marine quadrajet 17082515 on gm 4.3

[Zyen]

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.

[LouC]

I think the plugs look lean still to me.....

[Cliff Ruggles]

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......

[Mudsport96]

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.