351 backfire at 5000 rpm
I have just recently rebuilt my 351 Cleveland. I had an issue with her popping out the left exhaust and after pulling it down found she had a stuffed no. 5 piston. So I purchased a short off a mate that has flat tops and a mild cam, put a set of 2V closed chamber heads on it, they have been ported and polished some what, not over the top at all and they haven't been flow tested. I have an elderbrock performer on top with some porting done also, 1 inch open spacer and a Holley 600 vac on top. She has also got 4 into 1 extractors, electronic ignition, top loader and a 9 inch running 4:11's and locker.
She was running ok but would break down at 4000 rpm and found my float levels were not good so adjusted and she was sweet. She would pull hard in all gears to around 6k after that fix.
Then I started having a high tension miss, which I didn't know what it was so I changed the carby to a Holley 850 dp I have but was way too rich and poured juice out the exhaust. I also tried another Holley 600 vac from my dad but same issues. I then installed a billet full adjustable crane ignition and dizzy. But she still had the same issues. I then found a main wire from my alternator to starter had earthed. I fixed that, put on a new alternator as the other wasn't charging and the high tension miss was gone but now I have another issue.
Im running the 600 vac from my dad still and she pulls hard till 5000 rpm then I get a backfire through the carb. I have played with the curves of the dizzy and have it set to 12 idle and around 36 at wot. I have also changed the plugs and leads. Not sure what would be causing this. I have played with the carb some, checked all vac lines, played with the timing but still get this backfire. The best I have got is 5500 rpm before it happens but can't get rid of it. After the backfire she breaks down a tad also and feels like it did with the fuel bowl issue earlier.
Sorry for the long book but any help would be greatly appreciated as I have no idea what it could be. It has been suggested on another site that it may be my valve springs not coping but seems weird as it originally didnt have this issue.