Quote Originally Posted by evilstuie View Post
Well I've still got some misfiring issues I think. Going to hit the exhaust ports today with a laser thermometer and see if I can find a cylinder that's not firing.
I also bit the bullet and bought the HP tuners suite and scantool to help diagnose and tune. I installed a fuel pressure gauge, wideband and vacuum gauge, and can now see i've got between 7-11" of vacuum that bounces around wildly, and with the XR270HR cam in a 400SBC, it should be more around the 15-20" and very steady, so I have to go exploring after the misfiring is fixed to see what's causing it.

My next best guess is either a vacuum leak on the intake gasket, or something more sinister like a cam lobe being wiped, collapsed/spun lifter, valve seal or something else mechanical.
From what I've read there's also the possibility the low vacuum is because the valve timing is wrong. I've checked for vacuum leaks outside the motor, so maybe it is in the motor.
Looking to see if you're firing on all cylinders with a temp gun is very useful - correct if required
Check your initial and total timing. Engine can sound quite good but have a low vacuum.
How are you checking for vacuum leaks? What intake you using? - Perhaps a missed open port somewhere? (I've been fooled at least half dozen times )

Let me know,

Mikey G