Ughhh I need help. Tuning issues.

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340Duster247

340Duster247

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Ok guys I'm confused and need your assistance. I would like to say I'm pretty good at tuneing,but I'm a little stumped.
My 408 stroker with a QF 850 dp is giving me trouble.
So today I went to a buddies house and the car was running great. Outside temp was around 20 celcious. (68 for the American boys) and Still a little rich when I get on the throttle for the first little bit, but very happy and highway cruise was around 13.5. Afr. Idle was around 13.5-14.5. Anyways this is were it gets weird. Sun goes down and temp drops to 15 celcious. (59f) I flash the car up and idle is pretty lean. 15:5 16:5. Even when warmed up. So I drive away and the car runs as normal. Afr's are normal. I go to merge on to the highway and give quarter throttle and AFRs lean to 15:5 under load and the engine stumbles and misses. Let go of the gas and all is normal. Give really slight throttle and it's good. Give a little more throttle and rpm and leans out again and miss. .. I now come to a light and the car is still a little leaner at idle then it should be.... I don't get it. This entire time the fuel pressure is rock solid at 6.5.
I hope this makes sense.
 
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. When I gave 1/4-1/2 throttle the afr went to the 15+ ratio and the engine would sputter/thud/hesitate feeling.

Checked plugs when I got home. No signs of detonatin.
 
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What size are the high and low air bleeds. Check and make sure there isn't trash in one. Drive it again with and check your vacuum when it acts up. You may need a power valve that opens sooner.
 
Sometimes the fuel pressure is good at idle but drops under load from a restriction or a bad pump. Does the pressure hold while reving?
 
I'll check that justin. The fuel pressure stays when revved.
I just find it weird how it was fine but then 3 hrs later acting up. I check the air bleeds too
 
Locked out at 34.
I upped the power valve. I'll see later on if that makes a difference.
The confusing part is that it was also lean at idle that night ..... It's only done this once before. Which was a cooler night as well.
 
That's a ton of timing for a street car. You probably have detonation or other grossly advanced timing issues.
 
It'll take a pretty darn big cam to want to be locked out at 34 degrees!
 
At the strip its no problem. Good fuel, 20 degree start retard. Engine pretty much see's wide open throttle and loads at the flash stall which is where you'd have full time in anyway.
 
No signs of detonation issues at all.plugs look mint. Cam is 242/252 and just under 600 lift.
I have a start retard but it doesn't seem to need it. I have a pretty stout starter and the high timing does not effect hot starts.
Car runs better locked out. Actually was advised from respected guys on her too lock it out and see how the car likes it.
Car idles just under 1000. And when I had a curve in the distributor it was all in at 2000-2500 anyways.


My combo
408 11.5:1
Porter eddy heads
Super victor intake. Port matched
36-3800 stall
4:30gear
242/252 110LSA 596lift with 1.6 rockers.
Quick fuel bdq850
 
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Yeah the car is running lean in the colder, thicker air. I would fatten up the primary jets and use a larger accelerator pump squirter. The pump squirter should take care of the stumble when you hit the throttle.

I still think that is a pretty small cam to be locked out at 34 degrees. But it would want probably 20-24 degrees initial and then maybe the 34 total.
 
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First, check to make sure a vacuum cap hasn't popped off someplace. One of the little ones will make the car act like you say, but check the one that goes to the brake booster also. A vacuum leak anywhere could do it.

Idle is spreading between 13.5 and 14.5 at 68* fahrenheit? I would err on the side of 13.5, as it keeps it a little richer when the weather gets colder.

Have you checked your idle screws and PV vs. a vacuum gauge? Idle should be set at highest vacuum reading. PV should be 1/2 of "loaded" idle vacuum, rounded down.
 
If you are running in the 2500 rpm and lower range you are using the idle circuit.
Make sure the float levels are stable and in the middle of the sight glass, set idle mixture to 13.0 engine hot in neutral. Richer idle fuel mixture is a compromise for driving the car on a warm afternoon and going home in the nice cool evening air. I always get the pump shot close for street driving and fine tune it the track using 60' time.
It's not just the temp drop affecting your AFR, the relative humidity also rises after sunset and density altitude drops like a rock after sunset.
IAT, MAP sensors, etc do this in real time with FI. You have to find a compromise point with a carb on the street.
 
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