240z intermittently losing power

[pyro]

Hey everyone, 

I posted this on another forum but haven't really gotten any responses, so I'm giving it a go here too.

I have a 1973 240z with a Schneider stage 2 cam, Pertronix igniter, and an electric fuel pump with the original one removed.

Just got done rebuilding the engine, and was starting to have fun breaking it in. At first it was having the (seemingly common) issue of sputtering and backfiring at 4k rpm due to the Pertronix. Got that sorted out, but now I’m having a bigger issue.

The car has an intermittent issue of complete loss in acceleration during pulls. It goes into this state where there’s just no acceleration, feels like there’s hardly any power going to the wheels. The sound also changes, it starts to sound way lower and raspier. Just a low hum instead of the beautiful sound it normally makes. Seems to be more common going uphill, and from what I can tell, I can prevent it by keeping it under 3k rpm, although it’ll still eventually do it if I’m going uphill. When it’s in this state, it struggles to idle. It’ll sputter around then die.

I’ve done the following troubleshooting so far, yet the problem persists:
 

Checked to make sure the vacuum advance was working properly

Swapped back to points

Swapped carbs for my old 3 screw SUs that worked before (normally run rebuilt 4 screw)

Went from 3 ohm to 1.5 ohm coil and installed the ballast resistor

Checked float levels

Reset spark plug gap

Put heat insulation around the metal fuel lines

Richened up the fuel mixture

 

I’m thinking of using my borescope to look down in the tank to look at the pickup line and make sure there’s nothing on the end of it. It’s becoming super frustrating, especially after a nice full engine rebuild. Any help is of course much appreciated, thanks in advance!

Jason

[datzenmike]

Change your fuel filter. If this is not it, no big deal, they are cheap and probably due to replace anyway.

 

If you changed recently, pull the hose off the carburetors and direct into a suitable container. Turn the ignition on and you should get about 1.5 liters (or more) in one minute. A liter is pretty close to a US quart.

 

 

 

Vacuum advance is at maximum when decelerating and is at minimum when throttle is full open. Basically it decreases with engine load.

[Madkaw]

Where is the fuel pump mounted ?

What type fuel pump ?

 

[banzai510(hainz)]

you recheck your rocker arms gab on the valves.

 

to me if seems like your starving for gas

 

feel if coil is HOT( im guessing here since you did so  much swapping around you could have went backwards

[datzenmike]

3 hours ago, Madkaw said:

Where is the fuel pump mounted ?

What type fuel pump ?

 

 

Mechanical pumps are on the front right side of head, an electric pump is shown as an option. Perhaps they work together? Should be in line near the tank and look like the ones used on the 620 with air con and all 720s..

[Madkaw]

2 hours ago, datzenmike said:

 

Mechanical pumps are on the front right side of head, an electric pump is shown as an option. Perhaps they work together? Should be in line near the tank and look like the ones used on the 620 with air con and all 720s..

Yes sir - I know . My point was the pump in a place that can pull the fuel properly . Some might think the electric pump will work in the engine compartment. The original pumps were back by the pump and worked in conjunction with the mechanical 

[datzenmike]

Well they are similar to the 720 electric pumps. There will be a fuel filter before the pump and the bottom twists off and there is a filter there as well. Perhaps the mechanical pump cannot supply an L24 engine and the electric is to augment it. Maybe it;s simply not working? Can you hear the pump whirring away?

 

[Icehouse]

I would punch it till it does the power loss, then turn key off and coast to the side of the road.  Open up float bowls and see if if both are full.  I've had an issue with bent float that would sometimes stick and then you loose one SU.   If they are both full at lest you know the issue is spark.