Twin 489 strokers. Xm284hr cam. L29 heads. Timing 8 initial. 28 all in. Holley 750 vauum secondaries. One runs great. Other gets to 3000 rpm and backfires. Idle hard to set on that one as well. Was running fine. Changed vacuum spring and jetted up 79p 76s sec-86p 89s Found small tear in vacuum secondary diaprahm but the are not even opening when backfireing occurs. Whats up. 1 hour run time

is it happening under load? Or just when rev'ing up in nuetral?

alot of things could be causing this.from carb to 1 or 2 tight lifters or ignition problem.maybe pull plugs and compare them all.

U swap out carbs yet

Happens under load

No. We were going to but figured we wouldnt be able to turn without power steering

I have some quickfuel 750 dp sitting for another project we are going to toss on and see if it has the same results

Quick fiel 750 hr carbs fixed it. Still needs fine tuning Rpm acting weird on port engine. Everything is good to 4k then the port climbs with thtottle but stbd stays. Can hear both revving. Port oil pressure in neutral will raise to 65 at 4k. In gear 40 max. Possible grounding issues? Gaugrs are Ss comp

I'd put a mechanical oil pressure gauge on and verify pressure.as for the strb engine,,was it the one that was popping?

Sorry port was was that was popping and wont go up on oil pressure in gear or give consistent rpm