Throttle stumbling

[siteunseen]

Yep, cost me $5 for a Radio Shack volume knob. 

My '77 would spit and sputter until 2,500 rpm then go vroom vroom. After the Pot tweak it leaves out strong and continues to vroom vroom. 

[Jeff G 78]

4 minutes ago, siteunseen said:

Yep, cost me $5 for a Radio Shack volume knob. 

My '77 would spit and sputter until 2,500 rpm then go vroom vroom. After the Pot tweak it leaves out strong and continues to vroom vroom. 

Same here

[Lenbo211]

On 5/12/2020 at 8:38 AM, Zed Head said:

Do this coolant temperature sensor "tweak" and you'll probably get rid of the stumble.  It might run okay, or it might be gassy in other areas.  The low RPM, low throttle, stumble is a typical lean mixture problem, on many old Z's even with stock cams.

http://www.atlanticz.ca/zclub/techtips/tempsensorpot/index.html

p.s. and just for conversation, if I remember right, one reason big cams run poorly at low RPM is because the cylinders don't get cleared out well, and they tend to push air back through the intake valve before they start pulling.  So you can get lean cylinders and rich cylinders and dirty mixture cylinders all at the same time.  That's where the rumpity rump sound comes from.  It's actually just the sound of an inefficient system.  But, adding fuel can fix the lean cylinders so that at least they'll be firing.

Anyway, the "tweaker" is the simplest way to get richer.  No FPR adjusting or AFM fudging necessary.  Too bad Radio Shack isn't still popular, those little potentiometers are hard to find now.

Did this.  It definitely runs better now. throttle response in shop that I'd expect.  Can set idle up around 1000 rpm no issue. However, when I take it out for a drive, it stumbles when I add throttle under load and takes a second or two to take off.  

Something else is that when I'm on maintenance throttle going down hill, it is backfiring. (i think backfiring. I have hood off so not entirely sure if it is firing out of the air filter like it did in the shop, or if it is coming out of the exhaust.

This is the first time I've driven this car.  

Edited May 16, 2020 by Lenbo211

[Zed Head]

Keep turning the knob until it takes off like it should.  You should be able to make things so rich that the engine won't run, you can flood it.  Then go back and do something else to try to get the idle leaned out.  That's the game, tuning the untuneable EFI system.  

With the aftermarket systems you change things by computer.  You'll need to do mechanical and electrical things directly.

[siteunseen]

I played with mine for a week before I found it's sweet spot. Then zip-tied it all up.

[Lenbo211]

3 minutes ago, Zed Head said:

Keep turning the knob until it takes off like it should.  You should be able to make things so rich that the engine won't run, you can flood it.  Then go back and do something else to try to get the idle leaned out.  That's the game, tuning the untuneable EFI system.  

With the aftermarket systems you change things by computer.  You'll need to do mechanical and electrical things directly.

I have the dial turned the whole way to get it where it is.

[siteunseen]

But I put mine inside the cabin temporarily so i could turn it while driving. I could make it run cooler on the interstate and flood it completely out like Zed says.

I used speaker wire through the firewall and put the knob in passenger's seat.

[Zed Head]

3 minutes ago, Lenbo211 said:

I have the dial turned the whole way to get it where it is.

You need more ohms.  You could put another potentiometer of the same type in series.  Or find one that has higher resistance at the top end.  Or add a resistor to the circuit and use the potentiometer for tuning.

Old radios use them for volume control.  If you have any laying around you could scavenge.

At least you have a path to explore.  Might at least show the potential of something like Megasquirt.

[Lenbo211]

1 minute ago, Zed Head said:

You need more ohms.  You could put another potentiometer of the same type in series.  Or find one that has higher resistance at the top end.  Or add a resistor to the circuit and use the potentiometer for tuning.

Old radios use them for volume control.  If you have any laying around you could scavenge.

At least you have a path to explore.  Might at least show the potential of something like Megasquirt.

I was thinking the same thing.  When I bought it, I bought a pair so going to put another one in line.  

[siteunseen]

Mine is an Alpha A5K.

https://www.radioshack.com/products/radioshack-5k-ohm-audio-taper-potentiometer?variant=20332246341