Running a hp500, jumps up out of the hole ok, runs up to 3800-4000 rpm then starts to sputter and back fire; increase throttle and goes no where.. New plugs and fuel filter, tightened fuel line fittings today. Going to check rail pressure in AM. I'm thinking fuel pump, thoughts??

might be,how many hours on the engine,they need valve springs at app 250 hrs.

307 hrs, did springs in April. Has run strong all season. Started losing power while cruising, but was jumping up 300-500 rpm intermittently. Seems like fuel issue

pretty easy to check the fuel pressure.

I agree valve springs 200 to 300 hrs max on these motors as they need to be changed, even if they are not broken they become weak. Pull your valve covers to check them closely to see if the are broken or some other related problem. Also compression test and or leak down test would be a good place to start., then a fuel pressure test. You could do more internal damage if any valve train parts are not up to snuff if you keep running the motor. I am ruling out a fuel delivery problem but need to rule out the valve train- related parts first as they can be a real problem child with your app.. I have so many pics of broken springs over the years from Hp 500 carb and the Hp 500 EFI engines.

You already replied and see U done the springs - it takes a while for rne to type. So move on to your fuel pressure checks.

Dist cap and rotor would be the first thing I replaced or at least checked.

If it is the original distributor, change out the electronic module inside the distributor. The old module was not sealed, the new one is sealed. Had this problem on the way to the pool party during TX Outlaw Challenge. You can tell by looking at the connections, if has wires connected to nuts, it is old.

Sounds like its leaning out to me

Efi model?? headers??? check your headers,,make sure your not ingesting water..merc had a problem back in the day with the headers..

BTW they would leak internally and kill power..