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.

[LouC]

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.

[Zyen]

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.

[Zyen]

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.

[Mudsport96]

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.

[Zyen]

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.