Fuel System questions for Mercruiser 260 EFI Conversion

[Kareo]

I have a 1979 Mercruiser 260 (Chevy 350/5.7) engine that I am converting from the old Rochester quadrajet to the Holley Sniper EFI system. I'm doing the Rochester version that sits on the same intake, which also does not have a fuel pressure regulator built in.
I am struggling to figure out the fuel system for this. I've read a lot and watched a lot of videos, with conflicting information, so I want to see if any experts can weigh in! I want to make sure the whole system A: works, and B: is coast guard compliant.

I purchased a Holley 12-920 fuel pump, which is USCG certified. I have ordered A1 fuel hoses, and will double clamp all fittings with stainless hose clamps on any of the retrofitted equipment.

Normally you would remove the mechanical fuel pump and block that off, and install the electric pump. However, Holley says the fuel pump is gravity fed, and needs to be installed below the level of the fuel tank, which isn't really possible without putting it in the bilge. Also, marine systems need to have some kind of Anti-siphon system... I think the existing system has something on the tank valve. So it seems I need some kind of primer pump. Does anyone have a USCG approved one they recommend that provides enough flow? Another alternative, in one YouTube video I watched of a Sniper conversion they kept the mechanical fuel pump, and had it feed into the electric pump. This would be the simplest, essentially using the existing pump as a primer. But I'm concerned that it might not be able to provide enough flow to the system.

The next question is filtration. I currently have a stock fuel water separator. I was planning on putting a 100 micron pre and 10 micron post in line filter in as well. However, I can't figure out if there are any coast guard requirements other than mounting them (filters can't just sit on the lines).

As far as I can tell, there are no additional USCG requirements for a fuel pressure regulator, but if there is I'd like to know what I need.

I may not have a choice in some cases, but if I do, would it be better to use my existing hard line fuel lines, or A1 vapor guard lines?

I also may not have a choice, but for my return line I can either install it in my sending unit, about 2 feet from the engine, or the fill or vent line, about 8 feet away. Will it make any difference running the extra distance? The sending unit might be too small to fit a bung in...

So to sum up:

What do I need to to do to provide a USCG compliant pumping system that will provide enough flow for the Sniper?

What is my best filtration system solution, and are there any USCG issues I need to be aware of?

Which would be preferred for EFI, hard line or A1 hose?

Is a longer return line going to make a difference?

Is there anything else I need to do to make the entire system USCG compliant?

Thank you for any advice!

 

[dubs283]

If I were installing this system I would keep the oem mechanical fuel pump and water separator. The mechanical pump will provide plenty of flow for the high pressure pump as long as it and the supply system from the tank is sound. I would only use pressure rated, braided fuel line after the mechanical pump and not install any filters in line after either pump. The oem water separator should provide enough filtration for the system. The return line can be plumbed to the separator housing, just make sure it goes "in" the housing, arrows should indicate flow.

 

[alldodge]

What do I need to to do to provide a USCG compliant pumping system that will provide enough flow for the Sniper?

What is my best filtration system solution, and are there any USCG issues I need to be aware of?

Instructions from Holley state to place pump within 2 ft of sending unit, but this is because everything is based off the auto industry. Use Type A1 fuel line for all connections from tank to pump. You will have the tank antisiphon valve (barbed fitting on top of tank), to the 20 Mic filter, then the pump. Mount pump where ever it can be mounted safely

I would remove the mechanical fuel pump because with return to tank the pump may draw more then the Mech pump can supply

Which would be preferred for EFI, hard line or A1 hose?

The pump will run close to 60 psi so you now need HP fuel line which is not standard type A1. You need to find hose and connections that can support this pressure. There is flexible line which can be used, do the research

Is a longer return line going to make a difference?

