I have 2003 sportster that just had a leakdown done on it and there is 70% loss on the rear cylinder. I wanna up it to a 1200 or 1250 anyway so my question is,,, are the big bore kits for sale gonna help with this problem or do I gotta fix this issue first???

 

Depends: that kind of leakdown could be a combination of issues. You may have worn or gooked up rings leaking by as well as worn, leaking and/or burnt valve (s) leaking by as well. Need to more accurately diagnose what your issues are.

Leakdown test results are an indicator of general engine health; there is more to it than just looking at the number. Does air escape through the exhaust pipes during the test? Through the intake? If through the intake, is it really the crankcase vent or is it from the intake valve? If you open the oil tank does air escape from the fillvhole during the test? All of these things narrow the issue down further and point the direction one should go when tearing down to inspect/diagnose.

 

Boring to 1200cc's would fix it if in fact it was a cylinder/piston/ring issue.
But then it could be a cylinder head related problem, in which that too could be attended to as well at the same time the cylinders were being bored.
Keeping the 27T 883(?) trans spocket on a 1200cc conversion will allow you to SMOKE a standard 1200, FWIW.
Scott

 

with a 70% leakdown,it should have been easy to determine what the problem area was as you should have been able to easily hear where the air was escaping to,out the carb or pipes,valves/seats.out the timing hole or breathers,rings