Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: Zyen on September 07, 2025, 03:16:33 PM
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hello all.
been working on fine-tuning my new 4.3 in my boat. it currently has about 9.5 hours on it.
last 2 trips out (around the 6 hour mark), we have been changing props to get wot rpm in range (4400-4800), but have had no luck until yesterday.
when it did finally get to 4700rpm, it was backfiring thru carb. i cut throttle and stayed under 3500 for the rest of the trip.
changing props to change rpms dont make much sense going by the 'rule-of-thumb' for pitch/blade changes.
EDIT...adding props and rpms
with 1°/13° timing
17 pitch 4 blade 4300 rpm
19 pitch 3 blade 3650 rpm
21 pitch 3 blade 3850rpm
23 pitch 3 blade 3900 rpm
with 8°/20° timing
17p4b 4700 rpm and backfiring
21p3b 4100 rpm
did not bring the 19 this trip
did not try the 23 after low rpms with the 21
END EDIT
details and specifics:
boat is 19'1" bowrider, 15.1' lwl, 1.8' draft, 3280#, 1.76 drive ratio
carb is Rochester quadrajet 17082515. had 70/43b in carb when i got it (and ran rich in previous engine), but merc manual states 66/36b for this carb/engine combo. found and installed 67/36b to err on rich side, if possible.
also found 2nd rods were DG with an N hangar. should be DH. i had DR handy and they're very close, so installed them with an L hangar to get a bit more fuel when opening. secondary spring is 1/2 turn.
rebuilt with marine kit from cliff before putting on this engine.
idle is smooth as silk at 575 rpm.
fuel is 87 pump gas. pump does about 7psi max and 29gph free flow.
point gap is 0.019" and dwell is 38°.
base timing is 8°, total is 20° all in at 3200rpm. this spec is per my engine builder information (marine engines 4 less in florida) previous timing was 1° btdc base and 13° at 3200 per omc manual. increasing timing seems to have gained ~400 rpm.
i have a hard time reading plugs on the new fuel blends, but they look lean to me.
more views of plugs and (wonky, but best i could get) pictures of cylinders. the 'wet' appearance and 'sparkles' on top of pistons are a trick of light, for those that view and wonder.
https://drive.google.com/drive/folders/1utOENunwJ3SqC6nYHq_C1BAAJdQHXq2J?usp=sharing
i think my next step is changing jets/rods to 69/39b or maybe 70/39b to richen it up.
i am looking for conformation or other ideas.
thanks for reading.
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I think the plugs look lean still to me.....
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Complete waste of time to try to "read" plugs on this new fuel.
I could write a book here on why but just take my word for it.
Tune for best results instead. The primary side of the carb should be tuned first starting with idle mixture, then doing light throttle testing to make sure the jet/rod relationship is fine.
Then do some heavy runs primary side only and make sure it pulls well, not "flat", down on power, surging, etc.
Once you've dialed in the primary side move on to the secondary side. Make full power runs swapping metering rods with what you have available to you. You can move total timing some here as well to find exactly what the engine wants.
I find full throttle tuning easier with boats that cars because with a vehicle you typically find yourself very quickly running out of road and it's difficult if not near impossible to evaluate results for lack or test criteria, like a drag strip that shows ET/MPH, etc.
For boats you can almost always find a long flat stretch of calm water and make long full throttle pulls noting MPH changes to see what the engine really likes from the carb.
I noticed you mentioned 70/43 "ran rich on the previous engine" but you didn't state where it ran rich at, light engine load, heavy part throttle, etc......
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Couple of questions and comments.
1. Do you have an actual quadrajet 4.3 manifold or a spacer? If manifold, what part number? My buddy has an 18'6 Caravelle with a 4.3 and a square bore manifold. Wants to go quadrajet but not wanting a spacer.
2. Is the distributor in good shape? No play in shaft and bushings, and no sticking weights?
3. Previous mentioned Caravelle runs in recommend rpm ranges with a 1.76 outdrive ratio and a 21p stainless prop. I know weight carried and draught can affect water drag and engine load. But, if similar a 19 to 21p should be in your range.
And totally follow Cliffs advice on how to go about tuning the carb.
