ORIGINAL: Nik.
hey Ive got an STS .21 T Dadders mod stage II, and it just wont transition. Top end is fine, idles fine, but will NOT transition. Used brand new OS p3, and p7's. 20% trinity platinum. Mixture seems to be fine, only runs 3-5 seconds when fuel line is pinched. when throttle is applied, seems to be MEGA rich, but any leaner it quits and temps shoot up. No air leaks. Whats up?
Do the normal procedures to RTV seal your engine. This is a good practice, no matter what the engine or how many gallons it might have run through it. Always attack all air leaks and all possible causes for air leaks. Doing this will allow you to rule out certain factors (punctured fuel tanks, pierced fuel lines, engine screws not tightened properly, etc.).
While RTV sealing your engine, dismount the carb and thoroughtly inspect it. Take out all the needles and apply Associated Green Slime to the o-rings. In particular, check the LSN ant its o-rings. Does the LSN have a proper fit and feel when Green Slime is applied? Does it screw in and out normally?
Inspect the tips of all the needles and make sure they are not damaged in any manner. If any of the needles and o-rings are faulty, replace them.
Once you have done these simple things, reinstall the carb and try running it again. Does it still run away once it warms up? If so, check the LSN fit again. STS might have installed a faulty carb on your engine. The LSN hole might have been milled to big; I think teflon tape on the outer LSN o-ring could correct this. If not, try to call STS and get a replacement carb. They should send you a properly milled and machined STS big block carb.