What fuel line size for Merc 351 v8? Pickup to carb-

[fstarocka]

Hi all - re plumbing the fuel lines from fuel tank to carb (to be USCG approved).

Unfortunately the pickup is welded into the tank, it seems to be 1/2- IF i have a crack in the tube could I draw up fuel (with just enough to contact pickup) with mityvac and see if it holds in a clear tube? Im trying to eliminate fuel supply issues. If it doesnt hold up (assuming a crack in pickup) Can i shove down a replacement tube 3/8 instead and just cut it at a slight angle below?

Secondly - IF the pickup is 1/2", I guess I should try use that all the way to before the pump right? I think I have 3/8 in there now.

LUCKY for me the pickup sits right by pump and water separator/filer - so I should only need about 2ft of the pricey approved hose.

Now for the part from pump to carb - should I use 3/8 or try keep it 1/2?

What real world differences would I see? 1/2 seems a bit large on the output side -

Also I have a elec pump Im putting in at some point, except it seems to have a much smaller ID - somewhere around 1/4.. surely this wd invalidate having 1/2 after?

Thanks!

oh - this is a 22ft cabin cruiser - nothing racey.. dont like being above 3000 ont he ocean here anyway -

2 barrel rochester - runs like a champ and sips fuel.. 5 gallons gets about 13-15 miles on a nice day -

[Captsteve53]

The normal V8 engine has a 3/8" supply line, My sons truck he races (factory 3/8 fuel line) has a electric pump on the fuel tank outlet feeding a carter mechanical on the block, goes through top end at 4 lbs pressure at the carb, and produces approx 550 HP. (No signs of leaning out.)

Typically, at wide open throttle, full power, a typical 350 engine requires 0.5 lbs. of fuel per horsepower every hour. A gallon of gasoline weighs approximately 6 lbs.
Therefore an engine rated at 350 horsepower will require about 175 pounds
(29 gallons) of fuel every hour.
(350HP x .5 lbs = 175 lbs of fuel
175 lbs/6 lbs = 29 gallons per hour)

The bigger your line is, the easier it is to push the fluid without building pressure, so it really does depend more on the pump's ability to provide a specific flow rate at a specific pressure.

Regarding the pump issue:

A pump that can supply 50 GPH through -6 line at 4 PSI may not cut the mustard, but a different pump with a higher pressure rating may supply 100 GPH through the same line but it takes 8 PSI to do it and would be good enough. So pump specifications will make a difference

You may want to check what ever pump you employ will satisfy USCG regulations both in installation and actual pump regarding wiring/spark issues( Not all pumps do!)

Cheers Steve

[fstarocka]

Tx Steve, for both answers! Much appreciated...

[jimbunyard]

I'm confused. It sounds like you're describing the tank vent, shown near the filler neck in the foreground in your picture below.

The pickup tube is usually screwed into a boss, centered and welded on the opposite end of the tank, drilled and threaded for a 3/8" NPT fitting. I can't make out in the picture what it is that is in that location on the tank.

As for sticking a 3/8 'tube' into the existing 1/2" 'fitting', I don't know how you'd seal it, and I doubt you could get Coast Guard approval for it.

The hose should be the same size as the inlet on the fuel pump that is mounted on the engine, all the way to the tank.

I don't know if the rules have changed, but with mechanical pumps the line from the fuel pump to the carburetor used to have to be steel.

[Mojadito]

The regulation Fuel line is 3/8" for that engine. You should be able to purchase a withdrawl tube that will go into the 1/2"NPT fitting on your tank

http://www.blackbearsportinggoods.com/mobile/product.aspx?ProductCode=ELLMOEL03350024&click=12

This should have an anti-siphon device in the hose barb. That will prevent a leaking line from siphoning all the Fuel into the bilge. Be sure to use USCG approved A1 Hose for the application (B1 is for outboards only).

[fstarocka]

Originally Posted by Mojadito

The regulation Fuel line is 3/8" for that engine. You should be able to purchase a withdrawl tube that will go into the 1/2"NPT fitting on your tank

Black Bear Sporting Goods - Hunting, Archery, Fishing, Camping, Hiking, Boating, & more.

This should have an anti-siphon device in the hose barb. That will prevent a leaking line from siphoning all the Fuel into the bilge. Be sure to use USCG approved A1 Hose for the application (B1 is for outboards only).

Thanks yes will use approved line, i dont require a anti siphon as all my hoses/fittings are above the tank, its below the deck -

Will look into that pick up, I did originally shove a nylon tube into a brass barb from home depot to try resolve my issue - but I ended up having a darn seal of some sort inside the tank blocking it at random - I only managed to get this out once I had removed the whole tank

Im not sure IF the pickup is cracked, but if so at least I have a plan B - ill fill tank slowly till pickup has enough fuel to draw up a clear tube and suck it up with a regular pet pharmacy plunger and see if the fuel stays in the tube..