i had to re;engineer my SBP shifter kit to get the clearance an adjustment needed
frankly it was a pain in the ... (yes my engine is a 5/8th shaft engine)
I had the same problem with my SBP + 4G setup early on, but I simply switched to halflink chain to solve it.
Initially I needed 12.5 full links, but 25 half links worked better. After a couple years I switched the 17T jackshaft input to 18T and now run 26 half links. Running full link chain is impossible with my 11T->18T combo (always way too loose), but halflink chain does the job great. It was very tight with engine mounted as far back on mount as possible, but it ran awesome! The chain has since stretched/sprockets wore in a bit and now it has perfect tension. Over 2K miles on this setup.
anyway i'm guessing you are talking about the short
chain from the 4G drive to the jack shaft gear as maniac said it SHOULDN'T be that
tight so you need to make a new length of chain add 1 link
I don't think OP is even talking abut a 4-stroke setup tho, as the SBP shift kit was originally designed for the chinagirl and is so used on many many more setups here than our SBP+4G rigs. Also, adding a link may not help at all when using a full link chain due to the nature of full link chain and certain sprocket sizes and distances between them.
So today was the first day i started my MB. The problem is it is ridiculously hard to start it! It feels as if the chain will not let it start with me pedaling as hard as i can. Then once ive turned green and became the HULK i am able to start it but it doesnt want to stay on unless i am 100% open throttle. If i stop it, it stops.. So my newbie question is how tight should the chain be coming from the engine to the jack shaft on a SBP kit. Mine currently feels so tight that you would think its welded together. I know it is suppose to be "tight" but yeah. Can this be possibly causing my engine shutdown due to the inability to move freely?
The chain being very tight will definitely increase drag on the engine during geared operation and likely shorten the life of your jackshaft and jackshaft bearings, but as I also have run with an extremely tight primary chain without any adverse engine performance, I'm inclined to agree with Pablo that your problem isn't related to the jackshaft. My reasoning, assuming you have a 2-stroke with SBP kit:
With clutch lever locked on a 2-stroke kit, the jackshaft tension should have no effect on whether the engine runs or not. Remove sparkplug and check engine crankshaft for excessive tightness. Also check the "bucking bar", aka the clutch actuator pin in the cover underneath the clutch actuator lever. This pin/dowel must be greased regularly because if it gets dry, will reduce any freewheel ability. You will probably need to start the engine with bike on a stand; with rear wheel off the ground to make sure the engine runs okay before blaming it on the drivetrain.
Also, double check that all chains are straight and track true. Pics will definitely help!