I have changed my setup last fall and my car is as follows.

th350 trans with 3.73 from a 700r4 same 3.73.
11.5" 2500 stall from 10" 3000 stall in the 700r4.
car is 3200 pound iroc with 383 vortec stroker. comp cams 276hr 224/ 230 duration 540 lift with 1.6 rockers. rpm air gap intake. 9.3 compression. timing locked 36 degrees.

I am having a problem going from a stop to wide open all 4 barrels. The carb wont carburetor. The engine dies or stumbles badly at 40 degrees. I have gotten it to work ok off the primaries only but all 4 is not right. From a 50 roll things work good no bogs. cruising there is no surging leading me to beleive stock main jets are ok. Idles also fine once warmed up.

I have swapped to a green cam #2 hole on primary from blue #2 with the old setup. I am going to try green #2 on the secondary asap. shooters are 42 both sides.

My questions are 1.) with this combo or any average combo do mechanical secondary carbs typically take wide open from a stop with enough tuning? As it appears this has been sort of an issue throughout even with the old setup to some degree. 2.) Did i lower the effective first gear ratio to much at 9.4 to do what i want? 3.)would i really hate a vacuum secondary 750 holley over the prefered dp?
4.) Is the intake just to cold all the time in the cooler weather from no heat being plumbed in? I more less want it to be able to run from 40 to 90 degrees without retuning all the time. I will settle for less gas milage to do so.

What do you experienced dp guys think? when this car is right it will turn the tires 4-ever from a stop but that is typically 75 outside and i basically feather the throttle not just going wide open suddenly. As it really dislikes doing just that.

 

42 squirters? Wow, I have 40's in my 750 DP running E85!

Put smaller squirters back on. If you need anything over 37, you also need a 50cc accelerator pump to go with it.

For a given CID & CFM DP, WOT no-bog is dependent upon RPMs, not gears. 2500 RPM stall should be plenty for a 750 on a 383.

 

42 squirters! you're giving that motor a fuel bath everytime you hit the pedal. I'm running a 31 in front and 35 in the rear with a 30cc accel pump(no pv in rear) and it responds like fuel injection, transitions are nice and smooth. I'm running a fairly bigger cam than you are too.

 

I agree 42s are too big. I also happen to be running a 31 and 35. My accelerator pump cams are red in #1 hole. I've heard the blue #2 and green #1 work well.

 

thanks for the replies. I would say something is defenetly not right with the carb then. seems like the main sytem startup is delayed for some reason and the larger squirters are covering that up to some degree. Interestingly enough larger main jets also help the situation but a 20 degree swing in temperature and the thing is just too rich. That was up to a 73 main on the primary. I should not have to jet this thing daily for a clean operation.

I defenetly can't run small squirters at all. could it be do to the idle air bleeds being drilled out and the timing of the carb circuits fuel delivery is out of wack? This would be my guess. I really want a vacuum carb due to the less complexity, but i dont want to waste cash trying one only to find out i hate it. I like the throttle response of a dp when dialed in.

The carb liked the change to a larger squirter and green cam on the primary as it would bog slightly at speed when winged open just using the primary side. This is where i could have fixed the problem with a bigger main jet instead of the cam shooter change as it worked great going to a 73 jet. Temp that day was in the 60's. but doing so also makes the plugs much darker as in too rich at cruise.

oh and the squirters just limit the fuel coming through them based on a time frame not amount of fuel as the amount does not change with them. So i am not flooding the engine with them in my situation, Fuel is just their quicker. Example: a 28 squirter vs. a 42 squirter both flow the same amount of fuel but the 28 would hold the fuel shot back for some small amount of time. The 42 would empty the shot some amount of time less than the 28 shooter. And it is defenitly fuel starved to create a stall out when winged open. Its not chugging do to excess fuel thats for sure. The carb cicuit timing is somehow messed up. The mains dont start flowing quick enough is the problem but how do i fix it?

Apperently My shooters and cam choices are larger than what they should be, proving the system delay issue. Maybe ill just jet this thing up and run it that way. the only problem with that is the slight stumbling (different issue than mentioned) on and off the throttle at very slight throttle movement like a cough when in the 80s temperature and black plugs. Arrrrrrr.........getting frustrated I have been in the carb more than i care to talk about and i am not just guessing either i have been in the books . I just dont have the experience some of you guys have. keep the opinions coming... thanks alot.

 

update: I got this thing dialed in very well for the 45 degree temps.

this is what I did. 1.) up the float levels even more to a steady trickle out both primary and secondary. 2.) up to 73 primary jet. 3.) green cam #2 hole both primary and secondary. 4.) 45 shooter primary 35 shooter secondary. 5.) finally got around to install the dynomax 3" ultra flow muffler (back to the stock location) to replace the flowtech dragin off the tar.

This car just may need me to pay more attention to details. I feel with my combo and the auto trans this carb setup is ideal. I will just rejet to a stock 71 primary jet when the temps get above 80 in the dead of summer and i will be happy. Must just be the nature of the beast. oh and After doing research on the topic more, i probably need the 50cc pumps to be in more ideal shooter sizes.

