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megaladon6
12-06-2007, 03:06 PM
i have a holley 4175 (i know it's crap, but i have no money) and it has 2 problems. first, the idle adjustment is leaned out almost all the way. if i turn the screws 1/8-1/4 of the way in (to the stops) the engine dies. the idle speed is good (the speed screw is almost all the way out). the engine runs great, i would just like some tuning leeway.
second thing isn't so much a problem-just weird. after the car sits for a day or two, the fuel filter before the carb is dry and it takes about 20-30sec before i get fuel. i do get fuel from the squirters before cranking.
just looking for some ideas on what it could be.
i tried this question over at the g-body forum and got into an arguement with a guy who refuses to accept that my carb has a forward idle adjustment. please take my word for it (i've confirmed it about 12 times) i do have a forward adjustment not the emissions biased reverse adj.

thanks for your help.

whytry
12-06-2007, 03:28 PM
For the idle screws, I would turn them all the way and back them out 1-1/2 for starters. Then adjust from there. Do you have a way to check vacuum @ idle? That would get you an idea of where to go in terms of keep taking them out and how far to go. Make a 1/4-1/2 turn if vacuum increases then go to the next screw and do the same until the vacuum drops then go back to where the vacuum is highest. Then I would check your idle, it might be higher or lower then before.

megaladon6
12-06-2007, 04:15 PM
i went through all that. my best vacuum is with the screws almost all the way in.
the fuel pressur is 6psi and the carb was refurbished by holley

MrQuick
12-06-2007, 07:11 PM
make sure your throttle plates are not past the tranfer slots or it wont matter where the screws are.

whytry
12-07-2007, 08:29 AM
Good point Vince.

megaladon6
12-07-2007, 08:41 AM
throttle plate are almost completely closed.

MonzaRacer
12-07-2007, 10:09 PM
If you have the throttles almost all the way closed it sounds like you have unmettered air getting through.
Try this, flip carb over and look a small screw in baseplate like an upside down idle screw on the secondaries. you need to carefully work on getting it loose enough to adjust , now JUST barely crack the rear blades. The big reason for these are 2 fold, one to keep the butterflies from sticking and to give a slight signal break for the minor idle enrichment the secondaries do.
Now after reinstall ment, get it running, with your vacuum advance unhooked, try giving it anywhere from 12 to 18 degrees initial ( run my initial at top of the timing tab about 16-18 ) I try to get the car idling as low as possible. Also I use the lightest advance springs on the centrifical advance too.
Now after setting initial and getting timing and base idle set. Now think of it this way. initial with centrifical should be basicly around 34-36 (optimum performance race timing for most SBCs) now we add in the part that makes it run smoother and get mileage.
Hook your vacuum advance to MANIFOLD vacuum. If it doesnt idle up and smooth up some your advacne is damaged/bad or you have it hooked to ported vacuum, which is actually used in the good old 70s to open an EGR valve as the car is accelerating to curb lean mixture knock and lessen NOX levels.
ITs not for timing (even though some idiotds design did use it sometimes back then)sp dont use ported source.
Now you should be able to idle the carb down to a desent rpm and set your idle mixture.
Now if you are getting the mixture screws almost all the way in(shutting off fuel to idles circuit, you most likeley have either a bad power valve or some porosity in the metering block(s).
For the Powervalve with most performace street cams I have been using the 4.0 to 5.0 (read part number 122-45,122-45,122-50) and on some real wild cams I have even had to go as low as 3.0 to 3.5.
Remember you want the powervalve to open at around 2 to 3 inhg LOWER than your idle speed in gear idling.
BUT some engnes really need a little lower numericly to not bump open and closed at idle/in gear.
Also you will want to spend $10 or so and install a powervalve protector in the baseplate.
Now also make sure your floats are set at idle with sight plugs removed, itshould JUST be tipping out of the holes.
Setting the carb/timing is a dance and keep checking as things affect other things.Also idle speed should be set as low as possible in gear.
Now some radical cams require you to drill small holes in the primary throttle blades so the engine can still get enough air to run but allow the throttle blades to close enough to get the blades down past idle slots and give them enough signal to draw fuel through the idle mixture screws.
Now if you still ahve issues with the screws going in too far (shutting off fuel) you may have to bite the bullet and give one of the big carb shops a call and buy a replacement metering block. Or look for a matching used carb for parts.
Carb cleaner can be your friend looking for air leaks.
So:
1:After verifiying secondary ilde speed screw just opening
2:make sure all screws are tight
3: make sure base isnt cracked on one coener (can also cause problem)
Then:
1: unhook vac advance
2:initial timing 14-16-18,, if you can get tuning kit for dist use light springs and stock weights for street
3: get it running and set idle mixture/speed
4: Then hook up vacuum advance to manifold vacuum and it should rev up and reset mixture, idle speed hot in gear
5: just keep it adjusting, if the mixture screws keep gettting set almost all the way in(shutting off fuel) you may have misadjusted floats or blown out power valve.
Good luck and ask any questions you want Ill try to help you through the tuning set up.
Lee Abel
AFTERMARKET PERFORMANCE

MrQuick
12-07-2007, 10:14 PM
might have a vacuum leak, try this....stuff a rag into the rear bores and see if the idle drops.

