Inconsistent fuel pressure

41Draggin said:

I recently swapped the straight six and three on the tree for a sbc and turbo 350 in my 1980 C10. Everything went well except the fuel pressure is inconsistent. It's a Goodwrench 350 thats looking pretty clean on the inside. Stock manual fuel pump to a holley fuel regulator to an edelbrock 650cfm carb. Went with a single point GM distributor. The issue i'm having is with the regulator set at 6psi it runs nice and smooth, then it starts to lose pressure and runs sporadic with the pressure dancing between 0-3 psi. Eventually it will smooth out again for a while then back to falling on it's face. It's noticeable while driving like all the sudden its running out of gas then it picks back up. I've tried a different fuel pump, replaced the fuel push rod, bypassed the filter and even replaced the condenser (just in case)and nothing has changed. Also worth noting the is an intermittent clacking sound like a stuck lifter but all the plugs look great and all rockers and push rods look all clean normal and well oiled. I could really use some help with this one I'm running out of ideas. Anyone seen this before? Thanks!

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41Draggin said:

First off Thank you everyone for chiming in. Definitely needed a fresh set of eyes on this one.

Regulator is my trusted Holley 12-803, same one I've had on my Chevelle for years and it does it's job without incident. Fuel pump is stock carter for an early sbc, no return line. I went with the old school single point distributor cuz it's what I grew up with and if it fails it's an easy fix (this will be my daily driver).

Tanks are clean not seeing any debris, and outside of being underpowered, a three on the tree and a heavy exhaust leak, seemed to run fine.

Now @squaredeal91 and @Ricko1966 you both bring some good light to the problem. Yes I do have two tanks and sucking air through an old fuel line makes a lot of sense. Also there was an "evap canister" charcoal canister that I disconnected from the straight six that I did not reconnect to the V8. I didn't reconnect it because I really don't completely understand how it works (I'm so old school when it comes to wrenching). With that I have the fuel filter right before the fuel pump and it never seems to fill up. I'm thinking this could be the issue???

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41Draggin said:

First off Thank you everyone for chiming in. Definitely needed a fresh set of eyes on this one.

Regulator is my trusted Holley 12-803, same one I've had on my Chevelle for years and it does it's job without incident. Fuel pump is stock carter for an early sbc, no return line. I went with the old school single point distributor cuz it's what I grew up with and if it fails it's an easy fix (this will be my daily driver).

Tanks are clean not seeing any debris, and outside of being underpowered, a three on the tree and a heavy exhaust leak, seemed to run fine.

Now @squaredeal91 and @Ricko1966 you both bring some good light to the problem. Yes I do have two tanks and sucking air through an old fuel line makes a lot of sense. Also there was an "evap canister" charcoal canister that I disconnected from the straight six that I did not reconnect to the V8. I didn't reconnect it because I really don't completely understand how it works (I'm so old school when it comes to wrenching). With that I have the fuel filter right before the fuel pump and it never seems to fill up. I'm thinking this could be the issue???

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41Draggin said:

Ok quick update. I removed both gas caps off the truck and went for a 30 min cruise but only got about 4 blocks before the carb went completely dry, before the pressure would drop and the engine would stumble but it would pick back up and keep on running. not the case with the caps off. I had to prime the carb to get it running again.

Next I disconnected the vent tube from the tank to the charcoal evap canister. That seemed to help a bit but the pressure is still dropping intermittently, stumbling and then coming back. At this point the truck is drivable but if it's gunna be my daily i'd like to not have to worry about this issue.

Next I think I'll move the fuel filter. It's right before the pump and although I can see a small amount of fuel surging through the filter(wix clear plastic) It's nowhere near filling up.

Thanks again for all the help, I'm confident we will figure this out.

@Ricko1966 Congrats on your 66 that's one of my favorites! I've got a 68 I'm working on, it's a whole heap of fun.

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squaredeal91 said:

Just a note on clear plastic filters. There hardly ever completely full. I know that's probably odd but that's how it is.

Maybe that's why the asin pickups had fuel filters with inlet and outlet at the top??

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Ricko1966 said:

Yea, but my concern on his situation is that he's running it at the pump inlet,so if he hits a bump etc. he is possible sucking air,not fuel,for just a second. I'm not sure if that brief interruption is enough to lose prime on the fuel pump. If you position the fuel filter so the outlet is facing up it should fill completely,on its side air gets trapped because it rises to the highest point. Another little tidbit if you have a plastic and paper fuel filter,sideways and it's more than 1/2 full it means the paper on the bottom 1/2 is clogging and fuel is having to find a spot higher up to pass through the filter.

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