MPFI to Carb - fuel pressure and my frustration

[TacosnBeer]

I had a post a while ago regarding my truck running like crap but, I still haven't really solved the issue - just made it a little better with a new carb and fuel pressure regulator with the return line hooked up.

It is a 98 RCSB that had a 5.0 with MPFI and it was swapped out (before I got it) with a carbed 5.7 with iron vortec heads... I put on a new Edelbrock AVS 650 carb and a Holley fuel pressure regulator with the return line hooked up now. It just seems like it is still getting too much fuel. I have the FPR (which is new) set at 5.5psi but, it smells like it's running rich, the plugs are wet, and when I get on it, she bogs/dies out. I have set the timing at 12deg, and adjusted everything under the hood multiple times.... could it be that the stock fuel pump is overwhelming the FPR thus pumping too much fuel to the carb?

To go to an older fuel pump I would have to modify the wiring and lines at the tank to make it work... what other options is there other than a new tank and pump? What if I run a second FPR and step it down before it hits the carb? I have heard/read the FPR should hold but, I think the stock pump is 50-60 psi and I am dropping it to 5.5... that is a lot!

Any thoughts. the truck runs "okay" till she is under load it falls on its face. Being I have changed and adjusted everything else, I think it HAS to be the fuel pressure that is causing it to bog and run rich.

 

[PlayingWithTBI]

Are your floats adjusted properly?

 

[L31MaxExpress]

Is the engine up to full operating temp before you get on it? Choke opening all the way? Are the air doors on the back moving and are they properly adjusted? Have you properly jetted the carb with a wideband? Was the timing set with the vacuum advance disconnected and the engine at low idle? Is the timing pointer and balancer combination giving the correct timing reading relative to TDC? Does the distributors centrifical advance function correctly? Do you have enough compression, converter stall speed and rear gear for your cam?

Also as mentiones above. Have you checked the float level, the float drop as well as checked to see if the installed jets and metering rods are correct for the carb? I have fixed one that was bogging out of the box once by installing the missing rear jets. Also make sure the accelerator pump linkage is installed in the correct hole.

 

[L31MaxExpress]

I tore apart a quadrajet recently on a friends C10 that had a bog when you gave it throttle. Ended up fully rebuilding the carb and having to re-install a fuel feed tube that helps feed the rear barrels during the transition to WOT.

Helped another friend with a Holley a few years ago that Holley completely forgot to drill the carb behind the main jets for fuel passageways.

Just because it is a new carb does not mean it was built correctly or even functional.

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[Erik the Awful]

You did use a return-style fuel pressure regulator, right? While the standard-style FPRs look like they can be plumbed for return, they can't.

What's your fuel pressure when you get on it? Duct tape the fuel pressure gauge to the windshield if you have to. Been there, done that.

 

[Schurkey]

1. There's an entire sub-forum for carb or LS swaps. You might find what you need there. Perhaps a moderator will move this thread.
https://www.gmt400.com/forums/lsx-carb-swaps.14/

2. "Liquid-filled" pressure gauges are notorious for being inaccurate. Nobody tells you that you're supposed to pop off the rubber plug to burp pressure out of the gauge housing before you read the gauge. Temperature changes affect the internal pressure inside the gauge housing, if you don't burp 'em, they're maybe lying to you.

3. Edelbrock/Carter/Weber carbs are not known for being perfect. At least it's an AVS clone instead of an AFB clone.

4. What carb was on it before you installed the AVS? Why did you swap carbs?