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I have the OMC version of the same engine, same carb. According to them, the timing base timing was 6*BTDC if using 89 octane fuel (says it right on the flame arrestor) and 1* if using 87 octane. I have always used 89 or higher. These had a points distributor with mechanical advance (Prestolite, OMC and Merc used the same unit, just painted different colors) and at about 3200 rpm they would reach full advance which for the 4.3 was only 12* so 6*+12* advance = only 18* (+ or - 2*) total advance at 3200. I never found out why but the 4.3 is prone to detonation more than either the 5.0 or 5.7 V8s so they run much less spark advance.
My boat's a bit bigger (20'9") and a good deal heavier (approx 4200 lbs) I had to go with a 15x17 prop on the Cobra outdrive to get it to 4600, a 15.5x15 gave me approx 4900.
I actually weighed the boat on the trailer and was shocked at how much it weighs. Full load is 5050 lbs and the trailer itself weighs 850. So the boat is a porky 4200 lbs!
If I repower it one day I know it should have a 5.7.
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apologies for the delay. finally found my password and could log back in. :)
Complete waste of time to try to "read" plugs on this new fuel.
yeah, it's nothing like it used to be.
I noticed you mentioned 70/43 "ran rich on the previous engine" but you didn't state where it ran rich at, light engine load, heavy part throttle, etc......
it ran rich and sluggish at all throttle positions. >5s plane time no matter what prop i used. plugs and pistons were coal black. soot in exhaust system from heads to prop. bog worse once secondaries open.
Couple of questions and comments.
1. Do you have an actual quadrajet 4.3 manifold or a spacer?
manifold is from 3.8 and is for qjet
If manifold, what part number?
OMC 0983874
2. Is the distributor in good shape? No play in shaft and bushings, and no sticking weights?
4.3 marine dizzy in great shape. rebuilt with new bushings and springs. shaft, weights and advance are all free and smooth. lubed with graphite. advance follows factory curve (8° @1600, 12° ~3200).
3. Previous mentioned Caravelle runs in recommend rpm ranges with a 1.76 outdrive ratio and a 21p stainless prop. I know weight carried and draught can affect water drag and engine load. But, if similar a 19 to 21p should be in your range.
i think so as well, yet on both 19 and 21 my wot rpm is low
And totally follow Cliffs advice on how to go about tuning the carb.
absolutely i'll retune per cliff's advice once i wake it up from winter sleep.
run data from 8Nov25
lake was choppy, wind se ~8kt, 81°f & 78%rh
23p3b 3900 rpm @ 44mph (8% slip)
21p3b 4150 rpm @ 43mph (8% slip)
19p3b 4300 rpm @ 42mph (4% slip?)
17p4b 4600 rpm @ 35mph (16% slip !!)
tach is new in-dash unit. follows external tach (sears engine analyzer) within 50rpm
SoG is gps app on phone.
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had to post this separate, as it exceeded the character limit when including in previous reply.
I have the OMC version of the same engine, same carb.
mine is OMC also...you have the cobra, mine is stringer (800)
I never found out why but the 4.3 is prone to detonation more than either the 5.0 or 5.7 V8s so they run much less spark advance.
it's theoretical. marine is more-so due to constant heavy loading. everything I've seen talks about the lack of quench, but i haven't found any concrete evidence either way compared to similar combustion chambers in a gm v8 using dished pistons. 'they' say the dish can(will) "hide" the air/fuel charge from the flame front, which increases the risk of detonation in heavy loading/heavy throttle, but i haven't found anyone online talking about any type of pre-ignition or detonation damage in a gm 4.3, marine or auto...it's all v8's.
i have, however, found some data on 4.3l dyno runs where they were using 34° total advance. this is interesting, but dyno pulls are only full load for ~10 seconds...
My boat's a bit bigger (20'9") and a good deal heavier (approx 4200 lbs) I had to go with a 15x17 prop on the Cobra outdrive to get it to 4600, a 15.5x15 gave me approx 4900.
I actually weighed the boat on the trailer and was shocked at how much it weighs. Full load is 5050 lbs and the trailer itself weighs 850. So the boat is a porky 4200 lbs!
the cobra drive is (usually) 1.94 and uses a larger pitch (turning slower), so your engine would turn faster compared to mine with the same prop. factoring in the +1.5' and 1000# difference should even things out, which I've seen with the 17p prop.
