CamarosRUS

I have switched to carb from TPI and ditched the ECM.

My fuel pump is not priming with the key on and I need to work around this.

I have not cranked the car over yet so I do not know if the pump will turn on with oil pressure.

My questions:

Assuming the pump will turn on with oil pressure can I just not worry about the pump not priming since this is a carb application?

If, in fact, the oil pressure sender is FUBAR and will not power the pump can I safely hook up a switch to the priming wire on the relay and simply turn that on before each start and off after each shutoff?

I plan to hook up a switch to the pump relay anyway to ease setting the fuel pressure etc.

BigBadLou

iTrader: (3)

You sure could wire up a button to prime the fuel pump before start but for safety reasons, I wouldn't recomment using a toggle switch. A momentary button would be safer.

This button would send a 12V signal from an ignition source to the green/white wire on the fuel pump relay.

Hope this helps.
Lou

Jammin'Trey

Or, run the two end wires on the FP relay to a switch and short them to prime it or run it in general. Not reccomended though. Get an oil pressure safty switch and run it off that. (again shorting the two outside fp relay wires)

mark wells

You may also want to consider just installing a manual fuel pump on the block (if you have provision for one) on the fuel pump block plate you need 4 screws holding it on instead of 3. Not sure which one you have. The electric FP for the TBI is the same pump as the TPI and both run at around 45+ PSI and your carb will only need 6 PSI (aprox) or you can run a larger dia. return to the tank to compensate. Not sure if you knew this or not since you have not started the car yet.

CamarosRUS

Got the Mallory return-style FPR since I like the Walbro 255lph pump (if the check valve isn't broken). But, I am feeling I need a larger return line anyway...

mark wells

I have a friend who is running 2 electric pumps (for volume) on a carb but the 1st time he fired it up on 6psi the fuel could not get to his tank fast enough and was flooding it out. You will only need about a 3 second prime before trying to start the car so the push button style (hold down to energize) will work the best.

CamarosRUS

I think when I finalize the interior wiring I will now use a momentary push button switch for the fuel pump.

ownor

iTrader: (5)

nope, fyi the tbi pump is about 9-13 psi sorry, couldn't resist being a smartass.

Twin_Turbo

iTrader: (25)

Wrong too, the pump doesn't create pressure but flow, the regulator restricts the return and a such creates the pressure, how much pressure a pump can overcome depends on flow, the higher the pressure, the lower the resulting flow. The TBI pump can not maintain even a deadhead pressure for a MPFI system. The other way around, you can use a TPI pump on a TBI car since the regulator regulates the pressure and will do so unless the return line or the regulator itself becomes restrictive, but for that you need a massive pump.

The dead head pressure a pump is rated at is the maxim,um pressure it can sustain against a closed off end. Think of it as the pressure resulting from the amount of fuel (flow) the pump can cram in.

anyway, when it comes to hooking up the fuel pump, you can simply wire a timed relay in parallel to the 12V feed from the oil pressure switch. The relay should trip when the ignition is turned on, priming the pump for X amount of secs (depending on the relay, you can get time adjustable ones). If you don't have this, you will most likely end up cranking for quite a while before the oil pressure switch trips and energizes the pump.

BigBadLou

iTrader: (3)

Ok, now that we've clarified how fuel pumps work and create pressure, let's get back on topic.

The relay with delay is a great idea. However, it depends on the skills of the vehicle owner to decide which way to go. The relay might be too big of a project and the momentary button might be simpler (and cheaper). But I still love the delay idea. Good one, Twin Turbo!

Lou

CamarosRUS

I had thought about installing a timer. I had installed one in my old car for a p00r mans one touch sunroof and thought I removed it when I sold the car. I was wrong and since the fuel in the bowls will start the car and the pump turns on with oil pressure I can't justify spending another $20 for a timer that is basically pointless. I do love those timers, however, and want to find tons of uses for them

Now I need a larger return line...one ffin setback after another

Twin_Turbo

iTrader: (25)

The relay is actually how the stock GM system primes a fuel pump, the ECM triggers a normal (non timed) relay in this case but also during cranking and it shuts down after a couple of secs but since the engine is running and has oil pressure the fuel pump is energized through the switch.

It would also be relativly easy to wire the relay to trigger also when the starter is engaged. Electronics aren't very complicated, these relays are really simple to hook up.

BigBadLou

iTrader: (3)

Powering the relay with the starter turning is a good idea too.
It would have to be proven, though, that the short time of cranking would power the relay long enough to fill up the fuel bowls for a few seconds (before the oil pressure switch kicks in and powers the fuel pump).

Just as a clarification: the ECM also energizes the FP relay while the engine is running (when it detects tach pulses from the distributor). To prove this, you can disconnect the oil pressure switch and the engine will still run. That's because the oil pressure switch is a backup system, in parallel. One of the very few good ideas that GM had in its lifetime.

Hope this helps.
Lou