Idle tuning

[aochider]

1978 280Z, Holley 4160 (pn 8007, 390cfm), Arizona Z intake manifold

I'm having trouble getting the car to run at 800RPM. I have the timing set at 10deg (I tried 15, but maybe did something wrong, because the idle could not get very low at all when the timing was that high). The idle mixture screws are at about 1 turn out, and the idle screw is about half a turn in (any less and it ceases to move the throttle at all).

Since the idle screw is basically all the way out, I've been adjusting the mixture screws with vacuum hooked up. The lowest I can get it without the engine almost dying and then surging is about 900-950RPM. That produces about 16-17 in/Hg of manifold vacuum.

Possibly also of note: unless you give it gas fast enough for the pumper to put in a lot, the engine almost dies when the throttle is pressed. Not sure if this is a symptom of the idle mixture screws or if I just need to put on a more aggressive pump cam (I'm using orange, ie, 41R466).

I read Holley's guide to tuning idle, but the car just dies or almost dies and then surges (to about 1200) if I tune down the idle mixture screws.

Any advice please? Thank you for any help to a new-to-cars, really-new-to-carburetors person like me!

Edited August 3, 2014 by aochider

[porkbun]

The problem with advancing the timing is that it requires a leaner mixture. If you have a perfectly good running car and just advance the timing it will run very rich. Retarding the timing will make it run lean. Timing adjustments will require jetting changes. First figure out where your timing is supposed to be depending on your vac advance then after thats all set you can start jetting your car.

[aochider]

The problem with advancing the timing is that it requires a leaner mixture. If you have a perfectly good running car and just advance the timing it will run very rich. Retarding the timing will make it run lean. Timing adjustments will require jetting changes. First figure out where your timing is supposed to be depending on your vac advance then after thats all set you can start jetting your car.

i understand how to do each individual component of what you said (timing advance/retard, jetting, vacuum advance, lean/rich mixture) but i had on clue that timing affected jetting. do you by chance know of a good resource where i can read more about it? im having trouble finding important details like that and am instead finding mostly the same old "plugs are fouled, must be rich" kind of advice.

thanks!

[Zed Head]

I would find a small block chevy based forum, sign up, and ask a general "how do I tune my Holley" question. That carburetor has been around forever, but not many Z cars use it. I'd go to small block chevy land because there are more of those around.

I'm sure that I've seen Holley tuning videos on youtube also. Forget about 1978 280Z, or Datsun, or L28, it has nothing to do with tuning a Holley.

Here's a few that popped up when I Google "tuning a Holley carb".

Holley Performance Products Forums - Holley Carburetor Tuning Tips - Blogs

https://www.holley.com/support/

Holley Tune, Part 1

[porkbun]

I just learned it from tinkering around with my car. When I got the car it had a stock 390cfm but was also advanced as far as the distributor would go. When I went back to stock timing I noticed that the car was running really lean/barely at all. Another thing I noticed was that if you do end up advancing the timing more, try mid grade and premium fuel. When I got the car it ran best on premium. Not Im at ~20 degrees and it starts and runs better on 89 midgrade. Holleys youtube videos are great and will give you a pretty good idea of how your carb works and how to troubleshoot

[aochider]

great, thank you very much for the advice!

btw, just so im not chasing something totally unreasonable, but i should be able to get my L28 down to 800rpms without it dying or surging periodically, right??

EDIT: that is my near-term goal so it would be helpful to know if i can actually achieve it. i will, of course, still look at those resources and try to learn as much as possible.

Edited August 4, 2014 by aochider

[Zed Head]

Here's something you can check that might be part of your problem. If you have the vacuum advance from your distributor connected, make sure that it is connected to ported vacuum, not full-time vacuum. There's a lot of discussion out there about the pros and cons, but the two main results of using ported vacuum are that the timing is retarded at idle, giving a cleaner burn, and the engine idle speed doesn't bounce around as timing advance moves with intake vacuum.

One way for the engine to die,with full time vacuum advance, would be for intake vacuum to drop enough to reduce vacuum advance, causing lower RPM, ultimately leading to the engine dying. A vicious cycle thing, especially if static timing is set with the vacuum advance connected.

Just a possibility.