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Stagger jetting Holley HP

5K views 26 replies 10 participants last post by  Reelysalty 
#1 ·
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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#3 ·
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
 
#6 ·
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.
 
#7 ·
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.
 
#8 ·
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.
 
#10 ·
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.
 
#11 ·
>>>> 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.
 
#13 ·
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.
 
#15 ·
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
 
#17 ·
Maybe some video so we can hear what you’re talking about? And what is the octane in your country?
 
#18 ·
The fuel is advertised as 97/98 RON,
I have fixed the uneven fuel distribution by changing the carb, if I am driving at 45mph and squeeze the throttle it pings, if I am doing 70 and accelerate it does not ping (I cannot hear any), plug insulators still bone white with #77 jets, I have now fitted #79 jets and will go a step colder on the plug, hope to try it tomorrow, no video unfortunately but have photo of plugs
 

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#19 ·
The color change on your ground strap is saying too much timing. The carbon on the threads says that the heat range of your plugs is ok, no need for colder.

What happens when you disconnect the vacuum advance?

Float levels set to the middle of the sight glass, or if plugs so that fuel just trickles out with them removed?
How much fuel pressure?
What fuel system set up?
What air cleaner set up?
What temps does your engine run at?
 
#20 ·
Last time I disconnected the vacuum advance it made no difference to the pinging, it seems to be around 45mph cruising then hit the gas a little it pings if I do 70 and accelerate it sounds fine. Fuel level just trickles, mech fuel pump and holley regulator 6.5 psi, I have the thinnest filter I could find, I dont have a temperature gauge on it but my dual fans are on two stats and the second fan only comes on during hot weather in traffic at 185* I believe.
 
#24 ·
You need to limit the mechanical advance in the distributor, 10 initial will not cut it. You need closer to 20 initial and 10 mechanical. You need as much total as it will tolerate without pinging, then hook up your vacuum advance. If your timing isn't optimum for the engine and available fuel then you will never get a good Air Fuel reading.
 
#25 ·
^^^^ THIS first ..
Reading plugs only works if you turn engine off right after a WOT pull. Then ... you can read ground strap for heat range, base ring for jetting and porcelain will show if preignition or detonation happening.
Porcelain on top of insulator can be bone white and fuel mix still too rich.
 
#26 ·
Do you have or can you get a WBO2?

Where are you? just generally, US, Europe, whatever.

Read carefully whatever jmarkaudio writes. He is a known expert in this field. The real thing.
 
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