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Old 05-05-2019, 09:29 AM
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quick fuel carb tuning

I am a bit new to holley carbs but thanks to this forum I am learning a lot about them now. Anyway I am trying to get my new engine dialed in , so I will throw the spec's at you.

gm 350 with vortec heads , 9to1 compression
215.int / 223. exh. @ .050 , .473 lift int. and exh., 108 lobe sep. roller cam.
Hei. ignition , gm dual plane intake.

1977 blazer 4x4 ,(4500lbs or so), auto trans., 4.56 gears with 38 inch tire's so around 3.73 ratio.

HEI. , changed one spring to a light one and changed the vacuum advance can to a 10 degree max , hooked to full manifold vacuum. I am at 5,500 feet elevation and this engine only pulls 9.5 to 10 inches of vacuum at idle.

HEI timing with no vacuum adv. hooked up.
11 degree's at 780 rpm
29 degree's at 2,000 rpm
32 degree's at 3,000 rpm
35 degree's at 3,500 and up , seems to stop there.

HEI with vacuum hooked to full vacuum.
17 degree's at 780 rpm
34 degree's at 2,000 rpm
36 degree's at 2,500 rpm
37 degree's at 3,000 rpm

Quick fuel OR-600cfm-VS carb. , (basically an off road set up slayer) , when I took it apart the main body had 600 SL inside. So I paid 550 dollar's for a cross over vent tube and spring loaded needles, (bone head move).

Out of box
68 primary jets 74 secondary jets, (has a plate with jets, no meter block)
73/33 idle air bleed 31/31 high speed air bleed
Pink pump cam 31 primary nozzle 6.5 power valve
The IFR's have no markings so no idea what they are. Anyway this carb was way rich at everywhere at 5,000 feet so after reading on this forum I took it apart and went for it.

New set up
Squared up the t-slots on primaries , adjusted the secondaries 1/4 turn up from fully closed.
Put in 28 IFR's
Put in 64 primary jet's
Adjusted floats to half glass
Cleaned and blew out entire carb. , everything else is the same as it came.

So now the engine runs better but not perfect , (only have a 75 miles on it).

It seems to idle best with the mixture screws at 3/4 turns out at 790 rpm's but I feel like I am exposing to much of the t-slots. I marked the carb. and from .020 to .040 is a half a turn on the idle speed screw and it seems it wants to go more to idle better. So you think I should open the secondaries more than a quarter turn to help the idle?

I have a AFR meter set up and here is what I am seeing.

790 rpm 12.7 neutral
600 rpm 13.5 in gear
2,000 rpm 13.6 neutral
3,000 rpm 14.4 neutral
Driving around it runs 11.5 to 12.5 and if you floor it its 11 to 12 afr.
Seems like it want to stall in gear here and there and when you are going slow and give it just a bit of gas it hesitates and then takes off , but if you jump on it , it blows through the hesitation, (thinking more pump cam maybe).
Have not done any highway running really , just wanted to get the idle and slow speed worked out and then go from there. The vacuum adv. can is supposed to give me 10 degree's but it does not, (may modify that).I have the vac can that is all in at 9 inches, (checked it).

So what do you guy's think , open the secondary's more? Give it more timing?

I just don t want to get the t-slots to far exposed, Thanks

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Old 05-05-2019, 09:32 AM
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Sorry I forgot , the secondary jets are 74 in the quick fuel plate
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Old 05-05-2019, 09:46 AM
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From your post your pretty close to what you need and you seem to know your way around by changing out the idle feed restrictors and stuff and with your hesitations and stuff you carb has a pink pump cam and you might want to buy a pump cam assortment kit that comes with the different profiles and shooter sizes and helps to get the best tune you can on the accelerator pump circuit.

The pink cam has never worked for me on any of my engines regardless of what cam size I had the pink cam was to lean of a shot and too long of a duration of pump travel. In my experience cars that pick up rpms quick usually like a bigger quicker shot and I have always went to the middle of the ground of the pump cams and the orange one as always been my go to for my builds since all of my holley vacuum secondary name brand carbs came with them it always worked for me right out of the box. All of my quick fuel carbs I have used over the years I have changed out the pink to orange and fixes my hesitations all the time with the right shooter combo. Yours might not need an orange but that is why I recommend a kit as they have different profiles and its finding the right shooter size to work with the right pump cam profile.

