holley, stumble, backfire off idle..
 

I have a 502, that use to be a mpi, but converted it to carb motor, I changed everything, including ignition, to a thunderbolt 4. I have a 800 double pumper off a hp500 motor, but Im not sure what the factor jetting is for a hp500. Anyway, the boat starts and idles fine, it idles at 600 rpm. It has a issue though off idle, if you blirp the throtle, it will backfire though the carb, and die, if you ease onto the throtle it doesnt do it, but is kinda sluggish and ill reponsive till about 15-1800 rpm then it romps like a bat outta hell..any thoughts? What is the factory jetting for a hp500 motor?

Sounds like it needs a new accelerator pump diaphram. They should be the 50cc pumps.

What Griff said. Remove flame arrestor, and manually move the throttle from idle to say 3/4 throttle (engine off) and make sure you get a good strong squirt from the front and rear accelerator pumps. If it dribbles you need to replace it. Accelerator pumps are cheap, and usually available at napa or pepboys. You'll need to remove carb to change them though. 1/2 Hour Job at most though.

What Griff and M(w)ild Thunder said, but while you're changing the accel. pump...check to make sure clearance is correct on the pump lever/cam. I think the correct clearance is like .015", but I just set mine to where is almost, but not quite touching. Good luck

So what's the jets on a factory hp500? J know they run rich so I was going to run a size smaller.

You might not be getting enough timming advance off idle. That will cause a stumble off idle like that.

ok, went and checked the squirters, and they both have good steady streams of fuel, they dont dribble out, but what I did notice is that they dont squirt into the little like "o" they kinda hit it and spray fuel everywere, esp under the choke butterfly..is that a issue???

ok, I had a brain fart, the "o" things I am talking about are the boosters, where the squirter sprays into...its hitting those and spraying everywhere, and one isnt even making it to the booster, it sprays under it, right onto the butterfly...

Sounds like the nozzle may be pairtially blocked. The phillips screw holds the accelerator pump squirt nozzle to the carb, if you remove the screw you can take the nozzle off and spray it out. WATCH FOR A SMALLL GASKET under the nozzle that the screw goes through, sometimes it stays on the carb but be sure and find it and use it again. Spray out the nozzle assembly with Brakleen, spray the screw out also it has a passage in it, I also would pump the carb once to flush any debris and re-install the nozzle. There should be a stamped number on the nozzle, tell us what that is, probably a 40 or something like that. The main jetting has very little to do with the transition performance of the carb.

Try cleaning the idle air bleeds. They are very small and clog easily.