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Discussion Starter #1
Let me start by giving the engine specs. I have a 491 stroker engine. Bore: 4.290, stroke: 4.250. The compression ratio is 10.3:1. Crower mech. roller camshaft with .641/.636 lift, 256*/ 266* @ .050. Edelbrock Performer RPM 454-R aluminum heads, Victor Jr with 850 Holley carburetor. I have it running and have the timing set at 26* initial and 38* total with the MSD Pro Billet Distributor. My problem is even with the 26* initial timing I have to open the primary throttle plates too much uncovering the transfer slots. I need the idle to be around 900-1000 rpms. I have been reading on tuning Holley carbs. They suggest drilling 1/16 holes in the primary throttle plates and going up in size until I get the idle to what is needed. Does anyone have some other opinions?
 

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Hmm. You called the thread ignition timing but are asking more about drilling the throttle plates so I guess you are trying to find the balance between the two.

My test would be to crank the initial even farther and then try to back off on the throttle plates to get the idle down. At that point, read the timing with light to see where you are.

If this is successful and you have enough vac to make it operable, you may then want to add a vacuum advance to get you back in that neighborhood at idle and restore your initial and total back where they were (26*/38*)
 

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My problem is even with the 26* initial timing I have to open the primary throttle plates too much uncovering the transfer slots.
If u have 26 initial and assuming u dont have VA and the idle is 26 that is WAY too high.
Its as if u have set idle timing by highest vaccuum... u CANT do that...u can use the vac gauge only after u have the secondary butterflies set correctly which will give u about 1 1/2 turns +/- 1/2 turn on the primary idle speed and mixture screws


All assuming that u have the right PVC valve and no other vaccuum leaks
They suggest drilling 1/16 holes in the primary throttle plates
This is ONLY done after the sec butterflies have been set, idle advance dialed in and find that the carb is WAY too big for the application....ie secondaries adjusted till only a thou or so off the bore when closed and the primary idle speed is wound right open mixtures screws have little or no adjustment with idle advance around 16 to 20 deg...ALL of these.
and 38 total is was too much ball park will be around 34 at around 3000/ 3200 rpms... espec with that compression on modern pump fuels

There are many recent and old thread s describing in detail how to dial in your tuning specs, set up secondaries , timing etc, mod timing curves degs and VA units

So fire up the engine, start dropping the idle timing back, adjusting the secondaries, closing up the primaries, and tweaking the mixture screws into spec....When u have idle back in the 16/ 18 degs bracket and secondaries getting right u will find everything else will start to fall into place.
For non EGR engines ball park
initial around 8/ 12 degs
idle (initial +VA) 14 to 18 bigger the cam higher the idle advance.. very wild cam upto 22 deg
Total (Initial+ cent) 32 34 degs
All up (initial+cent+VA) dont go over 40/42 without dialing in with a knock sensor.
 

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2 choices, open up the secondary stop, btw, what Holley 850? If you have the 4781 old school 850, you will have to make a bent screwdriver to adjust the secondary. I ground an Allen wrench into a slotted screw driver, still a pain to adjust.

The second and better option is to pull the carb and see how far open you have the primaries, return to the correct square and then open up the rear to get it to idle, do not touch the front idle, as it is open as far as it should be. If you cannot get your idle speed desired, then you will need to drill the plates.

4 corner idle?
 

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Distributor- 18-20 initial, for a 34-36 total, if you are running a vacuum advance, and I do recommend it, limit it to 10 degrees, it will idle as low as 750 rpm, I like 900-950, 1,000 is just to high for me.
 

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Who is "they" that suggest drilling?
 

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lock down timing, no vac. advance... problem solved.... dont run a start retard it just makes it kick back and back fire through the carb. this is my experience anyway.
 

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Discussion Starter #8
Let me explain my problem. I am just starting a new engine. I have an older model Holley 850 carburetor (0-9380). I had both transfer slots adjusted to .020 when I installed it. Ok to keep the engine running I had to turn the idle rpm adjustment screw in until I had it to idle at 850 rpms. The duration of my camshaft is 256/266 @ .050. So I was going to set the timing at 26* initial with a total of 38*. I thought with the timing this much advanced, I would be able to adjust the idle screw back out some but it's not the case. I have a secondary throttle adjustment screw on the left side of the base plate that I turned in some to back off the primary. I read an article from the internet on tuning Holley carburetors. That is where I got the idea to drill the primary throttle plates to increase the idle so I could close the throttle plates some. Like I said the carb is a 0-9380 and has 4 corner idle. These were adjusted 1 1/2 turns out to start.
 

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Years ago I had a Holley 4780 800CFM carb on my 355 with a 264/274 @ .050" 0.630" lift roller. Drilling the primaries helped. The next carb on this motor was an 835CFM Race Demon with 4 corner idling. After some tuning it worked awesome without drilling.

Your motor is 136 cubic inches bigger, the cam is smaller and you have 4 corner idling. I don't think drilling the primaries will fix your problem. :beers:
 

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Discussion Starter #11
I am positive that all the timing marks are correct. That is the first thing I did was make a piston stop from an old spark plug. It works a whole lot better than a store bought one. Anyway I have an aftermarket pointer bolted to the cover. I turned the engine over until it hit the stop, marked the balancer and turned the engine counter clockwise until it hit the stop again, made another mark on the balancer. I made a mark in the center of the two lines which is "0" on my balancer. I believe the air bleeds on the air horn of the carburetor need to be adjusted, wouldn't that change the way it idles This is something I have never done. I also don't have the correct tools to check rpms except the tach in the car. I might have to take it to AED Performance and let them adjust the carb.
 

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What do the plugs look like?

I suspect it is running a little rich and those 2 plugs are starting to foul.
 

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Discussion Starter #13
What do the plugs look like?

I suspect it is running a little rich and those 2 plugs are starting to foul.
:confused: My plugs look fine for as much as it has been run. They have a slight tannish color to them. Is your answer for the right thread? What two plugs are you talking about? I haven't said I have any fouled plugs.
 

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I had both transfer slots adjusted to .020 when I installed it.
That is setting to basic ball park ...when one then istalls a carb dizzie what ever they then NEED to be dialed into THAT engine.
on idle circuits yes the carnb and dizzie have individual 'circuits.. like having separate carbs/dizzie for each job of
idle power and cruise...
so u start with 20/1000 set the idle avance to ball park also
on stock cam to mild street 8 to 12 , lumpy street 16/ 18/ wild cam that should be track upto 22 deg
Then adjust the carb idle circuits so the mixture is around 14.7 to mid 15s by changing the vaccuum 'leak' past the secondaries has the idle mixture screws and the idle speed screw are 1 1/2 turns out +/- 1/2 turn
If u cant get that either the carb is way to big for that engine.. which in this case very unlikely.....or there is a vaccuum leak somewhere....
I have an older model Holley 850 carburetor (0-9380).
And there is most proberly where your problem lays... worn butterfly bushers , and or incorrectly set bore to butterflies adjustment

It is not uncommon on old used BIG carbs to find the butterflies out of adjustment, due to someone playing with them , or someone having your issue, had the butterflies fully closed, car sits for a while, they jamb, bend a butterfly shaft , And wear the bushes...maybe even incorrect carb to base plate gasket.

It seems u know the basics and maybe more, of what u are doing .. checking timing marks ...setting slots etc...what should be happening isnt
I will lay odds the issue is an unwanted vaccuum leak, and most probably the old carb

Its that simple...

Then once u have established the idle circuit tuning specs u proceed to the power cicuits, get ball park AFRs 11.5 / 12.5 lock the cent set advance to 36, make runs and retard till sudden power drop.. couple degs above this now has your power circuits sorted... and your tuning specs set
Now just juggle/ adjust the cent degs , the VA degs and the VA vacuum range to meet the engine requirements

For get the drill hole crap, thats just a left over myth from way back in the 50s/ 60s when there was no or very little secondary adjustments..
 

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:confused: My plugs look fine for as much as it has been run. They have a slight tannish color to them. Is your answer for the right thread? What two plugs are you talking about? I haven't said I have any fouled plugs.
:eek: Wrong thread. Carry on, don't mind me.
 

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26 initial is way too high you flattened your advance curve and it will fall flat on its face under driving conditions. Verify the stop bushing degrees in your MSD and subtract from 34 and that should be your target for initial. Lets assume you get 16 degrees initial with an 18 degree stop. Make this change first

No need to drill primary butterfly use set screw to crack open secondary. A very very small secondary opening is worth two holes in the primary and same effects

Agree do not adjust idle screw beyond the square box. It's a wonder they even make this adjustable on performance carbs... Screwing with this corrects idle conditions and messes up the whole power curve. Makes no sense.

Opening up the secondaries will lean out your mix and raise rpm's to cover the drop you will get when you retard the timing. Next adjust your idle mix screws for best vac and use the secondary set screw to back off rpms as needed.

Finally plug in your vac advance and see if you are back to about 26. That's ok. If an automatic again put it in drive (have a buddy hold the brake) back off rpms with set screw on secondaries and tune to best vac on idle mix. Rev it up a little in between adjustments. When in gear be careful to not run yourself over.
 
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