Bonzen

You sayin chicks aren't fans of the "redneck carburetor"? ha!

I'll just add more sound system that usually does the job...

Bonzen

Reviving this thread from the dead because I have updates... installed the wideband AFR gauge & sensor. cnc'd custom adaptor plate...and lake tested it... AFR gauge says LEAN in flat spot. So looks like I'm chasing a weird fuel pressure issue at around 3/4 throttle where my pressure goes from 30-ish psi to 20psi instantaneously causing major stumbling/flat spot and then back to 30psi once I push through to WOT. It doesn't matter if I mash the gas or ease into it slow, the same thing happens. I can hold the rpms and the throttle in the flat spot for as long as I want, it never recovers until I let off or push though to WOT. I'm watching this happen on the fuel pressure gauge in real time as the issue occurs....I closed the motor hatch on myself and watched the gauge with a flashlight while a friend brought the boat up to WOT across the lake.

So I'm looking at a faulty/old fuel pump that cant handle the demand at that particular rpm range but then suddenly can at WOT...or...

I am looking at this return/bypass block thing that I have no idea what purpose it serves or what it actually does. It's plumbed to the fuel pump, and the main return line from the throttle body dumps into it. I'm wondering if it has some check valve or bypass inside thats diverting fuel from the pump via the small "USCG" hose which is causing my major drop in fuel pressure in that certain RPM range. Research is telling me it's some kind of vapor return setup to prevent vapor lock, which it still does at least once a trip.

Any ideas?

The plumbing goes like this: Feed line - Line out from tank(new lines) into water fuel separator(brand new) into fuel pump(old but still seems to work and obviously still works at WOT) into fuel filter(brand new) into the throttle body.

Return line - out of the throttle body into the "block thingy" and then straight to the tank.

Small crossover USCG tube from the "inlet" end of the fuel pump into the top of the "block thingy"

Also it has been verified that the fuel pump is not experiencing any significant voltage drop during the entire RPM range. The fuel pump and fuel system setup/plumbing is identical to my previous engine which was a standard run of the mill 454 330 hp. The new motor is .030over, bigger cam, 10:1 compression. The new motor runs GREAT except in that one rpm area where the flat spot occurs.

How could a fuel system run great, then stumble and drop pressure significantly, and then pick back up and run great again??? It is running fairly rich (another issue to get tuned) and then goes pretty lean on the wide band gauge and then back to fairly rich.

Black Baja

iTrader: (2)

Sounds like you need an intake and Carberutor to fix that mess.

Bonzen

Pics of setup/plumbing and RPM vs. AFR readings

SB

iTrader: (1)

Very odd. Fuel psi fine at 30, then gets to the troubled rpm spot and goes down 10psi to 20psi, then past that rpm spot and gains back the 10psi and goes to correct 30psi.

Alternator belt slipping or alternator issue at that rpm spot ? LOL.

Any fuel pump rupture or fume or vent tube hooked to vacuum by any chance ? Is the regulator hooked to vacuum in this set up, and if so, wrong vacuum port ?

Bonzen

haha we actually checked the alt belt tension and slightly tightened it just for the heck of it.

Alternator is charging at 13.5 volts off the back posts checked with voltmeter.

Fuel pump voltage stays the same throughout rpm range. so I don't think its being deprived of power.

The FPR is part of the factory TBI unit. I have changed diaphragms and spring rates and changed fuel pressure from 24psi to 32 psi with no success (FPR is adjustable)

SB

iTrader: (1)

Yes, an issue can develop from here as some of the return fuel is fed back to the inlet of fuel pump.

Any electric lines to this ? Any vac lines ?

Can you find out how this block works and what mechanical parts, if any, are in this ?

Bonzen

A guy over on the mastercraft forum says this is a vapor return system. It was some coast guard thing to eliminate vapor lock but it didn't really work well. He says it flows away from the pump. so the return line with unused fuel in theory should have enough pressure to pull all the vapor away from the fuel pump so it's always charged with liquid fuel.

Im wondering if something is stuck, or malfunctioning and that's causing it to starve my pump of fuel in that certain rpm range somehow...

Bonzen

SB -

No vacuum lines or wires going to the pump or the vapor return system.

Ended up buying a new pump, will see it it remedies the issue. The new pump supposedly puts out a little more PSI so that may be the solution if the current one is just old and weak or is acting up. At the very least i will have a spare pump.

After that my plan is to try to bypass this vapor return setup and see if eliminating that possible malfunctioning system will allow the fuel pressure to remain constant. From what Ive researched it doesn't seem to be instrumental to the fuel system or a safety issue.

SB

iTrader: (1)

I'd bypass the vapor return first, if there is noway to take it apart and see what's up.

Again, since it has a line that goes to inlet of fuel pump, if air or 'bubbles' from vapor or etc, this can cause fuel pump to cavitate and or displace fuel that should be going into pump.