So some folks say the 3/8" feed from the tank is adequate for for 502/500hp, and others say that only a 5/8" line is adequate. Whatever, but I would like to go ahead and upsize it so how do you go about replacing the siphon tube in the tank with a larger unit?

There was a thread on this a while back, not the easiest thing to do. Eddie Young explained how to do it and one of the members here posted the thread. I did not have to do it on my cat as I had dual tanks/tubes for one engine.

I would start by getting rid of the anti-drainback check ball in tank elbow and also make sure your pickup screen is not clogged.

Here are a couple of threads started that might have some info that is helpful to you:

http://www.offshoreonly.com/forums/g...na-575-hp.html

http://www.offshoreonly.com/forums/g...mech-pump.html

Also, might be some good info about fuel filters / separators on here:

http://www.offshoreonly.com/forums/g...drill-tap.html

Personally, at the 500 HP level I would just check the fuel pickup to make sure it is not cracked or anything, and then either replace the anti-siphon valve with a high flow model or ditch it in favor of a ball valve shutoff that will flow better. Then go to 1/2 inch line all the way to your fuel filter and mechanical pump, and possibly from the fuel line up to where it separates for each float bowl. 3/8 or #6 is sufficient going up to the float bowls from there for this power level.

Eddie can fab one for you. It's not cheap because a fair amount of labor is involved. You might consider doing the fuel line size upgrades first and then check your fuel pressure situation to see if that corrects it. If not, then you can go ahead and replace with a larger pickup because that will pretty much be the last thing to be replaced.

Have you verified that you do not have an issue with the tank vent?

Thanks, that's all helpful info. I can hear and feel the air whooshing out of the vent as I fill up, so assuming that's clear. I'll check the tube and check valve and go 1/2" from there. I'm fairly sure the stock pump is just to anemic and others seem to be fine with the fittings at this power level so just swapping the lines outa do it.

Hey rooster.

One thing I'd mention is to use a pump that puts out roughly 15 psi and run a return line with a regulator back to the tank via the fill hose ( they make a barbed stainless "t" that is 1 1/2" with a 1/2" bung in the side).

Use your return regulator to set the PSI at the carb inlet to 5.5 psi.

Dead heading a pump on an application where it may be asked to idle along for long periods of time isn't a real good idea.

3/8" line is plenty for what you're doing.

You could probably use a bypass regulator like this:

http://www.claysmithcams.com/by-pass-check-valve-an-6/

Add a "T" fitting at the line to the secondary float bowl, and then plumb this bypass in-line on the return line back to the tank. Your tank may very well have provisions for a return line (mine did), but if not you can probably just run it back to the input side of your fuel / water separator - most of those have two inlets / outlets.

You would probably do just fine with a self regulated mechanical pump that flows around 130 GPH and can maintain 6 PSI, but you are a little bit limited on mechanical pumps if your block does not have the fuel pump boss (and your cam might not have the lobe for it.)

So the simplest and cleanest install would be to tee the fuel line right before the carb, run it back to the Holley regulator, and into the extra input of the fuel separator. Is this a legit set up?