Alright I have built many SBC's and have done a lot of work on them. But this one just has me lost. First off it is a 1986 Chev 350 in a 1/2 ton pickup, We did a top end swap on it and cam upgrade. Went to a .490 lift cam 230 duration at .50. Also topped it with small chambered aluminum heads and matching holley intake. Well the issue it if you pull plug number number 1 and bring it to TDC according to the balancer it is at the top. Was not checked with a degree wheel but was looked at without the heads on. Well the engine with 12 degrees of base timing will not run for crap, backfires and make no vacuum at idle, only above about 1500 rpms it will make vacuum. It also goes very lean while trying to break the cam in. But the wierd part is when you shove it up to 57 degrees of timing it makes around 12 inches of vacuum at idle and revs fine. Not near as lean either. But it should make more vacuum then this, and the timing part is so confusing. It has a MSD Dizzy, MSD 6AL, and MSD blaster 2 coil. Any input on this would help greatly, I am running out of ideas. I dont really want to buy another cap just to cut it up and check rotor phasing if I do not have to. Also going to check the polarity of the magnetic pickup tonight, to assure it is correct.

Thanks
Adam

 

Your cam might be off a little bit. Did you make sure that you aligned the dots. You didn't degree it, so that might be the case. I worked on an OBS truck that had similar symptoms, and it turned out the timing chain jumped a couple of teeth.

 

Does it have the correct balancer and pointer? You need to fab up a piston stop and verify TDC and make sure it matches balancer and pointer. Possible that outer ring has slipped on balancer as well.

 

i would start there, sounds like you have the wrong balancer on it, or the dots are not lined up exactly!!!