No, and I would return to tank fill hose. Do not return to vent line because that will cause problems with the tank breathing and a vacuum may be created. Cut the fill hose and install a Tee similar to link. The one below is expensive and there are others cheaper just look around

https://www.cpperformance.com/p-13686-fuel-line-return-to-tank-adapter.aspx

 

[Lou C]

Unless Holley says specifically that this system can be used in a marine application I’m not sure I’d do it. What’s the big advantage over rebuilding a Quadrajet or even a brand new Edelbrock or Holley? Better cold starts and smoother cold running for all that expense & complexity? I just don’t see it. $50 to rebuild a QJet & $75 to replace a Carter mechanical fuel pump…..

 

[matt167]

Holley sniper does have a marine version. But it’s not a huge upgrade over a carb

 

[Rick Stephens]

Unless Holley says specifically that this system can be used in a marine application I’m not sure I’d do it. What’s the big advantage over rebuilding a Quadrajet or even a brand new Edelbrock or Holley? Better cold starts and smoother cold running for all that expense & complexity? I just don’t see it. $50 to rebuild a QJet & $75 to replace a Carter mechanical fuel pump…..

The Sniper system is fully marine rated.

 

[Kareo]

If I were installing this system I would keep the oem mechanical fuel pump and water separator. The mechanical pump will provide plenty of flow for the high pressure pump as long as it and the supply system from the tank is sound. I would only use pressure rated, braided fuel line after the mechanical pump and not install any filters in line after either pump. The oem water separator should provide enough filtration for the system. The return line can be plumbed to the separator housing, just make sure it goes "in" the housing, arrows should indicate flow.

The fuel water separator does have a spot for an in, but it currently does not feed back to the tank. Are you saying the return line could just go there and not feed back to the tank at all?

 

[Kareo]

Unless Holley says specifically that this system can be used in a marine application I’m not sure I’d do it. What’s the big advantage over rebuilding a Quadrajet or even a brand new Edelbrock or Holley? Better cold starts and smoother cold running for all that expense & complexity? I just don’t see it. $50 to rebuild a QJet & $75 to replace a Carter mechanical fuel pump…..

It's actually hot starts that are more frequently the issue. And idling too fast. Or dying at idle. And not restarting. And flooding. All issues I'm sure can be resolved, but I've had the carb rebuilt twice now in the past 8 years and the problems never really go away. I'm over it. I want something that never dies at idle, and starts every time with no questions.

 

[Scott Danforth]

Unless you have a heat exchanger to allow a higher temp thermostat to get engine temp up, EFI won't cut it.

 

[Kareo]

Unless you have a heat exchanger to allow a higher temp thermostat to get engine temp up, EFI won't cut it.

The ones that I've seen done, they had to update the software profile for the Sniper to start learning/tuning at lower Temps. Every story I've seen where people have done it to this engine, people seem happy with the results. Why won't EFI work at the lower temps?

 

[Scott Danforth]

Because at temps below about160, EFI reverts back to it's base fuel tables vs the self-tune tables and relies on the enrichment setting.

Make sure you always have the display running and watch the AFR so it doesn't fo lean

Good luck to you sir.

 

[Lou C]

It sounds like you had either an old worn out carb, or a carb that might not be worn out but wasn't really cleaned properly which is the most important part of the process. If done right with high quality parts you shouldn't have those problems.
I wonder how much of an improvement you would have seen with a brand new Edelbrock 1409 4bbl?

 

[Scott06]

you will need these to stab in oxygen sensors https://www.michiganmotorz.com/o2-spacer-stock-mercury-riser-wet-joint

scott is correct you need programming changed to marine temps to get it out of open loop (ignoring o2 sensor) warm up mode

 

[Kareo]

you will need these to stab in oxygen sensors https://www.michiganmotorz.com/o2-spacer-stock-mercury-riser-wet-joint

scott is correct you need programming changed to marine temps to get it out of open loop (ignoring o2 sensor) warm up mode

Yup. I'm half way through upgrading my exhaust system to the center risers to put one of those in. Had to find some longer bolts for the riser today to make it work!