ezstriper

you need to work on accel pump shooters and cams(accel pump), not enough shot or at right time for those open intakes...

Thunderstruck

Backfire out the carb is an ignition/timing issue in my experience. Maybe too much fuel from a power valve or needle and seat leaking but the motors wouldn't idle.

Easy to check the accelerator pump but I would think the motors would stumble and die or backfire out the exhaust if leaned out. Too much pump shot could cause an intake backfire but usually the holley carbs accelerator pumps are pretty close from the factory. I have 950s on 502's and never had to mess with the accelerator pumps.

Do the motors idle OK?? What happens when you rap the throttle when in neutral?? Does it backfire then or only under load?

What happens after you are on plane at low rpm and you push the throttles forward quickly??? What happens after you are on plane at high rpm 4K or so and push the throttle forward quickly??? (If accelerator pumps are the issue you should get a weird throttle response accelerating after you are on plane, not just coming up on plane.)

Do the rpms on both motors come up together when accelerating??

fbc25el

Pretty good size cam. It might need more ing. timming at idle.

jackhammer

I had this same problem... chased a lean pop all throughout the fuel (even bought new carbs). It was finally fixed when I replaced the ignition. I don't know what ignition part it was because I changed it all at the same time. It was one of those HEI ignitions with the coil built into the distributor cap. Went back to Thunderbolt and all is fine.

A word to the wise... don't look down into the carb to see if you're getting good squirts of fuel (while it's running). I can still see the flame as it started just below the carb... within milliseconds it reached my face with the force of a shotgun blast. Singed my nosehairs! This gets filed under 'stupidest things I've ever done" lol

Throttle Fever

Lean pops out the carb. Fat pops out the exhaust. Timing Timing and squirters. Get base timing up, readjust all four screws and idle. Keep working at it you will get it.

Sport 280

Will work on carbs again on Friday. Carbs are brand new, only changed power valve from stock 6.5 to 4.5 (figured with bigger heads and cam than a HP500 likely have less vacuum) and re-jetted same as an HP500. Not running lean, in fact running a little fat. Using Thunderbolt ignition which was working fine on HP400s I replaced. Hope to find the problem, been working on this re-power project for almost 9 months.

Thanks again for all the help.

Throttle Fever

What end are running a little fat on? I try and run timing 34 to 36 degrees all in at 3 - 3500. Then see were base is. Not sure with the Thunderbolts do not use them much. MSD wise with a cam that size we base time at about 16 to 18 degrees and 34 all out, or just lock 34. Some good advise here. Some times you can nail it rite off the get go other times it takes a little tinkering.

ezstriper

put a vacume gauge on it, power valve should be 1/2 the idle vac amount, don't think the 4.5 PV are helping you I would go back to the 6.5, you still should have over 12" vac at idle, just pay close attention to the the accel pump shot(not running as said before) if you can limit the amount of advance to run more initial will clean the idle up if that becomes a issue from the cam....Rob

Hammered

Having the same problem with my winter modifications. Started with stock 454's 330HP, Replaced the intakes with edelbrock RPM Airgap Dual plane, Replaced the carbs with Barry Grant Demons 750cfm and Replaced the ignition with MSD Dist, Coil and box. Cranks fine & idles smooth but have to be really slow coming off idle or it will backfire thru carb. once at higher RPMs it will rap up fine but try it from dead idle and it will backfire and usually die if you dont brimg the throttle back. Ease it off idle and once on plane boat runs fine. in fact I've picked up 4-500 rpm. Timing is set at factory 8 advance and looks like Silver Spacer in Dist which is 25 advance for total of 33 at 3-3500. I also replace the stock Mech fuel pumps with Aftermarket 130gph high flow mech pumps, new #6 lines and Areomotive regulators. Jets are 83 primary and 70 secondary. Acc pumps start squirting as soon as you touch the throttles and it smells fat so I dont think i'ts a fuel delivery problem. Maybe timing or need diff jet sizes.

Help Please