73 240z fuel starved with mechanical flooded with electric pump

[AndyStep12]

I have what must be a fuel delivery problem. When I bought my z it had a holly red field pump that puts out a boat load of psi which fed the stock mechanical fuel pump.
This combination resulted in frequent flooding of my carbs.  So while it was in the ship for a new clutch the mechanic bypassed the old mechanical pump. This is ran great, cold but would flood badly and stall following warm starts.

I thought over this setup and thought about how much I would prefer a ago k setup. Plus there was no inertial switch to kill fuel flow when the engine died.

This morning I disconnected the power to the Holly electric pump. Then I installed a fresh mechanical pump and routed the fuel lines back to it.  I compared the old mechanical pump to the new one and it seemed bad. The old pump didn't blow much air when I pushed the lever and the new one provided a nice puff. The car fired up nicely. But it drives horribly now. In 3rd gear the engine acts fuel starved (I think) and stutters, lurches, backfires with this new mechanical pump.

All that said, would fully removing the electric pump help?  Is it an added strain for the mechanical to suck fuel past my unpowered holly red fuel pump?  Or am I looking at carb or other problems? The drivability can be great with the electric pump, why can't I get the same results with a fresh mechanical pump?

 

[AndyStep12]

Oh yeah.  Stock engine. Some one in the past changed out the flat top carbs for round tops though. Which is nice.

[SteveJ]

If you are pulling through a dead pump, yes, it will reduce flow.

[Zed Head]

2 hours ago, AndyStep12 said:

 boat load of psi

You could measure.  Always a good idea.

Carbs are supposed to have valves that determine when fuel is allowed into the float bowl.  Flooding is usually caused by bad valves or bad floats.  

Food for thought after you bypass the old electric pump.

[Mark Maras]

The original problem may have been caused by an elec. pump "putting out a boat load of pressure" that was forcing fuel past the float valves. At this point in time I'd do a fuel pressure and fuel volume test as well as checking the float valves and the float height.

[siteunseen]

Mine runs great with the mechanical pump only, 2 round top SUs.  Yours should too after you get the floats back to normal.  It's hard to imagine a high pressure Holley on my 240.  Fuel would be leaking out everywhere. 

I had the Blue ones for my boat motor, set at 3.5psi.  They were fantastic pumps, noisy as hell but I knew it was running. 

[AndyStep12]

This morning I routed the fuel lines around the Hugh pressure Holly pump. Now I'm all mechanical. No more flooding. Pretty anemic in power compared to having the other pump. But the mechanic tinkered with the carbs just a bit to get it running well (when not flooded) with that air fuel mixture. I'm sure I need to adjust the carbs now. It's all new to me but I'll read guides and such. I'll try to balance them and whatever else I need to do. I think the floats might be fine now. Everything I read made it sound like that field pump would push past good floats. Forgot the exact Psi but multiple times spec.

 

Thanks for the help guys. It got me to make progress today.

[Zed Head]

2 hours ago, AndyStep12 said:

No more flooding. Pretty anemic in power compared to having the other pump

A properly spec'ed and functioning fuel pump should have no affect at all on engine power.  If it does, something is, or was, way out of whack.

Don't try to piece-meal carb work.  I think that you have to be even more methodical than working with EFI to get these carbs to work best.  You're probably not going to find one big thing that makes things better.

Good luck.  Progress is good.  Carry on. 

Edited August 20, 2018 by Zed Head

[bartsscooterservice]

On ‎8‎/‎18‎/‎2018 at 11:55 PM, AndyStep12 said:

I have what must be a fuel delivery problem. When I bought my z it had a holly red field pump that puts out a boat load of psi which fed the stock mechanical fuel pump.
This combination resulted in frequent flooding of my carbs.  So while it was in the ship for a new clutch the mechanic bypassed the old mechanical pump. This is ran great, cold but would flood badly and stall following warm starts.

I thought over this setup and thought about how much I would prefer a ago k setup. Plus there was no inertial switch to kill fuel flow when the engine died.

This morning I disconnected the power to the Holly electric pump. Then I installed a fresh mechanical pump and routed the fuel lines back to it.  I compared the old mechanical pump to the new one and it seemed bad. The old pump didn't blow much air when I pushed the lever and the new one provided a nice puff. The car fired up nicely. But it drives horribly now. In 3rd gear the engine acts fuel starved (I think) and stutters, lurches, backfires with this new mechanical pump.

All that said, would fully removing the electric pump help?  Is it an added strain for the mechanical to suck fuel past my unpowered holly red fuel pump?  Or am I looking at carb or other problems? The drivability can be great with the electric pump, why can't I get the same results with a fresh mechanical pump?

 

 

Carbs are not designed to have an electric pump feeding them ( unless the manufacturer wanted it ).

Many people don't seem to understand how carbs work instead of fuel injection these days...

Yes the fuel lines need to go from tank directly to the mechanical pump... , remove the electric pump.

Don't forget the spacer plate between the block and mechanical pump..

Sorry to say this, but your best bet will to rebuild the carbs to OEM spec, THEN adjust them properly.

 

[Mark Maras]

Datsun did add an elec. fuel pump to the 73 and 74 Zs to try and alleviate fuel percolation and vapor lock problems with limited success. Regardless of the number or type of pumps the fuel pressure at the  carbs needs to be somewhere in the 3.5 - 4 p.s.i. range. More pressure than that will force fuel past the float valves (needle and seat) and over fill the float chamber.