first place i'd go is fuel delivery. check hose connections, pump push rod, and pressure.to my knowledge all mech pumps that don't need a regulator are marginal and some are horrible, like the stock replacements. I modify carter 6903's and I think they're borderline for 450hp. i'm just too stubborn to go electric/regulator. mech pumps do better cold than hot. they'll lose a pound or so in psi when warmed up. I have one of those high pressure carters, 4862, that's a real gusher but it would have to be regulated. finding regulators that work good with pulse pumps has been an issue for me.