Throttle response.
 

Got a 355 thats not stock but not nasty either that I can't seem to get right. I put on a proform 750 double pumper carb and from 3/4 throttle to wide open I have no change in rpm or speed. It seems like I need more jet. It runs real poor at wide open but good at 3/4 or so. 70 main and 79 in secondary's. plugs look brownish. It also stumbles and sometimes spits or back fires threw the carb pretty good when I'm running about 3500 rpm and go wide open but I think I can figure that out with a bigger squirter. Any ideas?? I don't think his exhaust are cracked and timing is 35 total 12 idle. Thanks

check your fuel pressure at that rpm..low float bowl ,,wrong power valve just to name a few things..i have the same carb on a 355..4.5 power valve's,,78 primary,82 secondary. 36 squirters front and rear and idle air screws 1/2 turn out on all four corners..

id say it,s jetted to small,try 80/88 jets and see what happens.

I have 79 and 88 jets that were in it before. I changed thinking a little better fuel economy. Thanks

how did it run with this jetting?

You don't have 2 plug wires crossed do you? That's about how mine ran when I did that.There are 2 that fire very close I had them wrong.

Those jets were on a crate racing motor I had the carb on. A friend has a motor in his boat similar and I thought I would pull back the jets a little bit to help with fuel consumption. He had a stock 350 260hp q jet carb on it and it would run good to about 85% throttle then the same thing. Just sounds like it is not getting enough fuel.

Payton. I will double check the plug wires. Thanks

Jet for max power, not fuel economy, unless you abosutely, positive know how to do that.

BTW: Vac secondaries way more fuel effecient than mechanical secondaries, even if set up for max power, simply because of one accelerator pump vs 2. That difference alone is huge.

Anyway: keep adding jet until boat goes the fastest and then add one more jet# for safety.

You'll need a wideband and vac guage to see if you have rich 'holes' down lower in throttle/rpm range

BTW: You may want to check for fuel supply issue if carb tuning doesn't help.

BTW#2: You could be setting yourself up for engine burn down if it is lean a/f that's limiting your WOT. Be careful ! Can just take a second and sizzle.