Choosing the Right Universal Fuel Pump

[Scootrader]

Hi All,
I'm new on the forum but i have been lurking here for about a year or so.
Here's my story.
I have a 86 Pace Arrow, 27ft motorhome. I drove across the US last year, from Georgia to Colorado.
No issues at all until I started climbing in altitude and hills. Stumbling on climbs. Got to my destination and I'm just now getting around to fixing her.
No issues starting, drives fine at a mile high altitude, but figured it was fuel pump related.
So I did my research, read just about every thread on fuel pumps, in tank, outside of tank, regulators, relays and switches.
I started by replacing fuel filter in the carb and the big one on the fuel rail.
Dropped my tank down, and started replacing old fuel and vent lines.

Now my RV does not have an external electric fuel pump nor an internal tank pump. just the mechanical on the motor. There has never been a hum from the tank when starting so I know there is nothing in the tank. No extra connectors, And there is no fuel pump relay in the engine compartment as shown in the manuals or anywhere to be found.
I read another thread where someone else had the same situation on their 27ft Arrow.

So now I want to add an external electrical pump near the tank, have researched the Carter Electrical Pumps, most have 1/4" NPT nipples to fit 3/8 fuel lines, and the Rv is setup with 1/2 fuel lines so I guess I'll need to switch out the adapters to 3/8" NPT .
I've been looking at the Carter P4602RV setup with a bypass check valve to flow through or around the pump. [size=78%]https://www.summitracing.com/parts/crt-p4602rv/overview/[/size]
Just wondering what others have found as good option for the diameter of the fuel line and the pumps on the market.
Thanks for listening, hopefully I can finish this modification before we start exploring more backcountry of Colorado.

[tmsnyder]

1/2 inch?  Hoses are identified by their inside diameter (ID)  Are you measuring the outside or inside diameter?   I would expect 3/8 ID not 1/2 inch.

[Scootrader]

Yes, fuel line from the tank is 1/2 inside diameter.

[Rickf1985]

The only time you would have 1/2" inside diameter lines is if you had the in tank electric fuel pump. Being a 27 foot it probably did not have the electric pump. A bypass pump is a good idea as long as you have a good mechanical pump on the engine. This seems to be the problem, it sounds like your mechanical pump has died. You have to either have a good solid mechanical pump to pump through or you will need to bypass the mechanical pump and use only the electric since if the diaphragm goes bad in the mechanical pump you will be pushing gas into the oil pan. Have you tried replacing the mechanical pump with a brand name replacement pump or a good racing pump from Summit? That will probably solve all of your issues IF, Big IF here, you do not actually have 1/2" line. If you do then any residual air in that line will absorb the pulses from the mechanical pump and you be hard pressed to get gas to the pump. Once primed it will be fine but priming it means you need to have the nose downhill. If you have 3/8" line this should not be an issue.
85-86 is when GM started putting fuel pumps in the tanks on the longer motor home chassis and 1/2" line was part of that upgrade. So if you have the bigger line and no pump or regulator in the line then things have been changed around.

[Scootrader]

So the mechanical pump is original and seams to work fine, I have not checked the pressure output, if I just replaced the mechanical pump with a newer version,
what PSI rating should it be?
I may add a electrical pump at the back as a backup on a switch.

[Rickf1985]

Stock fuel pressure at the carb should be between 5-7 lbs. The stock setup is still the best and that is to run a pump at the tank that will put out around 15 PSI. and then add a regulator to drop it to 5 PSI before it gets to the mechanical pump. The main issue with the setup that they use on the motor homes is where they put the regulator. They put it all the way at the rear right by the tank. This is not the optimal setup to prevent vapor lock which is what the in tank pump was originally put there for. Keeping the line pressure higher is the best way to prevent boiling and then dropping it before the mechanical pump so the diaphragm is not damaged in that pump.

[tmsnyder]

If you put in a lift pump on a switch, I would strongly consider adding an inertial cutoff switch in case you get in an accident.  You don't want the electric pump continuing to pump gas toward your hot engine after getting into an accident.  If a line breaks, you'll have a bad fire very quickly.  I think they are like $25, cheap insurance.