Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
marine quadrajet 17082515 on gm 4.3
Mudsport96:
If you changed your primary rods, you will need to go out and retest. It is possible will the different rods that you required throttle angle at cruise will change. That will change manifold vacuum.
Cliff Ruggles:
Primary metering rods aren't a big player when tuning a Marine carburetor. About the only time they are down is idle and light engine load. Any heavy/full throttle and they are going to be up on the .026" tips.
As for secondary metering rods they are the key player at WOT and full load. Secondary rods vary considerably is upper section diameter, taper, tip length and the included angle leading to the tips. So the hanger each rod is used with plays a role in fuel delivery.
If you can come up with a nice assortment of secondary rods and s decent range of hangers it will help with WOT testing to get the best results.
Once you get the primary side "dialed in" do NOT change anything there.....
Zyen:
next update. (had to heavily edit to make the 3500 char count...guess im just long-winded)
ebay seller claimed original 4.3l qjet. got it in last week. it has 71/41 mains with lightest spring and DG/P 2nds.
makes sense, as mine came off a 3.8l with 70/42 (not 43b; better magnification showed 42b).
changed to 70/39 for next test.
--- Quote from: Cliff Ruggles on April 27, 2026, 04:10:37 AM ---Primary metering rods aren't a big player when tuning a Marine carburetor...
so if i am understanding this, it is preferable to keep the 0.030" (+/- .002 or so) jet/rod difference and tune for best jet / response on mains?
...
If you can come up with a nice assortment of secondary rods and s decent range of hangers it will help with WOT testing to get the best results.
i dont have a 'nice' assortment; DG, DR, DP and V, N, P, L. currently running richest i have.
Once you get the primary side "dialed in" do NOT change anything there.....
understood.
--- End quote ---
went out yesterday to get some data.
temp ~81°, 65-75%RH or so, winds 3-5kts, lake choppy...not idea for testing.
we put all all 4 props to get some readings, after cruising for ~20 minutes to warm everything up.
new phone, and havent thought about gps app, until i needed it...
tested in order below:
Prop Cruise Vacuum WOT RPM 2ND AIR SCREW WoT Vacuum (changed at same time as prop)
17 14" @ 3500 4400 1/2-5/8 turn < 1.5 (hard to get good read <1.5)
19 14" @ 3400 4300 3/4 turn ~ 1.5 - 2
21 9" @ 3500 4300 7/8 turn ~2.5
23 7" @ 3100 4100 3/4 turn ~1.5
i dont understand why wot is lower now...previous runs using the 17 would hit 4700 or so.
seems like im heading in the right direction in regards to jets. the 23p is way overpropped, so i think it's no longer a contender unless rpm come way up on others.
21 still 'feel' the best, but overpropped with current tune.
19 was (for the first time since i've had it) quite responsive.
17 felt like there was no load. from idle, hit wot and immediately jumped to 3800-3900 with bow way in the air. started popping thru intake so i didnt try that again. thinking about it, im sure with the 0.400" float dimension i was uncovering the main jets with that bow rise.
in all cases, plane time was ~3.4-3.5 seconds. seemed to pull about the same with all props; i couldn't tell a difference. will have to time 0-20 on mains to compare.
made couple more runs with 21 and 3/4 turn. cruise vacuum ~10" hg @ 3100.
still bog/lean...if hold wot > 3-4 seconds, it would backfire thru intake.
next step is richer main. i have 70,71,72,74.
downside is i cant (wont) do carb work on water or at (usually busy) ramp...so major tuning is at home on fridays, then the hour drive to the lake on saturday or sunday.
i am using the richest 2nd set i have, and it seems i still need more fuel. as this v6 cant open the 2nd air door more than ~40% and all the rods are +/- 0.002" at that range, the only way to get that richer is bending the hangar to pull rods up more. are there any other options?
thanks for the replies.
Zyen:
oops...i meant to say the 2nd air door spring was changed at same time i changed props.
Mudsport96:
What secondary rods do you have available?
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