351 Cleveland rebuilt, new MSD ignition start but won't run

I rebuild a 1971 Ford Mustang Mach 1 351 Cleveland 2v heads roller chain decent cam I don't remember the exact specs on the can bored out 30 over with roller rockers 1.73 new push rod 150 compression all the way around New MSD ignition and coil I put cvf racing water pump and front serpentine belt 1 wire alternator. Kit the problem I have I can get the turn over and start will not run backfires seems like timing but I have checked at too many times to tell but it's right is right. I can get it the fire up sound like it's going to run I can get the barely run but it will backfire through the carburetor and that's all I can get any suggestions?

I also put on a new brawler 650 carburetor.

Thank you in advance for any ideas at all I have 12 volts at the coil on the Run position also on the start position. I called MSD they said everything is right with the distributor and coil. I got my power through the red wire with the green stripe off the relay.

If I remember right. The red wire with green stripe has 12 volts with key on but drops to 9 or so when cranking. I think that is the wire that went to coil from factory with a built in resistor to drop voltage. Make sure you have 12 volts all the time to that wire. When I installed my msd box, I cut the resistor wire out from under the dash to get full 12 volts.

Thankyou much definitely didnt think to check constant voltage. I ASSUMED

Will check after dinner tonight.

I know you said you checked timing, but you can't really set it with a timing light if it's barely running. To me it's either the cam timing is off, the distributor is at least a few teeth off or the firing order is wrong. I'd check those first.

Is it a ready to run dizzy or do you have an ignition box? To make sure you enough voltage run a jumper wire from the battery to the coil and dizzy. On my ready to run I fought the same thing you are until I got full voltage.

Firing order? 13726548

Counterclockwise rotation on distributor.

Heres the update...but first thankyou for all your ideas.

I did a bleed down test holy crap it was every where and not within 10 or 15 % diffrence.

Found 2 valves are sticking (after heads redone second time)

Going to lap them and hope for best.

I have full 12 volts at coil start and run positions, i do not have a power box. (Didnt think i needed it.) According msd also.

I found 2 collapsed llifters one lobe looks off from other on cam.

Comp cam 32-246-4

Grrrrrr this thing is making me learn?

Torn the motor back down to start from beginning i put it together 2 yrs ago and maybe i missed something. Im grasping. Im sure.

Thankyou all again.✌

Cam spec are .....cam adv dur 274. 286

Dur. @.050 230. 236

Lobe sep. 110

Valve loft. .562. .565

I have several engines NOT know it all guy but have never delt with such a hard start up.

Im sure how i messed this up. Used all break in oils and additives.

Oil smelled of slight fuel when i drained it, was wondering if just trying to keep starting wiped out something.

Lapping won't help with sticking valves, the guide liners are too tight (use a different machine shop). Leak down variation: was block bored/honed with a deck plate, rings correctly gapped and properly installed? Stop buying Comp Cams valve train components. Chuck

Lapping won't help with sticking valves, the guide liners are too tight (use a different machine shop). Leak down variation: was block bored/honed with a deck plate, rings correctly gapped and properly installed? Stop buying Comp Cams valve train components. Chuck

Click to expand...

Good tip on the valve guide liners and rings, but Dang Man - why so harsh on CompCams?  All my stuff is CompCams and is running like a champ!  

I thought i had a good machine shop till I found out they send out their balancing. I also took for granted the boring was done right. I'll check the tapper I did the gapping for rings. First time Ive done that. ill check those to be sure. I took my time when I did those but here I am so right now anything is posible. the lftets I got with the cam had rust on them so the machine shop gave me the ones I used. Im not sure what happened there.

The rings were installed per instructions and correct, though the gap could be in question.ill check them out tonight. Ididnt mention the piston are kieth black hypereutic(s/p) error im sure.

As far as the vaves they were replaced and new guids had a second machine shop checked

The heads. They did say the springs were to much for the cam.

So the inner springs weren't put on.

Again thank you for all the input.