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Discussion Starter · #1 ·
I have a 502 gm motor, Holley 4150 750. Just bought myself a wideband kit.
The problem is that when I do modifications I get the opposite reaction than what I would`ve believed.

Here`s my start,
Idle 14:1
Cruising 12-12.3:1 /leans even more out after passing 75mp/h
WOT: 17-18:1 but richens after a short while 14:1 ( I think this is when the secondaries open??)

I wanted to go:
Idle 14:1
Cruising: 14,7:1
Wot: 12-12,5:1

Ok, I got down from 78 main jet in the primaries to 73. Cruising went up to about 14:1. But spitting, poor gas response occurred. I know that I should use the accelerator pumps nozzles toget rid of this. The problem was that holding the throttle down for light acceleration for a while was showing very lean condition.

Then changed the secondary jets, from 83 to the biggest i had...99. This way should`ve seen when/if the secondaries open. I also tries different springs with marginal results - total lack of that wild acceleration I had before i started jetting.

The result: Cruising went down from 14:1 to 12:1 again. Poor light-medium acceleration, shows as lean. full throttle indicated a super lean condition 18:1 +++

So then I thought that the secondaries did not open and I put the lightest spring in there, still showing as totally lean at WOT. I`m gonna double check the secondaries with the paperclip trick next time.

So what am I doing wrong here guys ? This is my first Holley carb and I`ve done my homework but have no experience with this carb..
Why do I have a rich cruise but going uphill makes it go to 14:1 ?
I may also have damaged the spring/gasket/vacuum secondaries rubber. I guess this could be one of the reason why the secondaries probably not are opening.

Strange, please help me out here guys, I`m going crazy !!
 

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I'm not an expert, so I'll tell you what I would do. ;)

Sell your 750.
Call Patrick at ProSystems. Tell him your specs.
He will sell you a larger (I think) carb tuned to your needs.

From his setup, you will then need to make only minor changes to get where you want. With a 750, I think it's the wrong carb which will mean you need too many changes to get the most out of it.
 

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Ok first of all that carb is a bit small for a 502. The changes you are making are dramatic. First get your cruise ratio right. You said 78's were too rich @ 12:1 during cruise so try 75-76's. Jetting the secondaires try to keep the stagger around 8-10 jet sizes. For example if you have 75's in the front run 83's in the back. If you can find a 850 vacuum secondaries or even get a Holley 870 Street Avenger you will get it much closer.

Dave
 

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Discussion Starter · #5 ·
I had a feeling that that carb was too small, It may be short on power compared to others but shouldn`t it be possible to make the A/F ratio right anyway? If tuning this carb is hopeless (I`ll give it a fair try first) I`ll buy a new/bigger carb.

I`m away from home now but I`m jetting the primaries and secondaries closer when I get home. start off with your ideas dave, thanks.

But why would my carb show lean condition at WOT when I`m at the biggest jet available in the secondaries ?
 

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With a 99 jet in a vacuum seconday carb, the secondaries probalby aren't even opening, because its just pouring in the fuel in causing it to not pull the vacuum to allow it to open. You also mentoined you might of tore the vacuum secondary diaphram, so you may be only running on the primaries.

Dave
 

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What carb is this? With 78's primary it sounds like the Holley 850 that comes with the 502's.
 

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I would not give up on the holley carb.

I would go back to the basic jetting that the carb comes with to establish a baseline and make sure the secondary is functioning properly. Also make sure bowl fuel levels are correct. Check your vacuum diaphram. If the assembly is off the carb just extend the diaphram and hold your finger over the vacuum hole and see if it holds.

Do you still have the power valves primary and secondary.

If one is removed then compensate by adding 6-8 jet sizes.

Do you know that your vacuum secondaries are working.

And go to the innovate forum to get some tech support.

With factory carb settings you should be pretty close, at least close enough that you don't need to make extreme changes.

If it is the usual 4150 style with both primary/secondary same size I would try to keep the jetting similar front and back especially for wide open throttle.
 

