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=sharingi 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.
