Carbureted LSX forum - LQ4 carb size
07-01-2012, 12:21 AM
I have a carb'd 6.0 LQ4 317 heads custom grind cam .605/.605 236/244 split 111/114 LSA i have a holley 750 double pumper in a 91 firebird. Is the 750 a proper size for this motor it keeps going thru plugs real fast! I did some research on here and it seem like everyone is running 750 or larger carbs so idk what my problem could be. I am not too educated on holley carbs to much so there could be something im missing. I've tried down jetting, smaller squirters and white cams front and rear and its still running rich. I do have a wideband AFR guage on it and it reads about 10.5-11 at WOT and 11.5ish in the primary circuit any suggestions as far as carb sizing goes or anything to point me in the right direction would be appreciated.
07-01-2012, 06:12 AM
750 is the sweet spot for most. Lean it out. 12.7 at WOT, shoot for 14.7 at cruise and idle.
07-01-2012, 08:48 AM
750 Should be great!! I have an 825 in mine. Just tune it properly.
07-02-2012, 07:15 AM
I'm running an Ly6 with a similar sized cam in a heavier C10 pickup. Running a 770 street avenger. It ran great out of the box, but I do plan to fine tune it one day
07-02-2012, 02:10 PM
Give us some details:
What model number 750 Holley?
Primary Jet Size?
Secondary Jet Size?
Two corner or four corner idle screw?
How many turns out are the screws from closed?
What heat range plugs?
07-02-2012, 09:57 PM
thanks guys sorry it took so long to reply i heard that 750cfm carbs are perfect for this motor but i had a few people say different so i thought i would ask all the mad scientists on here that probably have done this a few times lol
its 4150 holley fuel pressure set at 6 psi (just put a brand new regulator on)
intake= edelbrock performer
4 corner idle screw
as of this being posted it killed another set of plugs only lasted 2 weeks
the idle screws have been set with a vacuum guage and their at were it pulls the most vacuum
right now its got
72 jets in the front with a 3.5 power valve
84 jets in the rear with 6.5 power valve
07-03-2012, 12:48 PM
First, I would like you to check your choke horn for an actual production number. For a standard 750 mechanical secondary Holley, it should say 4779 dash something. For example 4779-3. This number is important as we can use the Holley reference sheet to see what the baseline jetting - etc. should be.
It is somewhat uncommon that a 750 Mechanical Secondary has a rear powervalve. I typically remove them as they are not good for much in all but oddball applications. I think the rule of thumb is to jet up 4 to start when removing the secondary power valve. But, I would like to know the exact model number first as defined above.
The first thing I would try is to adjust your idle mixture screws equally on all 4 corners until you can achieve an idle mixture between 13.8 to 14.7. Get it as lean as you can without poor idle or off idle stumble and tell us how lean you can get the AFR.
Here is the reference sheet we can use to determine your baseline calibrations for your particular carburetor production model:
07-06-2012, 11:35 PM
sweet! I'll check the part number and adjust those idle air screws and get back to you thanks!
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