Ignition drops voltage while cranking/starting

[HaZmatt]

Hey guys I am currently working out some issues with my 75 280Z. The EFI has been removed and replaced with carbs. Lately I have been having some random starting issues. It doesn't happen all the time but once in awhile it will crank but not turn over. I have narrowed it down to the fuel pump cutting off during starting/cranking. Currently the fuel pump is wired directly to a 12v ignition wire from behind the ignition switch. I hooked up a volt meter to this ignition wire and usually 9 times out of ten, it only drops to 10-11 volts during cranking...but every once in awhile it drops to 0. When this happens I can look at my fuel gauge under the hood and it drops to 0-1 psi, which causes the vehicle to not start and just keep cranking. 

What i've done so far:

tested for other 12v ignition sources - same result

replaced ignition switch - same result

replaced voltage regulator - same result

oh, and all the wiring has been removed, cleaned, terminals replaced if needed, etc...

Anybody have any suggestions?

[wheee!]

Bad ground at the pump?
Nice to see you still have the car!

[Captain Obvious]

There should be enough fuel in the bowls to run the car for about a minute with no fuel pump at all, so... I'm thinking it might not be a fuel pump issue at all?

By any chance, is the ignition coil power on the same circuit as the fuel pump? If so, I suspect lack of spark, not lack of fuel, may be causing the issue?

If that's not the case, and your bowls are actually drying out between starts... I'd be interested as to why that is happening.

[HaZmatt]

2 hours ago, wheee! said:

Bad ground at the pump?
Nice to see you still have the car!

Yep still have it. Working out some small issues now. Maybe that will make me want to keep it!

[HaZmatt]

1 hour ago, Captain Obvious said:

There should be enough fuel in the bowls to run the car for about a minute with no fuel pump at all, so... I'm thinking it might not be a fuel pump issue at all?

By any chance, is the ignition coil power on the same circuit as the fuel pump? If so, I suspect lack of spark, not lack of fuel, may be causing the issue?

If that's not the case, and your bowls are actually drying out between starts... I'd be interested as to why that is happening.

Yes, I will double check tomorrow, but I am pretty sure its the same circuit that I have my MSD ignition system wired to. I will try and test spark tomorrow as well. Thanks

[Chickenman]

MSD???  It's possible the Red Box has struck again...

[dutchzcarguy]

check connections  and ground in your distributor..

[HaZmatt]

9 hours ago, Chickenman said:

MSD???  It's possible the Red Box has struck again...

Has this been a problem in the past? 

[HaZmatt]

2 hours ago, dutchzcarguy said:

check connections  and ground in your distributor..

Will do, thanks

[Chickenman]

30 minutes ago, HaZmatt said:

Has this been a problem in the past? 

MSD boxes are infamous for out right failing and having intermittent problems. The older model 6AL's and 7Al's  are particularly bad. Might be an idea to disconnect the MSD from the Ignition switch power though and temporarily wire the small red wire ( +12v switched) directly to the Battery positive terminal. Use a small switch and a fused line. Large red wire should of course be connected directly to the Positive battery cable with a 20 amp fuse or Circuit breaker. 

It's best to run the switched +12v wire from it's own separate relay. They can be sensitive to voltage drop from loads on the ignition switch. 

MSD shorting internally could cause the voltage drop to 0. 

Edited June 28, 2017 by Chickenman

[Tammy]

Hello, I had the same issue a couple weeks ago. Check your ballast resistor.

[Chickenman]

Quote

 Currently the fuel pump is wired directly to a 12v ignition wire from behind the ignition switch.

That's not a good way to do it.  The ignition switch cannot handle a lot of amperage. Fuel pumps can draw significant amperage, especially when used in a returnless system and when they get older. The factory wiring also uses too small a gauge of wiring to the F/Pump and that also increases current draw and Voltage drop. 40 year old wires don't help either if they are still stock. The addition of the extra Fuel Pump amperage draw, plus all the normal devices connected to switched ignition, could be enough to burn the contact points in the ignition switch. 

Connect the Fuel pump through a proper relay.  A Bosch 30 or 40 amp relay is better than the factory EFI F/Pump relay  Use 12 gauge wire minimum to the Fuel pump from the relay. Both on power and ground side

The MSD is also sensitive to voltage drop on the small +12V ignition feed. That's why I recommended isolating the small +12v switched power lead and running it though a Relay to the battery. 

Edited June 28, 2017 by Chickenman