Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Need Tuning after dyno run
leeklm:
Cliff, I will be calling you for another parts order. In preparation for that call, what are your thoughts on the above specs? It feels like i must have something wrong here, as have never had these issues on my previous qjet builds. (All 2 of them :))
73 Z28:
--- Quote from: leeklm on February 21, 2013, 05:33:45 AM ---
Primary rods = 44 (bought from Cliff :)) Measurements above are what I took from the fat & skinny part of rods
Idle Tubes = .036 (I think I installed these w/o changing when purchased in my kit)
Down Channel = .05
Idle bypass air, which I drilled into the baseplate passages = .098
Lee
Lee,
From your description of the problem and your calibration info. It looks like you need to increase the main jet size to 74's and open the idle down channel to .062 and increase the idle tube to .037-.038 Tipping in the choke and getting it to run better indicates your too lean on the primary side. This is quite common in big block engines with mild cams.
Paul
--- End quote ---
73 Z28:
Two other things. What size did you drill out the idle mixture screw holes to? They should be minimum .095 and then you can adjust the idle mixture screws at about 4 1/2 turns out from fully seated. Your apt should be set 3.5 turns up from fully seated to start with. This will give you a good starting point for dialing in the qjet to your big block.
Paul
leeklm:
Thanks Paul. I did drill my idle screws out according to the book. If my memory is correct, .095 sounds about right.
Since I am running lean and down at least 30hp at full throttle, it seems that my primary & seondary flow needs help as well. At full open throttle, how much of the CFM is coming from the primaries as compared to the secondaries?
As I prepare to do more testing, what would reccomendations be for extra primary and secondary rods to have on hand?
Thanks again!
73 Z28:
--- Quote from: leeklm on February 25, 2013, 07:11:34 PM ---Thanks Paul. I did drill my idle screws out according to the book. If my memory is correct, .095 sounds about right.
Since I am running lean and down at least 30hp at full throttle, it seems that my primary & seondary flow needs help as well. At full open throttle, how much of the CFM is coming from the primaries as compared to the secondaries?
As I prepare to do more testing, what would reccomendations be for extra primary and secondary rods to have on hand?
Thanks again!
--- End quote ---
Lee,
The big block q-jets I've calibrated, I typically use 74 or 75 main jets and 44k or 45k metering rods for the primary side.
On the secondary side, I prefer CV, AU or CK rods. Typically the qjet flows 250cfm through the primarys and 500cfm through the secondarys, but as GM made 2 (well technically 3) sizes of q-jets. The primarys on the 800cfm q-jets are closer to 300cfm. When calibrating a q-jet for a big block, the carb is trying to deliver fuel to approx. 50 to 100 more cubic inches of motor so more fuel on the primary side will be needed.
Paul
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