I need to prime my carbs and set float heights and wanted to do it before starting the motor so that I can check for fuel line leaks. Bought an inline electric pump that will temporarily be installed to accomplish this.
Question- can I hook it up between fuel tank line and mechanical pump, or should I bypass the mechanical pump and run the line to the vapor separator?
Thanks

My thought is to bypass the mechanical. That will get you close. You'll need to consider that there's a big chance that the two pumps will nave different pressure and the float height will be different. If you have a pressure regulator, I'd think you'll be fine setting them. Also as the engine is running, it will lower the level. Good luck how ever you go.

Hey, Pynzo. Would you ever post any pictures of your car and do you plan on doing any car shows in Philadelphia? I would like to meet up at a car show someday. I'm trying to save up my money to buy that $300 clutch fan. Might settle fort the $200 one. Thanks.

Go to a local pharmacy and get a syringe they give out for liquid medicine. Fill through the vent tube to get it primed. Works great.

If it's a fresh fire, I start with the floats real low and then come up. The alternative is not always pretty.

My ol' rusty Bee isn't pretty enough to show. I've got no interior, just getting it running right preparing to have it professionally restored. Body work is not my strong suit, I'm more mechanical. Stop by sometime I'm in Lawndale, near Cheltenham. I always have beer on ice-

I thought about priming through the site holes and decided I'd rather pressurize the system to check for leaks, so already have an electric fuel pump to splice in temporarily. Id rather not find out it was leaking while behind the wheel starting it.

It works! Spliced the electric pump between tank line and mechanical pump and fuel flowed to all 3 carbs. Now I can set fuel level in bowls without the motor shaking!

Edit-
Glad I did this, center and rear outboard float levels were low, and I found a couple of fittings that leaked. Good to go now. I bought the pump at NAPA, 4-7 psi w/ 32 gph flow rate. I had a junk tire inflator around, pulled the power cord from it, and spliced it to the pump wiring. Nice to have an on/ off switch at your fingertips. Used a 5 amp fuse in the power plug. About $60 for the pump and 2 3/8" hose fittings. It's going into my toolbox.

As a side note, an old stock eliminator racer from N. Jersey taught us that a small (Carter) electric pump just after the tank, along with a mechanical pump works great. When you turn on the electric pump, it will flow directly through the mechanical pump to the carbs. I mentioned this on this site in the past, and received a bunch of "that won't work" from the crew. We run this set-up on our 10.80 N/SS car as well as my 11.50 streeter. On the street car, I typically don't need the electric pump for normal driving. If I'm pulling a hill or standing on it I will need it. The mechanical pump regulates the pressure, so no regulator is needed. The race car sees 6-7PSI all the way through the traps. It's a cheap way to get it done. I was told this works all the way down to 10.0's. The fuel lines are 3/8th inch all the way from the pick-up.

Only the street Hemi and some 440 HP cars came with 3/8 O.D. fuel lines, those are around 5/16 I.D. The rest of the Mopar cars have 5/16 OEM lines and those are around 1/4 I.D.. My 1963 Plymouth M.W. Belevedere would run quicker(.03 to .04) and faster(.2 MPH) in the 1/4 with the Holley Blue 110 GPH pump bypassed and shut off than with it hooked up through the stock 5/16 lines That test was done with the stock alternator modified with all the diodes remove so there was no drag from the alternator, NHRA required all stock class cars to have the original charging systems on the cars back then, they did not require that they work I found out later that car would run faster and quicker with the alternator working Many myths and false information about drag racing out there, not all combinations respond to the same mods. the same way As far as the mechanincal fuel pump allowing fuel to be pushed through them by a small electric fuel pump it depends on which valves are open when the motor is shut off, if the outlet valve is closed, suction stroke, you won't be able to push fuel through it usually

I just use a cooking syringe and fill them thru the vent tubes. Never had a problem.

Me too but the OP wants to set float levels and check for leaks. The turkey baster won't help him do that.

Why does it have to be so difficult...you should have lightly primed the carbs; pulled the plugs and spun it over for about 30 sec. Then open the sight plug and see where you are at. The pump that was utilized does not put out the same pressure as your stock fuel pump therefore your float settings will be incorrect...

As for the the extra electric pump I just don't understand why. I ran a 416 six pack that produced over 400 hp at the rear wheels on the road course hitting speeds of 125 mph on the straightaways and never once did i have any fuel delivery issues. 3.47 miles per lap. Carter Street pump 5/16 lines. I won't want even to begin to ask if you base-lined the set up with the six pak tuning guide.

First off, it's been done over a week ago. And it wasn't difficult at all, far less work than pulling plugs and everything else you think I should have done.
And it worked perfectly, a 4-7 psi pump was capable of achieving the 5 psi called for in the FSM.
And when I started it up for the first time with 3 brand new Holley's, float levels set, no leaking lines(9 fittings all new), it settled into a steady idle and only had to set idle and solenoid speed. That little pump was pure peace of mind.
I could have done it your way, have so in the past with Carters and Edelbrocks. I'm not a hobby mechanic. 35 years working on Mopars. I respect my car. And my garage is under my living room and Do Not need accidental fires due to careless attempts at tuning.
Not this time with $2200 worth of multi steel fuel lined new Six Pack setup.
And these out of the box Holley's were set low. More than 3/4 turn low.
$60 for a pump that'll sit in my toolbox, maybe get used twice?
Peace of mind is priceless.

You only need to prime the center one....