Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

marine quadrajet 17082515 on gm 4.3

<< < (2/2)

Zyen:
apologies for the delay.  finally found my password and could log back in.  :)


--- Quote from: Cliff Ruggles on December 06, 2025, 04:44:24 AM ---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.

--- End quote ---



--- Quote from: Mudsport96 on December 06, 2025, 05:24:18 AM ---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.

--- End quote ---

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.


--- Quote from: LouC on February 02, 2026, 06:52:33 AM ---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.

--- End quote ---

Mudsport96:

--- Quote from: Zyen on February 25, 2026, 05:43:40 AM ---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.

--- End quote ---
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:

--- Quote from: Mudsport96 on March 07, 2026, 09:17:57 AM ---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.

--- End quote ---

Navigation

[0] Message Index

[*] Previous page