noted... one will have #122.5 main jets, another will have up to #127.5 main jets. That tells me that the airbox is slightly more restrictive from one model to the next. How did they get 10 HP difference between models?

It's the valve timing that is different in the J model vs the LTD and the CSR. The two latter kept the milder cam timing from the previous era.