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I'm no expert on this, but am in the process of working through my carb as well. First tune in your idle mixture. You say you're at 14:1. I'd lean it out a bit from ther even. 14.7:1 is stochiometric, and at idle there is no load on it. I'm looking for about 15:1 there. Next set your cruise. Get back to your 78 main jets in the primaries and 8-10 higher in the secondaries, provided there's no power valve in the rear. If there is a power valve jet equally throughout. I believe from your initial post that that put you to about 14:1 at cruise. Then it's time to address your power circuit. You say you're going way lean. What size are your nozzles? If you're lean down low and then get reasonable, it's in the nozzles. They are restricting fuel flow to the point where it goes lean. Get a torch drill set at the hardware store and start drilling them out 2 sizes up at a time. Once that is dialed in, then you can see where it is at for high speed WOT and adjust your acc pumps and cam.

Devin
 

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Discussion Starter · #10 ·
ok, the seller said the car had an original 750vs ?? I guess if that carb is to small I prolly have a 850, especially if the jetting sounds like 850.
I`m trying 76 - 84 jetting today, and checking that the vacuum diaphragm holds vacuum to. I think i might be missing the cork gasket to. Maybe that is my problem with not getting the secondaries to open..
 

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Carb being small for the engine should not effect the ability to tune it properly, the effect should only be inability to rev as far as the larger carb would. I am wondering if your running lean at wide open is fuel supply unable to keep up with need, thus float level is low at WFO. Might start with a dash mounted fuel pressure gauge measuring at the carb inlet
 

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Discussion Starter · #12 ·
Here`s my progress so far.
Put in 76 jets in the primaries.
Got another vacuum diaphragm, my previous failed it`s vacuum hold test, probably due to a leaky gasket.
Let the 98 jets in the secondaries stay, in order to easily watch when/if the secondaries open.

Result:

Idle: still around 14 (drifting)
Cruise : 12-12.5:1
Light acceleration: 14-16:1
WOT: 15-16:1 but goes down to 12-12,5:1 after
the secondaries have opened, a small second.

I see no signs that the powervalve opens. I think this might be why the carb leans out when giving it a light throttle. And also why i have a wot ratio at 12,3:1

But OMG !!!!!!!!!! how much it helped to fix that vacuum diaphragm !! the car runs amazingly strong at WOT now. Burns rubber straight up to 5500rpm`s in second. :) and had no chance of doing a 0-60mph test without spinning tires. Think I`m investing in some drag radials soon.

What would you do to make that cruise ratio go up to 14,7 ?
And how can I check my powervalve? I suspecting that it is not working properly, and that that is why I`m not way rich at WOT. Agree?
 

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Look on the choke tower and let us know the #'s as this is important. Does the carb have a seconday powervalve? If its been removed it may be causing that momentary leanness with the secondaries.
 

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That quick lean spot could be if you are missing the check ball in the secondary housing also. Light acceleration the power valve won't open either. Give us a carb number too.

Dave
 

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Discussion Starter · #15 · (Edited)
here are the numbers on the choke housing.
80531
0140
So this means I have 850 after all, that`s nice.
 

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80531 = 850 vac sec. Put it back to stock and go from there. 78 primary, 82 secondary. Front and rear PV's 4.5/3.5 and a #40 squirter. Try a 6.5/5.5 PV combo as the 4.5/3.5 is better for lower vacuum motors.

Where is your timing at - specifically?
 

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I again agree with Vince, a stock jetted Holley is usually the best set-up. I would get rid of the rear power valve and put 88-90's in the rear.

Dave
 

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Ok... with the wide band O2 you have LOTS MORE visibility into what is going on in the engine. But to make sense of it you also need LOTS MORE understanding of what/how the carb works. And once you get there you may just give up and convert to EFI B)

Ok... off of the soapbox. Your cruise mixture is affected significantly by the idle circuit fueling. You are actually running largely off of the transition slots in this mode. This circuit responds to manifold vacuum, so it is really perverse in it's fuel delivery: when you need more you get less. This is why you get some leaning as you roll into the throttle, and why you probably do not want to get too wound around the axle about exactly hitting that magic 14.7 number. You need to leave some room for this enevitable leaning to occur.

