jetting for a chevy 350

[Lightfoot]

This question actually relates to a boat I am working on swapping a motor into. Its a 350 inboard with a 4bbl on it. I believe its a holley, but it has no markings that I can find at the moment. The long block that came out of the boat was rated at 245hp. The newer long block that is going in is rated at 280hp. According to the engine builder, the only difference is in the cam. I called their customer service and asked them If I would need to rejet the carb for the increase in HP, and if so, how much larger should the jets be. The answer I got was that it is usually not necessary because the cam does not have any more lift, just more duration. Im not completely satisfied with this answer, so I thought I would run it by you all on here. Im thinking that more hp means more fuel, so it has to come from somewhere. and what exactly does duration vs. lift have to do with a need for larger jets? That could be a 100% valid answer, im just hoping for an explanation.
Thanks
Lightfoot

 

[eflores]

If it's a Holley the tuning opinions could go on forever....variables to include what cam heads comp ratio altitude etc.....look on YouTube for a Holley tuning vid and go from there. If u can give me the info from above I can give u an idea. If u run it as is u can try a couple jet sizes bigger for starts and check the plugs after a while of running it. Keep in. Mind there is more to tuning a carb tha. Just jets, if it's a vac secondary u have more parts and don't forget power valves and such

 

[eflores]

Since your relatively close u could bring it down and we can go cruise around pueblo reservoir and I'll tune it for you

 

[Andy in Pa.]

If it's a Holley the tuning opinions could go on forever....variables to include what cam heads comp ratio altitude etc.....If u run it as is u can try a couple jet sizes bigger for starts and check the plugs after a while of running it. Keep in. Mind there is more to tuning a carb tha. Just jets, if it's a vac secondary u have more parts and don't forget power valves and such

X2...
40HP isn't that big of a jump, but in a boat its a good idea to check the carb out to make sure its up to the task. It would help to determine what Holley part # it is so you know the carbs SCFM/ factory jet size. I would have to guess that it is a 600-700 SCFM range for that engine/horsepower (and application). If you want to be on the safe side, jet it up slightly, run it and read the plugs to see if your too lean/rich. There is some great stuff on how to read the plugs on the interwebs. If this is a fresh engine (that you just spent a nice chunk rebuilding), it may be wise to just put a new carb on it too. This will simply remove the "unknown" of how the old carb is set up.

 

[eflores]

But even a new carb doesn't mean it's right.....this is a boat not a ford galaxy...