Need Carb Advice (long and detailed)

[280zwitha383]

The more I mess with my 670cfm Street Avenger carb the more I want a 700 or 750 double pumper. However, I'm trying to save money for a wedding, honeymoon, and house . So, I've been trying to get the one that I have running to where we can get along for now. My problem is that when I punch the throttle while moving I usually get a little bit of a flat spot at slower speeds and a very noticable flat spot at higher speeds. Once whatever catches up it's all smiles, but until then I'm pretty disappointed. Besides the flat spot I've been pleased with the performance of this setup. So here's what I have.

 

383

10:1 compression

Dart IE 215cc heads

SLP51010 cam (.490/.50? lift and 280* duration roller/ hyd. lifters)

Victor Jr. Intake

670cfm Street Avenger

Small tube headers going into a single 2.5" pipe w/ cutout

th350

3.90 rearend

stock mechanical fuel pump (ZX)

 

I haven't tried a whole lot of things but I have played with the vacuum springs a little and I just can't bring myself to put in anything stiffer than the stock spring because it feels like I lose power up high. With the lightest spring if I punch it the car almost literally falls flat on its face. I'm pretty sure the carb has stock jets, power valve, etc. Now I would think that some bigger jets might help stuff out a little because it's a borderline carb for the engine and I've read that with a single plane intake you don't get as much vacuum and should install larger jets (like 2 sizes on both). I don't think I'm running rich and I've thought about trying to change out the power valve but I'm pretty sure it's a 6.5 and at idle I have around 11 psi of vacuum and in gear while idling I have 8 or 9 (so that shouldn't be the problem right?). I have never been able to get any kind of vacuum changes by adjusting the idle mixture screws unless it's under half a turn and trying to die, and I've tried the mixture screws everywhere from half a turn to a turn and a half without any noticeable change in performance (seat of the pants is difficult to judge because of the extreme gearing). I'm positive that I don't have any vacuum leaks and the only thing I have that's kind of weird is the check valve immediately after the carb which I would think doesn't ever send a low pressure signal to the tranny. I need to know where to start. I can't imagine that this thing could possibly want stiffer springs because everyone always says in a car this light the lightest spring should be used and my only thought is that maybe the carb is just that much too small for the engine and it pulls the secondaries open quicker than most people's since they are probably running a larger carb on their engines. From the paper that came with the springs it doesn't seem to me that anything stiffer than the normal spring will ever completely open up since I don't take the engine over 6k rpm. Any advice will be greatly appreciated. Thanks

~Lance

[Alf]

sounds like you do not have the correct power valve installed or your acelerator pump system is not tuned properly. if the jetting is close, try installing a 5.5 pv then a 4.5 pv and see what happens, 11 inches of vacuum is pretty low. another option is squirters and pump cams. try mashing the gas from a low speed in first gear, if the car goes initially then bogs, you need a larger squirter, if it bogs and then goes, you need a smaller squirter. the pump cams basically just vary the shot of fuel dumped into the carb. when you change the vacuum pod springs you are only tuning the secondary side, if i was you i would try to tune the primary side first and then the secondary. that victor jr. intake probably isn't helping much, i would recomend a performer RPM air gap or another good dual plane that better suits the limited RPM capabilities of the 383. i recomend getting the Holley Carburetor Handbook by Mike Urich. it covers all 4150 and 4160 style carbs in great detail. good luck.

[dr_hunt]

several things come to mind.

 

1. Accelerator pump shot not providing enough fuel or not fast enough.

Cure: change accelerator pump cams, rare occasions change squirters.

 

2. Vacuum secondaries opening too soon, try a stiffer spring.

 

3. Timing

When you nail it you have no vacuum so your timing will be retarded from what it would be at cruise. That's why msd's are sooo nice. Try setting timing at 35 degrees total advance with the vacuum advance plugged and see if the stumble goes away.

[280zwitha383]

Is there any way to tell whether your jettings are close without an a/f sensor? I do have my timing set at 36* total and 14* initial. I will try the accelerator cams and a new power valve and see what happens. Should I get a dual stage power valve or a single stage? Alot of the reason I haven't tried anything with this carb is because the parts stores around here don't have the parts I need like acc. cams, dual stage power valve, or assortments of jets. So I would have to order them (or look around more here) and I just haven't had time lately.

[grumpyvette]

the guys are correct!

your symtoms point to the engine getting a LEAN mix ,durring the rapid increase in air flow, probably because the accellerator pomp and or power valve need to be set up to provide a richer A/F ratio durring the transition from low to the high air flow states

Id set the accellerator pump up to give the richest and longest durration shot you can as a first test, then Id sellect a stronger spring to slow the rate the secondaries open,it won,t cost you any power simply because the engines fully able to suck the secondaries fully open to the match the rate of flow it needs with most of the springs available

BTW ID also make SURE the ignition timing and advance curve were set up correctly , ON MOST SBC engines, youll need to have about 6-9 degrees at idle and have the ignition curve SMOOTHLY increase to give full advance by about 2900rpm and reaches the full 36-38 degrees of advance and check for vacuum leaks

[dr_hunt]

Yeah, you read the spark plugs. To do that with a new set of plugs, you run it out in high gear WOT, let off the gas, kick it in neutral and kill the ignition. Then pull the plugs and see what they look like.

 

To check jetting, you jet for highest trap speed or best ET. Best ET is usually alittle lean and best MPH is alittle rich. The two are almost never the same jets wise.

 

Rule of thumb is stock jets plus or minus a jet for every 2K altitude in feet. But as John Scott has mentioned its really about density air, not physical altitude which most tuners have known inside out for many years but they never share cause they want to be the only tuner in the land.

 

The problem with too big of a DP holley is that you don't get good signal on the boosters and they don't work well, too much pump shot and too big of jets make for a dull boy and lazy car. Vacuum secondaries are great for traction limited cars like the z and they make good power like anything else if tuned right.

 

The dual stage power valve is much too intricate of a tuning choice right now. I'd stay with the single stage until you get it running right then play with that aspect to further tune the fuel curve if need be. There is a chart for accelerator cams that tells you volume it delivers, just pick a color that increases that volume after you make sure that there is no play in your accelerator pump mechanism and that any blip of the throttle causes a "bleed" at the squirter. You want instant squirter action when the throttle is even slightly moved.

[280zwitha383]

Yeah' date=' you read the spark plugs. To do that with a new set of plugs, you run it out in high gear WOT, let off the gas, kick it in neutral and kill the ignition. Then pull the plugs and see what they look like.

 

To check jetting, you jet for highest trap speed or best ET. Best ET is usually alittle lean and best MPH is alittle rich. The two are almost never the same jets wise.

 

Rule of thumb is stock jets plus or minus a jet for every 2K altitude in feet. But as John Scott has mentioned its really about density air, not physical altitude which most tuners have known inside out for many years but they never share cause they want to be the only tuner in the land.

[/quote']

 

Dr., please keep in mind I'm trying not to spend any more money than I have to and I don't have a lot of time to work on this thing (yeah, I know, I'm in the wrong hobby). With that said, I have to take off my headers to change out my spark plugs because the heads are angle plug and the headers run too close to the inner spark plugs to get a socket on them, so maybe I'll try that with a few new spark plugs on the outer cylinders. Second, the track that we have in this town charges $20 to run your car and it's an 1/8 mile track which in my mind is way too much. And last, how do you know what the air density is in your area? The weather channel?

 

I really appreciate the help guys and I'm sorry if this is somewhat of an elementary post.