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apologies for the delay. finally found my password and could log back in. :)
run data from 8Nov25
lake was choppy, wind se ~8kt, 81°f & 78%rh
23p3b 3900 rpm @ 44mph (8% slip)
21p3b 4150 rpm @ 43mph (8% slip)
19p3b 4300 rpm @ 42mph (4% slip?)
17p4b 4600 rpm @ 35mph (16% slip !!)
tach is new in-dash unit. follows external tach (sears engine analyzer) within 50rpm
SoG is gps app on phone.
So speeds are GPS verified that was one of my concerns. Are all props from the same manufacturer? I know outboard props of the same pitch and material can have different speeds from different cupping and rake. And something is way off with the 17p I would almost say it is damaged or marked wrong. Typically you get LESS slip with lower pitch. 15p@4600 is 35mph at 6% slip is why it think the prop is the problem there.
Okay back to the engine.
One thing I am curious about is the balancer. I myself, would pull the number one plug and use a piston stop to positively verify TDC lines up with the mark on the balancer. Over the years I have seen a disturbing number of balancer that were more than 4 degrees inaccurate. I am not saying that is definitely your issue. But, in the off season it doesn't hurt to check. PLUS, if it is wrong, you are possibly fighting two issues. Carb and timing ( that looks correct but is not.).
Another thing to try is if you have a buddy you trust to slowly block the carb and see if richening it up helps speed, richer secondary rods.
Thats all I can think of, and I may be way off. I like to try to help, but online diag sucks, I prefer hands on.
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So speeds are GPS verified that was one of my concerns.
right, i see a lot of posts that use the 'wish-o-meter' in the dash. gps is the way to go.
Are all props from the same manufacturer?
no, they are not. all are good condition ebay specials. size and part# are factory stamped and visible on all. no visible damage on 17, 19, 21. 23 has some slight edge scraping, but no bends or cracks. 21 appears new; no rub marks on inside splines when i got it.
I know outboard props of the same pitch and material can have different speeds from different cupping and rake. And something is way off with the 17p I would almost say it is damaged or marked wrong. Typically you get LESS slip with lower pitch. 15p@4600 is 35mph at 6% slip is why it think the prop is the problem there.
yeah, i agree...and it's a 4 blade, which makes it worse. also, this prop acts funny, like it's not 'catching' water. sluggish, hardly any 'pull'....and the 19p feels the same. starting to wonder if the hub is slipping. will have to look when i get time.
Okay back to the engine.
One thing I am curious about is the balancer. I myself, would pull the number one plug and use a piston stop to positively verify TDC lines up with the mark on the balancer. Over the years I have seen a disturbing number of balancer that were more than 4 degrees inaccurate. I am not saying that is definitely your issue. But, in the off season it doesn't hurt to check. PLUS, if it is wrong, you are possibly fighting two issues. Carb and timing ( that looks correct but is not.).
the balancer is new (came with engine) and i eyeballed it before i put the heads and intake on, using feel from btdc/atdc piston movement. my thumb isn't exactly a positive stop, but it's damn close.
Another thing to try is if you have a buddy you trust to slowly block the carb and see if richening it up helps speed, richer secondary rods.
that's easier said than done...i have one i trust to help, and our schedules clash. i'm working on teaching my oldest daughter (15) to pilot confidently enough that i can mess with engine at wot....but it's a slow process.
Thats all I can think of, and I may be way off. I like to try to help, but online diag sucks, I prefer hands on.
for sure. i appreciate the time and suggestions.
