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Old 03-03-2009, 03:59 PM
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Correct Jetting on Holley 750

After idling for about 15 minutes with new plugs, I pulled them out and they were pretty black. I had jsut went up 1 heat rang on the plugs but im thinking I need to go down a lot more in the jetting. I have dual 750 Holley carbs with vac sec. This is on top a 8-71 BDS blower with about 8-10 lbs of boost. Right now I have #68 jets in there. My elevation is about 5000ft, so i was thinking of going down to #65. Is that too much of a jump?

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Old 03-03-2009, 04:34 PM
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If they are fouling on idle, fix that first, don't change main jets.
It could be that you are running rich on idle, then fine or even leaning
out in higher RPM's.
The idle mixture screws on the sides of the metering block are what
you need to adjust for idle.
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Old 03-03-2009, 05:03 PM
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I had the same problem. Dual carbs on my 6-71 Blower. Black plugs at idle and little response from my idle mixture screws. My fix was to lean out the idle feed restrictions in the metering blocks. My metering blocks are billet so all I had to do was go down a couple of idle feed restriction jet sizes. After this my idle mixture screws became responsive. If you don't have billet, adjustable metering blocks, I read somewhere that you can put a piece of wire in the idle feed restriction to lean it out.

If you have removable idle air bleeds, you can also try to enlarge the idle air bleeds to lean the idle mixture out. If you don't have removable/adjustable air bleeds, Holley says to leave the air bleeds alone. You will ruin your carb body.

Another issue I had was that my pcv valve was sucking oil into the blower which fouled my plugs. I had to get taller valve covers with better baffles.

Hope this helps......
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Old 03-03-2009, 05:10 PM
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Thanks everyone for the help, this gives me a really good place to start. I order a O2 sensor from summit a couple days ago and Im hoping it will be in by the weekend.

I dont have removable idle air bleeds so i'll have to look in to other options.

The cam is a solid roller Int .650 exh .660 235 ish duration. Its around that I havent looked at the cam card in a while. I know that info should be permanently burned into my brain.

I'll have to check the model numbers on the carbs too.
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