 

Just the idle, or mains as well? If the mains were drilled out, that's probably where the problem lies, and everything else is just a crutch.

I don't see the DP as any more complicated than VS. In fact, simpler IMO.

The size of the squirter affects the rate of the pump shot. The size and adjustment of the pump diaphragm & mechanism control the amount of the shot.

If the main bleeds were drilled out, that would do it.

See the "crutch" comment above.

You never mentioned PV (unless I missed it).

 

for one you have too much cam for such a little stall. You need to up it to 3500 or 3800 and it'll help tremendously. Secondly vortecs don't need 36 degrees timing, try it locked at 30 or 32 and it'll run better as well.

 

Besides having extremely large squirters, your setup is not the best for that
camshaft.

Compression is too low.

Stall is too low.

What is the installed intake centerline ?

The torque converter should be in the 3500 range, and the camshaft could
probably use some advancing to increase the cylinder pressure at low RPM.

That alone would really wake up that motor.

Stick with the mechanical Carb. You just need to take some time to set it
up. I suggest brining it back to stock config and re-tune.

What part number/series is the 750?

Lastly, if the bleeds are drilled you can insert rods (well secured) for a temporary fix until
you get the proper size. Then have the correct bleeds installed.

Good luck, I think there are enough suggestions in this thread to keep you
busy for a while.

Change one thing at a time, and document the changes. That will help dial
in the carb.

 

thanks for the replies. I still dont understand anyones logic on the shooters being oversized. They make perfect sense due to the combo at hand.
Engine cranking compression is 205 psi
carb is 4779-6 and was new.
no the main bleeds are not drilled. Idles were drilled to lean the idle without having less than 1 turn out on the mixture settings. Power valve 8.5 Idle vacuum is 15 -16" hg in gear 800rpm or so.
I agree a higher stall would be great but i have enough stall already imo. I got sick of the 3000 stall before but that was really probably more of the 11+ effective first gear though oh well.
The 32 degree timing spec is that your opinion or do you run vortecs? Just curious because i have looked into it significantly and i find the dyno likes more timing with a good camshaft involved.. The compression is as high as you can go on 91 octane on those Iron heads when the temp is 90 outside and while in traffic ect... The camshaft is not anything but just right for a 383 motor. this thing has plenty of torque at 2000 rpm and great power to 5500. The intake centerline is 106. 4 degrees advance in the camshaft. That cam works with compression around 9.1 according to comp cams. I have read articles through chevy high where they have run the comp 282hr in the twister engine series and thats the direct step up from what i have.

Its dialed in and flies. It takes full throttle right away like i would expect. My problems were the little things i had mentioned above with the carb. Its like a new car. like what you would expect for the build. I guess ill need to jet when the temp swings to much, but not everybody is real fussy like myself either.
I have been rereading my notes and when the 73 jet made a problem of themselves was 88 degrees not in the 60's as stated i goofed. knowing that i feel that a rejet at that temp is probably appropriate.

 

No, they don't make sense, not even close to perfect.

Think about what they are supposed to do: When at idle, there is no flow through the main fuel circuit. When you open the throttle, the fuel flow through the idle circuit is now insufficient for the air flow, and the fuel hasn't made its way from the fuel bowl to the booster venturi. The accelerator circuit is there to make up for the lack of fuel until the fuel can start flowing through the main circuits via the booster venturi. Similarly, when at cruise, and you open the throttle more, the additional fuel through the main & power circuits takes a little time to get to the booster venturi, so the accelerator pump makes up for that.

Having to use so large of squirters, especially on the primary, indicates you're crutching something else.

 

load on the engine will dictate that shooter size also. The time needed will increase for the mains to start due to gear, stall, weight, rpm air gap intake, vs. a more stock type intake, ect.... I am not disagreeing completely with you i have built 1 carbed car and have no other experience with other setups to really disagree. I depend on what i read and try that recomendation. I have seen people on here with shooters as large as mine and they could i suppose be covering that large hole in carbeuration also for an imperfect carb setup? Maybe.
I just see nothing wrong with it at this point and honestly beleive that retuning the carb will not provide better performance no matter how you slice it. It just runs too good to be wrong currently.

 

You won't be able to get optimum performance at 40F and 90F without changing jets. You will need to bump up your jetting to get it to run right when it is that cold.

Secondly, the air gap manifold makes it even more difficult to get the fuel to vaporize properly at 40F. I assume you are running an open element air cleaner as well? When you stomp the throttle the manifold vacuum falls, the carb gets a big gulp of cold air and any vaporized fuel in the manifold will condense on the cold surfaces. A larger pump shot will help with this, but only so much.

What temperature themostat are you running? If you are running a 160 you might want to try a 180 for the colder weather and see if it helps.

FYI: I have a 750 DP on my car with a Weiand Team G intake, I run #35 squirters front and rear. I agree with the others that you should not need to run that large of a squirter nozzle to get your car to run correctly.

 

engine temp was 200 degrees fan off. thermostat 180. i am at sea level also. I have also run a 35 shooter in the dead of summer without issue. Maybe the intake like mentioned is very finiky to cold air.