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update...weird sh!t and im a dumb@ss.
woke the boat up for this season, added vacuum gauge and took it out 2 weeks ago. ran 21p prop. plane times are down (around 5s or so) and still bogging at wot. cruise seems great, 3200-3400, but gauge is showing steady 7". wot showed 0", so i tightened the air door spring from 1/2 to ~7/8, then 1-1/4 turn. this did nothing.
removed carb that night and checked it. somehow, the spring came off the air door and was wedging against the rod it's supposed to hook on. when secondaries opened, there was enough vacuum to flop the air doors fully open, and the vacuum break would close them when i cut throttle. after i found this (on the bench), i tried for almost an hour to recreate it so i could get pics/video of it, but had no luck.
reset spring to 7/8 turn and put in DG rods with an L hangar. top of air horn to center of hangar holes was set to 0.64" +/- 0.01" or so (hard to see at an angle).
checked idle mix and found they were 5 turns out, so warmed up engine and rechecked using vacuum gauge. settled on best vacuum at 19" and 3.5/3.6 turns out.
took boat out saturday for another run. put 23p on for snits/giggles. plane times still low, cruise was smooth at 3200/3300 with 7" vacuum and ~35mph (guessed...didn't actually check).
wot still bogs, but now backfires thru exhaust (that sht sounds weird as hell in a boat). vacuum at wot was showing 1-2". played contortionist to loosen air door spring to ~5/8-3/4 turn or so and put 21p prop on. plane times still >5s, vacuum at 3300 still around 7", wot now showing 0-1" vacuum and bogging but no popping.
got aggravated and went home after an hour or so. weather was great, lake was smooth, but not much room to adjust what i need to with everything installed. cant watch carb while driving, either, so been playing the guess/test/cuss game. did not check speed or rpm; was more focused on vacuum gauge.
get home and pull carb again. this time, open it up to see what's going on.
damn...i had 39b rods with those 67 jets this whole time (since last year). i coulda swore i put the 36b rods in there. had to get out the big magnifier to see those little stamps.
changed to 69 jets and kept 39b rods.
set secondary side to DR rods with a V hanger. i don't know what's more important on secondary rods...sitting completely down when closed (rods bottomed out in jets), or holding that 0.641" when open. since this lil 4.3 will never open the secondary side completely (maxing around 40%), i opted for completely down when air doors are closed. this combo has ~ 0.70" from top of air horn to center of holes.
i have (what appears to be) the blue power spring from cliff's marine kit. it seems to be a 'medium' spring, compared to others i have.
also, while i had it on the bench, i measured the MAB at 0.051". they dont appear to have been molested but are brass inserts. the IAB are the same on all counts.
so, the questions i have now are these...
is 7" manifold vacuum ok at cruise? i expected it be more like 10-12", but have not seen any references for 3200-3300 cruise rpms.
is the blue spring ok for this? searching shows any "blue" qjet power springs are for 9-12", in which case this would run with full enrichment at cruise.
should secondary rods be completely seated in jets when air doors are closed?
if not, how important is that 41/64" dimension when the air doors will never be completely opened?
thanks for reading.
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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.
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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.....
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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.
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?
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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.
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.
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oops...i meant to say the 2nd air door spring was changed at same time i changed props.
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What secondary rods do you have available?
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If i remember you had DG rods in it. They are a bit richer than the DRs. Could always try them again
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i dont have a 'nice' assortment; DG, DR, DP and V, N, P, L. currently running richest i have.
the problem i see currently is i *think* the mains arent tuned properly yet. still working on that.
however, as this 4.3 will never open the 2nd door past ~40% and all the secondary rods are +/- 0.002" at that setting, i don't know how much difference that will make.
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I would use the DP rods and tweak the hanger a little to lift them higher.
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I would use the DP rods and tweak the hanger a little to lift them higher.
while i appreciate the advice, i don't see how going to a much leaner rod can fix a lean bog on the secondaries. at 40% the DP is 0.1157" whereas the DG is 0.1075". even bending the hangar to get the rod to 60%, the DP is still 0.100" fatter.
am i missing something?
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My mistake. I was lookin at the specs for CP... not DP. :o ill be over in the corner....
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LOL.
i understand...get cross-eyed at times looking at all these things.
rebuild kit came in yesterday for 'new' carb. i wound up stripping couple air horn screws (which were very close to gone already), so gonna clean/install/tune new carb. may very well be 'starting over', but this core is in much better shape than what's on there now.
gonna put it back how i received it with 71/41 and DG/P.
will update soon.
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P is about the leanest hanger there is.
Most V6 marine Q-jets i have seen has an E-H secondary rods hanger (with DH secondary rods).
If you want richer use a B-H hanger with your DG rods.
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so, i got off the internet, pulled my head outta my ass, and pretended to be the tuner i say i am. i *thought* previous 70 jets were rich because (as it turns out) some PO had the choke wired half-closed. i remember removing ties and adding divorced choke linkage. the 66 jets from merc specs are for the merc carb, which has smaller primaries than qjet. the change to 70/39 didnt help much so did some math and found that the 67 combo was running an estimated 17 afr and 70s were around 15.7 afr. the 70/39 and the 'new' 71/41 were both jetted for a 3.8v6. changed to 73/43 for an estimated 13.2 afr @ 4k, and 8° base using a nib/nos cobra dizzy. today's run on a 19p ss prop gave me 4600 wot @ 41mph on gps. idle to 3600 was crisp and responsive. 4200-4600 was slow and lazy. this gives me a good baseline to work on fine-tuning.
i cant advance timing much more because of the advance 'hump' at 1600 in the timing curve; anything above ~8° base puts my LPCP around a dangerous 10-11° atdc at 1500 - 1800 rpm and not much better till 2500+, but the LPCP at 4800 on 8° base is a lazy 25+ atdc, as seen by the 1.5-2 second climb from 4200 to hitting a wall at 4600.
however, did a good 2-3 minute run at wot with no problems. hard to see <1.5" hg with the gauge im using, so estimating < ~0.5" hg at wot.
so, unless anyone wants details, i'm gonna say this is closed as fixed.
thanks for the help all.
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I've got a question if you don't mind my asking. How did you calculate the LPCP for your engine? And since yours is a marine engine, is it any different than one in a vehicle?
You're not using a vacuum advance if I remember correctly, so maybe the calculations would be different on an engine with vacuum advance?
Thanks for any help you can offer.
Rick
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If Mercruiser calls for 8 degrees that is where I would put it. You aren't trying to win races ya know. The problem with trying to get street curved ignition timing on a boat application is that load varies sooo much o. The water vs pavement. That is why the ignition curves are "lazy" from the factory. The odds of losing traction while cruising down the street is exponentially lower than spinning a hub or ventilating the prop. At which point revs go wild obviously.
You could play with weights and springs to get every last bit of timing. But, if you need every last bit of power the 4.3 has... I'd drop in a 305 or 350. As for fuel usage. I have two close friends with almost identical boats. One is a 4.3 16.5 footer and one is a 350 18 footer. And cruising together on the same route, the get within .5 gallon per hour difference.
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mudsport, your's is a bit easier, so i'll respond to it first. yeah, most 4.3 stock is 8° base to keep the timing peak around 1600 under control while giving the most power/torque to get on plane.
you're right...it's not a race boat, i don't time it, i don't run anyone in it, and im not trying to get a street curve in the ignition...that's a sure-fire recipe for disaster. the curves are lazy to account for the multiple variables in boats and their respective owner/operator. since i can control some of those variables, i can account for them and reset accordingly. i dont have room for a larger engine, nor am i looking for an outrageous output from this one. the mileage is negligible. dont' really care, but want it reasonably efficient and somewhat close to the 205hp it's rated for.
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cadman, no marine engine should have a vacuum advance. that's way too much timing for marine loads. LPCP calc's are the same for any gas engine. the advance doesn't really matter, cause advance is advance in the math. im about to head to bed, but will give a breakdown on how im figuring it tomorrow. and, i may be wrong.
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How did you calculate the LPCP for your engine?
Rick
Here's how i did it, using my current wot and ignition timing as example:
crank rotation: 60,000 ms / (RPM * 360°) = ms/deg
4600 rpm = 0.0362 ms/deg
~12.8 afr using 87/E0 and ~90% VE (~0.5" hg) = ~1.3ms burn time to peak pressure
~0.3 ms from spark to flame propagation. = 1.6ms
Total burn from start to peak pressure = ~44.2° of rotation.
timing is set to 8° base and hard stop of 10.5° advance @ 3200 = 18.5° TA > 3200 rpm
burn time from ignition to peak pressure - spark advance = lpcp
44.2° burn - 18.5° btdc start = 25.7 atdc lpcp
-Allan