Near-empty fuel filter?

[ByStickel]

I'm trying to get my early-'71 all dialed in, and had noticed some running issues. It would sputter under load when I tried to move it while cold. It felt boggy when revving, and would backfire on the overrun.

I drained and cleaned out the tank. The supply and return lines are clear. I pumped air through the supply, return, and fuel rail. I also used a vacuum pump to draw fuel through the supply line, and it seems to come readily.

I bought a new fuel filter, and upon installing it, it filler up to about 4/5 of capacity, but the running issue remained. After re-checking the lines and adding a bit of timing adv. the car  started running well, but the filter is now nearly empty.

I also ran the car off of a small glass jar (9oz)of fuel, and while running, it drained that bottle in less than 10 seconds at idle, so it seems like the pump is  returning fuel to the tank just fine.

Right now, while warm, the car revs freely, with no backfires on overrun. I'm not sure what it'll do after it cools off (will check later), but wonder is anyone knows what's up with that filter. I'd feel better if it filled normally.

[David F]

Weak fuel pump.

[Zed Head]

It it a mechanical pump or electric at the tank.  One pulls and one pushes.  Could tel you something.

[ByStickel]

Original-style mechanical pump, so pulling through the filter.

[ByStickel]

If anybody was wondering, it looks like L16/18/20 fuel pumps are 2-bolt vs the 3-bolt on the Z. I thought I could grab another pump to install. Nope.

[David F]

I will expand on my earlier 3 word reply.  I had the same issues.  Tried two new pumps only to have them fail or pump insufficiently almost immediately.  So, I installed an electric pump and left the mechanical pump in place.  The electric pump is controlled electronically which means it needs a tach signal to stay pumping after a 3 second prime period when power is applied (i.e. key on).  The electric pump solved the empty fuel filter issues and cold starts after prolonged period of non-use was great.

But, problems soon became evident.  After the engine reached operating temps and got a little warm due to idling in traffic, the engine tended to flood due to fuel overwhelming the carb float valves.  So, my current solution was to disconnect the tach lead from the electric pump controller.  This prevents the pump from running while the engine running; thereby, relying on the mechanical pump.  But, the electric pumps still runs for 3 seconds when the key is placed in the one position; thereby, priming the carb fuel bowls.

When the engine is warm to hot, it will stumble slightly on hard acceleration due to inadequate fuel from the marginal mechanical pump.  I cannot seem to get the fuel delivery just right.  But, an occasional stumble is better than flooding, so living with it for now.

Edited March 27, 2020 by David F

[Mark Maras]

 How much pressure is the elec. pump putting out. Carbs only need 3-4 psi. Higher pressures can overwhelm the needle valve. You haven't mentioned what carbs you're running but are you sure the float level(s) are correct? My early 71 (mech. pump) would run fine at all rpms with the filter 1/3-1/2 full.

[ByStickel]

Thanks for the full reply, David. I have a buddy coming today to buy a 510 from me, and he's bringing a couple of mechanical Z pumps for me to try. I pulled the existing one apart, and the diaphragm looks good, so I dunno.

I should check the floats, too. The car ran fine before coming completely apart for resto. I didn't change anything with the carbs, but you never know with SUs.

[Zed Head]

Is the arrow in the picture correct?  And the orientation while driving is as shown?  And is the picture taken with the engine running?  There's no fuel even at the outlet of the filter, to pass through to the carbs.

You can get a bubble in the filter if it's on its side, but fuel will still get through, like Mark implies, if the filter outlet is covered.  The carb's float bowls will fill until the float valve seals.  Try to separate your filter problem from the running problem.  They might not be related.

[240260280]

I videoed one running at idle: