Stagger jetting Holley HP - Chevelle Tech
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post #1 of 27 (permalink) Old Jan 5th, 20, 2:36 PM Thread Starter
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Stagger jetting Holley HP

One side of the Edelbrock RPM manifold is leaner than the other, from reading other threads I see this is not unusual.
I am 3 jet sizes between left and right already but it still wants more, is 6 sizes between two sides a sign of other problems, most threads I have read say two sizes? As it is vacuum secondaries, will I have the same issue with them? Will the idle mixture screws be out of whack also? The main venturis are nearer to the front which seems to favour the front 4 cylinders and the rear cylinders lose out.
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post #2 of 27 (permalink) Old Jan 5th, 20, 3:21 PM
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Re: Stagger jetting Holley HP

6 sizes sounds extreme.

Check out Fig 6 for this stagger jet setup.
Edelbrock Air Gap Performer Intake Manifold

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post #3 of 27 (permalink) Old Jan 5th, 20, 4:32 PM Thread Starter
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Originally Posted by MadmanMark View Post
6 sizes sounds extreme.

Check out Fig 6 for this stagger jet setup.
Edelbrock Air Gap Performer Intake Manifold
The one you reference is a double pumper on an air gap, the air gap has a notch between the plenums, my vacuum secondaries dont come in during normal driving and the RPM I have has no notch between plenums, apart from the shared vacuum passages it runs like two separate 4 cylinder engines, due to differing shape, volume and size of runners and plenums, main jet requirement may be different either side, I have not heard of anyone with such a large discrepancy from left to right, maybe if I fatten the idle mixture screw on one side that might help out a little.
This is all theory on my part, I have very little experience with carburetors, hope someone can shed some light for me
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post #4 of 27 (permalink) Old Jan 7th, 20, 2:22 PM Thread Starter
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I have increased passenger side jet to 78, this is 6 sizes larger than the other side, the plugs fed from that plenum are still bone white. I give up! I have another identical holley hp on another engine, gonna stick that on and see if that does the same.
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post #5 of 27 (permalink) Old Jan 7th, 20, 8:36 PM
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Re: Stagger jetting Holley HP

Put a 1" open spacer on it. With the performer rpm air gap which has the notched divider, I do not see such color variation on the plugs.
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post #6 of 27 (permalink) Old Jan 8th, 20, 4:45 PM Thread Starter
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Re: Stagger jetting Holley HP

I fitted another Identical carb and I am now getting an even colour across all spark plugs with the #73 jets installed, unfortunately it is white and still pinging so I have upped the jets to the baseline how Holley would have supplied it, #77, will test drive tomorrow. Obviously staggered jetting is a thing but when you increase jet size by on one side by 6 and nothing changes there are bigger problems within, it would seem that my carb has an internal problem, maybe that is why the last guy sold it, brand new in box, maybe he tried it and took it straight off again!
I took off the secondary metering block and these jets have been changed to #76, should be #84 from the factory. I still got to figure out what is wrong with it as I now have an engine with no carb on it. Both these carbs are Holley HP 750 VS. Buyer beware.
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post #7 of 27 (permalink) Old Jan 8th, 20, 5:07 PM
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Re: Stagger jetting Holley HP

If it's pinging under full throttle consider an issue with total timing or jetting, provided there are no air leaks, if it pings on the level or with "lean tip in" (like starting up a hill and SLOWLY easing into the throttle) consider unplugging the vacuum advance (if you have one) to see if the pinging stops. If the pinging stops you may wish to acquire a Crane adjustable vacuum advance and cut back on the amount of advance in the vacuum advance unit. Hopefully you have ruled out pre-ignition due to too high a compression, wrong plugs, poor fuel, etc. Might help to know what carb you are using and current ignition timing. Good luck.
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post #8 of 27 (permalink) Old Jan 8th, 20, 7:00 PM Thread Starter
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If it's pinging under full throttle consider an issue with total timing or jetting, provided there are no air leaks, if it pings on the level or with "lean tip in" (like starting up a hill and SLOWLY easing into the throttle) consider unplugging the vacuum advance (if you have one) to see if the pinging stops. If the pinging stops you may wish to acquire a Crane adjustable vacuum
advance and cut back on the amount of advance in the vacuum advance unit. Hopefully you have ruled out pre-ignition due to too high a compression, wrong plugs, poor fuel, etc. Might help to know what carb you are using and current ignition timing. Good luck.
The only things I have ruled out are vacuum leaks from the carb base to the heads, the manifold itself. It is a Holley HP VS. 32* total timing, I am using the best fuel available in my country, I need to get it jetted right before I can do anything else, thanks for your input.
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post #9 of 27 (permalink) Old Jan 8th, 20, 7:26 PM
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Re: Stagger jetting Holley HP

Quote:
Originally Posted by ivorchevelle View Post
I fitted another Identical carb and I am now getting an even colour across all spark plugs with the #73 jets installed, unfortunately it is white and still pinging so I have upped the jets to the baseline how Holley would have supplied it, #77, will test drive tomorrow. Obviously staggered jetting is a thing but when you increase jet size by on one side by 6 and nothing changes there are bigger problems within, it would seem that my carb has an internal problem, maybe that is why the last guy sold it, brand new in box, maybe he tried it and took it straight off again!
I took off the secondary metering block and these jets have been changed to #76, should be #84 from the factory. I still got to figure out what is wrong with it as I now have an engine with no carb on it. Both these carbs are Holley HP 750 VS. Buyer beware.
Take both metering blocks off, remove the air bleeds in the body, and blow carb cleaner than compressed air through all passages including the boosters, etc. Verify the bleed sizes. Confirm the emulsion passages are not blocked.
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post #10 of 27 (permalink) Old Jan 9th, 20, 12:53 AM
 
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Did you verify the float level is correct? If its too low your jetting change is not going to be accurate. Also check the idle screws, can they be turned all the way in, with the engine running? If yes you have a vacuum leak or throttle blades too far open and your running on main circuit. The carburetor has slots in baseplate, if you can see the whole slot your butterflies are too far open. Check vacuum with a good gauge, and use this to set idle mixture screws. When you get them set, check your power valve for correct size. It should be (Example)around 6.5" power valve for a reading of 8" vacuum idle in gear. If power valve is not correct it will affect your plug readings.
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post #11 of 27 (permalink) Old Jan 9th, 20, 1:02 AM
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Re: Stagger jetting Holley HP

>>>> I need to get it jetted right before I can do anything else, thanks for your input.<<<<

Ivor, this is not right. what you need to get right first is the ignition. What's the base timing, amount of mechanical advance, amount of vacuum advance.

If you don't know this stuff the chances of it being right are between slim and none. There's point whatever trying to optimize the carb without knowing exactly what's going on with the timing.

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post #12 of 27 (permalink) Old Jan 9th, 20, 11:31 AM Thread Starter
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Quote:
Originally Posted by pockets View Post
Quote:
Originally Posted by ivorchevelle View Post
I fitted another Identical carb and I am now getting an even colour across all spark plugs with the #73 jets installed, unfortunately it is white and still pinging so I have upped the jets to the baseline how Holley would have supplied it, #77, will test drive tomorrow. Obviously staggered jetting is a thing but when you increase jet size by on one side by 6 and nothing changes there are bigger problems within, it would seem that my carb has an internal problem, maybe that is why the last guy sold it, brand new in box, maybe he tried it and took it straight off again!
I took off the secondary metering block and these jets have been changed to #76, should be #84 from the factory. I still got to figure out what is wrong with it as I now have an engine with no carb on it. Both these carbs are Holley HP 750 VS. Buyer beware.
Take both metering blocks off, remove the air bleeds in the body, and blow carb cleaner than compressed air through all passages including the boosters, etc. Verify the bleed sizes. Confirm the emulsion passages are not blocked.
I did all of this and found a manufacturing defect with one emulsion bleed, i fitted a brass plug with .027 hole in it but this didnt cure it.
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post #13 of 27 (permalink) Old Jan 9th, 20, 11:43 AM Thread Starter
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Originally Posted by Tom Mobley View Post
>>>> I need to get it jetted right before I can do anything else, thanks for your input.<<<<

Ivor, this is not right. what you need to get right first is the ignition. What's the base timing, amount of mechanical advance, amount of vacuum advance.

If you don't know this stuff the chances of it being right are between slim and none. There's point whatever trying to optimize the carb without knowing exactly what's going on with the timing.
10* at idle 22 mechanical all in at 3900rpm, 10* vacuum advance. I would like to bump it up to 34* total all in by 3000rpm as my starting point but while it is pinging I thought I would wait until I found the problem, I have just fitted a known working carb but it still pinged, I have returned it to factory settings but need to try it again to see if it has stopped it pinging, I hope to advance it another 2 degrees if it will allow me.
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post #14 of 27 (permalink) Old Jan 9th, 20, 12:59 PM Thread Starter
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Originally Posted by Kirk View Post
Did you verify the float level is correct? If its too low your jetting change is not going to be accurate. Also check the idle screws, can they be turned all the way in, with the engine running? If yes you have a vacuum leak or throttle blades too far open and your running on main circuit. The carburetor has slots in baseplate, if you can see the whole slot your butterflies are too far open. Check vacuum with a good gauge, and use this to set idle mixture screws. When you get them set, check your power valve for correct size. It should be (Example)around 6.5" power valve for a reading of 8" vacuum idle in gear. If power valve is not correct it will affect your plug readings.
The pv is currently 4.5 as supplied by Holley, I did try 8.5 and 10.5 but some plugs were still bone white, the carb was faulty, I hope with this replacement carb I can tune it without pinging.
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post #15 of 27 (permalink) Old Jan 9th, 20, 1:59 PM
 
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Are you sure on actual timing? Do you check it with vacuum advance disconnected from distributor and line plugged? Verify balancer hasn't spun if old factory one(verify TDC).....the two piece balancers will spin the top ring and not be accurate, and certain years have keyway in different spots
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