Lee, don't you want advance on a ported circuit?

megaladon6
12-08-2007, 02:05 PM
if i had a vacuum wouldn't i be adding fuel not leaning it out?

MrQuick
12-08-2007, 08:57 PM
if i had a vacuum (LEAK) wouldn't i be adding fuel not leaning it out? No, its just as Lee said. Its un metered air. Or air that is introduced to the system with out addition of fuel.

megaladon6
12-08-2007, 10:07 PM
obviously i don't know carb physics too well so this may be stupid. i'm used to working with factory efi, and a vac leak causes a lean condition(ignoring the resulting feedback from the o2 sensor).
if i'm losing airflow at the carb (less signal?)shouldn't that pull less fuel? one of the things i've been told to double check is that the car dies when i turn the idle-mix screws the last bit in/lean cut off--and it does. i've looked for leaks and can't find anything so if it is there it's pretty small.
just to clarify--i am not arguing or doubting, i just prefer to ask question and double check everything.

MonzaRacer
12-09-2007, 06:14 PM
As for advance if you use ported vacuum your going to cause advance only when your accelrating or when your timing should be in the performance zone (ie small block 34-36, ask most dragracers)Vacuum advance is for mileage and driveability.
You want vacuum advance at cruise speed and light accelration, not under hard acceleration.
As for unmettered air your not going to see a fuel delivery problem as the small amount of air simply disrupts the A/F ratio.
It will also give lots of tuning issues.
OK basics 101
Run your idle mixture screws all the way in and back them out 2 1/2 turns for initial setting.
Then using a vacuum guage adjust the idle mixture screws for the highest manifold vacuum readings.
I also have been setting my idle and mixtrue wil idlling in gear then fine tune in park.
And dont forget timing and carb tuning go hand in hand. If you advance timing the engine "sees" it as being richer.
Now if you want basic principles of carbueration here we go.
Basicly (assuming we are at sea level the barometric pressure is 29.92mHg) a carb is simply a fuel metering and vaporizing device.
Now as we all know the engine is simply an air pump and creates a negative pressure event under the carb, inside the intake.
Now with a negative pressure under it the atmospheric pressure is pushing fue lthrough the carb.
Now at idle, air and fuel is metered through the idle mixture circuit, and will uses the idle air bleeds to mix with idle fuel and keeps the car running.
Now on the Holley they have transition slots to allow extra fuel including the idle mixture and allows transsition to the cruise circuit and if the vacuum gets low enough the powervalve opens (as the vacuum is holding it shut, when it goes low enough it opens to give enrichenment, including the main metering (ie jets).
Carbs do not put fuel into the engine they meter it as its pushed into the engine. This also explains how positive boost operates and why carbs are a bad metering device as they are ment to meter from a slight pressure to a vacuum, boost puts them into a different opertaing mode and they have to be modified to operate properly.
Here is what I set my truck at:
16-18 deg initial, centrifical set up with stock weights and light springs, and vacuum advacne hooked to manifold vacuum.
I unhook the vac advance and get car idling and adjust intially the idle mixture and speed.
Then hook up the vac advance and readjust the idle speed and mixture.
I have found this to work on pretty much any engine so far.
Most people have never been through an Advanced Engines class like I have and one of the principle I learned was exactly what timing is supposed to do.
The only reason you would use ported vacuum was back in the 70s for egr or timing retard. But it wasnt the greatest idea even back then.
After going through an emissions certification class, we had a 78 6.6 TA (400 Pontiac) that could not pass emissions for nothing.
so after we put it all back(stock) it was still failing.
so I went through and set it back up better(yet hiding the mods on the vacuum system.
After several tries at getting it we found thatthe carb had the wrong jets and metering rods and wrong secondary metering rods/hanger too.
After fixing that and getting the right egr valve and getting the vacuum setup right and it came right into spec.
Then I showed them that a lot of the crap tacked on wasnt needed and it ran even better too.
As for fuel injection I had one the other day they just about had me flustered but I figured it out.
After switching from suspension tech to diag tech is a long hard jump but so far it going good.
Even got through an class clalled Advanced Engine Management Strategies Workshop,,, 2 weeks of learning or relearning diagnostics and repair.

megaladon6
12-15-2007, 04:25 PM
i've noticed that everyone uses a 750 cfm carb on the 383's. i ran the formula and it comes out to 664cfm at 100%VE and 6k rpm. my carb is a 650 because of this. now if 750 is the proper size, is it possible that the airflow through my carb is at too high of a velocity and that is screwing up the idle signal causing it too pull more fuel?