4.3 gl backfires/lean pop intermittenly

[Lou C]

A couple of your plugs look so lean it’s as if they were never used; I think that carb needs to be gone thru and cleaned out. And those V/P timing specs don’t really make sense to me, that spec seems to have the level of advance jumping up very early. I looked at the chart for my old ‘88 pre Vortec 4.3 with the 4bbl and points Prestolite distributor. It starts with 6* base timing at idle, @ 1500 its at 12* total @ 2500 its at 16* total, and it peaks @ 3200 with 18* total. Are those specs from iBoats or right from a VP factory shop manual? I can imagine that if you have Vortec heads you might be able to run at more advance but that seems like a big difference.
as an example I was looking around to see what specs I could find on Merc rigged 4.3s and found this;
https://www.mtboats.com/mercuryignitionadvancemodules.html
look at the top one for the V6, 14* of advance, so if you start with a base of 6* which is typical on the 4.3s you wind up with a total of 20*

now if your engine ran well with no popping for a while and then this started the question is what changed?
couple of possibilities:
intake valves not sealing well
intake gasket leak
carb gasket leak
defective distributor cap
I would for sure do a compression test and leak down if possible

keep in mind that exhaust system problems (leaky manifold to elbow gasket) can cause corrosion on valve stems and seats due to water intrusion....

 

[boatlife123456]

Timing spec is right out of the manual, Carb was COMPLETELY gone thru, new jets ,powervalve and accelerator pump diaphram, new float valve needle and seat, new carb gaskets, new distributor cap. leak down/compression is next.So frustrating. Really dont want to pull the heads off of a brand new remanufactured engine.

 

[alldodge]

The distributor should provide around 20* of timing (give or take)
If distributor is set to 21* then the distributor will provide more timing if it is working. Giving a motor much more above 21* will cause detonation.

A motor running lean will cause backfiring at higher rpms. If your motor has a knock sensor and the sensor detects a knock then timing will be retarded.

Right now I don't think its a motor issue yet, still leaning toward fuel or timing, but could have a burnt valve because it doesn't take long once detonation starts

 

[boatlife123456]

compression and leakdown test were fine ( thank God) and the WINNER IS drum roll please ------- LOU C ---!!!!!!!!! bad manifold to elbow gasket- Pulled it off today and it was in ruff shape and looking down into exhaust manifold I see water, pull plug #3 and #4 and both tasted salty( yum). So thinking back the boat runs fine off the line and once it planes out around 3 grand runs fine then when giving it more gas ( rpm increase) she pops- I could assume the water pump spins faster forcing some water into the exhaust manifold, and then the issue when turning the boat makes complete sense. The water pooled at the bottom of the exhaust manifold rolls into the cylinder causing the popping. All speculation at this point but the tell tale sign for me was the water in the exhaust manifold. The risers are in good shape no clogs anywhere( only 2.5 yr old) but the gaskets were gone. So new gaskets here on friday and I will update. Hopefully this is it, and thanks to all who gave me some assistance.Anyone ever up in the Newport RI area, I'm buying.

 

[Lou C]

Makes sense and I know that when I had water in a cyl from leaky head gaskets I definitely had a misfire. V/P had issues with some of the gaskets used and they were revised a few times along with the installation procedure. So it depends on which gaskets you are using (VP or aftermarket). When I installed my updated exhaust (replaced the old NLA one piece units on my OMC with the later style used by OMC and later Volvo) on my 4.3 I used a Barr aftermarket exhaust kit and used their gaskets, coated in Permatex Aviation. So far no leaks, installed in '17. If you use the original equipment VP gaskets follow their instructions....Good thing you caught it!
Newport RI, we used to head up to Camp Yawgoog in Rockville RI every summer when my son was involved in Boy Scouts, we went from '07 up to '18 actually. Went to Newport one year and loved it.

 

[LegacyT]

Disregard, I see you’ve discovered the issue

 

[Lou C]

here's what the new V/P gaskets look like....