As your manifold vacuum continues to drop, at some point the idle circuit completely shuts off. This is where the power valve is supposed to open up and compensate. It is also supposed to convert the mains from efficient 14.7 type operation to power 12.5 type operation. So the size of the power valve restriction will control the amount of this step, at least for the primaries. These parameters will dominate fueling in the lower WFO ranges before there is enough airflow to begin to activate the secondaries.

You should also be aware of the limitations of the wide band O2 sensors, in particular with respect to larger cams around idle. The sensor reads the level of oxygen in the exhaust and how far off it is from a norm. If you have a cylinder misfire not only will you have a bunch of raw fuel going down the pipe (which the sensor ignores) but you will have a bunch of unused oxygen going down the pipe. This "excess" oxygen will be intrepreted by the sensor as a lean condition even though you are not lean at all, in fact you just dumped raw fuel through ;)

Your results of leaning out above 75 MPH correlate with my own results even though you are running a Holley and me an Edelbrock Performer. Goes to show that all carbs work off of many of the same principles. This leaning is the result of the balance of air bleeds and jets. As air velocity increases through the venturi and boosters, the vacuum signal generated does not go up in the same linear proportion as the air mass (and thus fuel requirement). One of the functions of the air bleeds is to reduce the vacuum/signal gain and better match the two response curves. I have not determined exactly what the tuning correction is here but I theorize that a slightly smaller air bleed AND jet are needed.

To lean out your cruise you may need to lean the idle circuit. How far open are your idle mixture screws? Nominal here is about 1.5 turns and if less than 1 it further indicates you are too rich in the idle circuit. You have to be careful here though because if you go too lean you can easily get tip-in stumble. Note also that the idle fuel restriction is a very small hole and it will not take much tweaking to make a big difference. Typical practice is to insert small wire strands to reduce jet size.

This all gets even more complicated because of the overlaps and concurrent operation of idle/primary/secondary fueling circuits.
 

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Discussion Starter · #19 ·
Thanks for all that info Steve. I`m really looking forward to understand more about my carb, the innovate wideband helps me monitor every change i make. I definetely have a lot to learn, especially in the idle circuit which is far more complicated than first expected.
I`ve found an article I`m trying to understand. Please post more links to articles that can help me understand this better. Bought the Holley dvd, didn`t make me at all smarter.. If there`s a video online that`ll help me please post links to those also..

I might also put the carb back to original specs as you guys said although since I have to do a lot of tuning anyway I`m having trouble seeing why I really should as it was far out when I first started. At least the WOT is not hurting the engine now while beeing to lean, which is what I`m most conscerned about.

I have jets for the main circuits, springs for the vacuum diaphragm.
What other parts do i need?

And by the way I`m very happy and thankful for all the help I get from you guys, I really appreciate it !
- I`m a newbie (as you`ve probably noticed by now :)
 

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where do you have the sensor? hanger on the tailpipe or in a bung up front? If it';s on the end of the pipe you'll need to wrap a rag around the pipe and sensor to get a good idle reading. Performance cam will cause exhaust pulsing in the pipe, suck fresh air in and booger the reading.

be aware that with a large carb it will probably be at least 2500, maybe 3000 in steady-state cruise before the main system kicks in and it starts metering. under 2500 you're still running on the idle circuit, changing the main jets won't make any difference. If you want to know the RPM where the mains take over put 10 sizes too big jets in the front and drive it, crowding the RPM up slowly, keep the vacuum above the value that causes the PV to open. gradually increase the RPM, when it goes pig rich you've found the RPM where the mains kick in. Put the jets back in.

What the other guys said, put the carb back to stock calibration as close as possible, then with your new-found knowledge you'll be able to make good decisions on what to do.

BTW, my car runs great on the freeway at 16.5:1 AFR. There's nothing magic about the 14.7:1 deal. Really helped the mileage and keeps the plugs clean. It's a Q-jet so I'm able to lean out the cruise mix without leaning the power mix.
 
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