Also is your accelerator pump arm lever set so there is no slop in it and when you just barely touch the throttle fuel will start to dribble out the shooter? If that has any play in in that can cause issues like that as well.
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Old 05-05-2019, 09:49 AM
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Also with your AFR gauge don't always try to get the leanest magic number that everyone says you should have as the engines with the bigger cams won't always run on lean numbers for mileage wise to a certain degree and always need a slightly richer mixture cause of overlap of the cam and the reversion and lesser carb signal and lesser vacuum. I have always just went with what makes the engine run its best and leave it there and not always try to get the leanest tune as it always left to much little issues here and there. You want each circuit to transition smoothly between each other as best as possible with out any issues when they do.
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Old 05-05-2019, 10:01 AM
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Thanks eric , I will fly to summit and get some pump cams today. What do you think of opening the secondary's a little more to bring the idle up a bit as to not expose the t-slots on the primaries to much?

By the way thanks for all your post's on carbs. , I have been reading them for a week now.
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Old 05-05-2019, 10:33 AM
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What’s your initial timing?
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Old 05-05-2019, 11:43 AM
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11 degree's , with vacuum can hooked up 17 degree's. My vac can is supposed to give me 10 but it does not , may have to grind it out a bit to get 10 , but I don't know if I need it.
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Old 05-05-2019, 12:09 PM
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On the primary throttle screw idle setting I make notes that you can go up to half to 3/4 of a turn and your transfer slot will not be exposed to much and the most you can get away with is one full turn past contact on the the primary without your transfer slot being open up to much past about .040. On my engine with a 900 rpm idle in park I have 3/4 or a turn open on my primary and 3/4 open on my secondary and my transfer slot is squared still enough to not be pulling fuel out the transfer slot and making my idle mixture screws unresponsive. Just depends on how much air and how rich or lean you idle circuit is tuned to where you need your butterfly idle setting front and back set at.

My lesser cams normally would have about half to 3/4 of a turn on the front and a 1/4 to half open on the rear but when I get in the higher [email protected] or more duration and more seat to seat duration of a bigger cam I have to have the back side open more. That is the nature of the beast as the more cam the more higher of an idle and air that needs to be let past so it can idle and run.
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Old 05-05-2019, 12:20 PM
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10 4, thanks , I may try and open the secondary's a bit more to help my idle a bit. I am on my way to get pump cams as we speak.
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Old 05-05-2019, 01:02 PM
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Quote:
Originally Posted by hirschdalechevy View Post
11 degree's , with vacuum can hooked up 17 degree's. My vac can is supposed to give me 10 but it does not , may have to grind it out a bit to get 10 , but I don't know if I need it.
Are you setting initial with vac can unplugged?
If so and your 11 is accurate then you likely need to add 8 degrees to your initial.
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Old 05-05-2019, 06:30 PM
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11 degree's vacuum can unplugged , 17 degree's with vacuum adv. on full manifold vacuum.

Do you mean 8 more degree's with out vacuum plugged in? or add 8 with a vacuum adv. can.

I was staying at 11 , initial as to not over advance at total , distributor has 22 degree's , that's a total of 33 degree's and I thought that was about right for vortec heads. Do you think I need more for 5,000 + feet in elevation?

Actually it goes a tick more than 22 degree's at high rpm , 35 degree's at 3600 + but seems to stop there.
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Old 05-05-2019, 07:32 PM
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I’ve always run way more initial when at elevation.
With stock or modified motors.
However a vortec might need a different approach.
I’ve used more initial to get the fire started earlier. Something that’s needed due to air density.
Vortec might be more effficient with less timing.
I have no experience with them.
I always think due to higher elevation you need an optimized timing curve.
That’s why I suggest 18 initial. You’d need to limit your distributor to 14 though.

The most significant reward I had for optimizing my timing was never needing to fiddle with the carb other than mixture screws.
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Old 05-05-2019, 09:38 PM
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Advice on how to easily limit mechanical advance on the HEI distributor is in the HotRodders Wiki, about 3/4 down the page for the "Hotrodding the HEI distributor".

https://www.crankshaftcoalition.com/...EI_distributor
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Old 05-06-2019, 07:26 AM
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Thanks guy's , I may have to play with the timing a bit. I went to summit and got a pump cam kit and a fuel pressure gauge and pulled my carb off again as well , then it started raining , bummer. Looks like I have some more reading , thanks again.
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Old 05-06-2019, 07:49 AM
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Here’s some more reading for a rainy day!
Where should I set the timing on my performance engine?
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