 

I actually think now that my float levels for whatever reason were below the ideal level for quick main startup for about the last yr. I am guessing my attention to detail when setting them was not there. Or i was not on level ground?
So that last summer if felt as if it wouldn't take full throttle due to that alone. And now thats its freezing outside like 40f the air gap is a fussy pig and wants lots of pumpshot, main jet, green cam ect..
I bet in june the carb will tolerate smaller everything just fine.. We will see.

 

Sorry to say that most off-the-shelf cams are not 100% perfect for any
motor. They are mass produced for generic combos, and not made specifically for every motor's requirements.

Having said that, the 106 centerline is helping you, otherwise the DCR
value would be unacceptable for your static compression.

Do you know the vacuum reading at idle in Park/Neutral? Post that up
when you measure it. That will indicate how much the torque converter
is loading the motor in gear.

From what I see now, if you have that much vacuum at idle speeds, your
induction side of the motor is severely choking flow at higher RPM. I'll bet
my left nut and a sandwich that your torque curve peaks early and noses
downward as RPM increases.

As for the squirters, the reason they appear to be correct is because
your power valve is too low for the vacuum measured...which means
power enrichment is delayed. When the power valve and engine vacuum
are properly matched, they should be about 2-4 in./hg. different for best
performance.

If you had the proper set up, the engine would take a 28-31 squirter
and increase the pump shot duration (which is what you need).

Lastly, increasing the jet size for idle tuning will get you nowhere fast.
You may improve the idle circuit, but part throttle response and WOT
power will suffer. There are several ways to tune the idle circuit which
do not affect the main circuit



If you're only 1/2 a turn out on the idle mix screws, there's a chance
the throttle blades are well out of the transfer slot. You should also
do a basic tune and clean on the carb before doing any tuning.

clean the bleeds, set the bowl levels, check fuel pressure at the inlets
and get the power valve dialed in properly.

As you can see from the diagram, if the air bleed is dirty, the engine
is going to suck more fuel from the bowl and richen the idle mix.

You posted about an engine bog, everyone is trying to help you , but
it seems you're not willing to try any of the suggestions.

The motor is not going to improve otherwise. Worst case, look at your
notes and restore to the settings which produced the best results.

 

I have that much vacuum at idle because i have the timing at full locked 33 degrees currently. Otherwise its 11" or so at idle with less timing. The power curve does not peak and fall as described i Have run 87mph in the 1\8 and have only improved the car since then. The intake is not restrictive if any its the vortec heads being efficient with the 170cc runners helping to. Also 383 cubes dont hurt the situation.

You must be misreading entirely. I have never tuned the idle circuit with the mains. I am not following you on this one. I am not a half a turn on the mixtures not even close. That was with the small idle air bleeds before i enlarged them. I have them set at 1/1/4 to 1/1/8. The idle is clean currently and can idle for many minutes in traffic. 90 degrees and a 73 main jets make the plugs less than ideal but they never foul out.

I am at the best results. Maybe you read over somehow but it has no more bog period. Its fixed. Why do i want to retune? I agree a power valve increase would be worth a shot and actually bought them last fall. It was at a 6.5 and i went to a 8.5. I kept the 10.5 handy to try at some point and that may be soon.

 

May I ask why the timing is locked? Is this a strip only car?

Do you have a start retard box for the ignition?

Have you tried the total timing upward of 36 degrees? 33 seems a touch
low...but if that is what the engine needs, who am I to disagree.

I'd be curious to see a dyno graph, or 1/4 mile trap speeds if you have them.


You mentioned in a couple of posts, and one other member mentioned to
increase the main jets to tune the idle circuit. Bad idea as explained in
my previous post.



I think (know) there's more power hiding in the engine based on what I'm
reading. I may have jumped a few posts, however the power valve and
squirters are still holding you back.

A smaller squirter doesn't determine the pump shot timing, but rather the pump
shot duration. IE: A 28 will stream a fine shot of fuel over a longer duration,
whereas a 42 will dump it out over a shorter duration.

The pump cams setup the timing and the amount of fuel delivered.

If this was my car, this is what I'd do:

- set the base timing for around 20 degrees.
- Unlock the distributor and ensure the full advance falls around 35-36 to start
(adjust in small 1 degree increments until you get the best trap speed, or
peak HP in this case).
- Check vacuum at idle in Park and at temperature
- Install the correct power valve
- Throw in a 31-31 squiter combo
- Go for a drive

Give it a shot

 

I'd have to agree with lukn4trbl. You may be masking a too low powervalve with the pump shot. My combo nets me 9.5-10" of vacuum in gear at a choppy 750-800rpm idle, so I run a 7.5 power valve. With that cam I'd say 20 degrees of advance would be on the low end of the spectrum. If your idle quality or throttle response suffers signifigantly after changing from the locked timing to a curve you could try welding half the advance slot so you would have approx. 26 at idle 36 total. I run locked timing with an ignition toggle switch: Kept getting better idle/throttle response the more initial I ran all the way up to 36 degrees...