Holley help. 390 running way rich.

clutchdust

Millionaire Playboy
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Mar 27, 2008
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This is the 390 Holley on my little 240z.
For the most part I've been really happy with the way my car has been running even though I've felt it was running rich the whole time. Pulling plugs every couple hundred miles and they're coated with a pretty thick layer of black carbon. Not to mention puffs of black smoke just revving it in neutral
Today, I finally got the O2 sensor connected, at least temporarily, so I can prove my theory. Sure enough, the sensor shows it just about pegged on the rich side under all conditions short of lifting off throttle. In neutral, when I lift off the throttle the meter comes way back, almost (but not all the way) to lean until it hits idle speed and then goes right back to rich. While in gear, lifting off (engine braking) will put it just about in the middle of the "stoch" range.
The only other time it appears to come off full rich is right at throttle tip in and then it goes just to light side of rich. I'm thinking that tells me about the only thing that's right on this set up right now is the pump shot.
So I just put #52 jets in it, it had #54, and that made almost no difference. I have the idle mixture screws turned almost all the way in. I mean literally, I think they're turned out maybe 1/8 turn.
I set it up with the initial settings in Lar's paper but it still runs way rich. Since I have done several things to lean it out but so far nothing is working.
Any thoughts?
 
I"m catch hell for this but.....










i'ts a HOLLEY.....fuggetaboutit....




:crutches:
 
Do you have access to a vacuum gauge?

Warm up the motor, connect a vacuum gauge to a non-ported source. Turn the idle mixture screws in all the way (Gently!) then turn them back out 2 1/2 turns. Adjust the idle mixture screws for maximum vacuum, re-set idle speed screw as needed. Remove the vacuum gauge.

Is the choke opening up all the way when it is warmed up?
 
Last edited:
Yeah, choke is fully open.
Didn't really think about using vacuum. Like I said, I made the initial settings per Lars' paper. Worked well on the 750vs on the Vette. But I've thought I was rich on the Datsun ever since I set it up. Now I can prove it. I'll give it a try a little later today and see what that does for me.
 
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