4.3L mercruiser with points. Timing question.

[kbreks]

I have a 4.3l mercruiser with a prestolite distributor. I understand that it should have thunderbolt but instead has points. Timing spec says it should be 8⁰ BTDC according to manual for thunderbolt. Total timing at 22⁰. What is confusing me is that with aftermarket HEI distributor with 22⁰total. My boat falls on its face and wont plane. However if i install the points back in, 8⁰ BTDC i get 32 ⁰ total advance and i have lots of power. According to the manual i have, v6 mercruisers with points should run 12⁰ base and 20⁰ total advance. How can the same engine have such different advance needs depending if its points or HEI. Im not trying to blow up my engine or melt pistons. Im only digging into this because my distributor is falling apart and if i cant get it rebuilt, i will need to find a replacement with the correct timing curve. So i guess my question really is. What should my timing actually be at base and total without melting my engine?

 

[Scott Danforth]

12 degrees of base timing plus 20 degrees advance is 32 degrees. That's how math and timing work

 

[kbreks]

12 degrees of base timing plus 20 degrees advance is 32 degrees. That's how math and timing work

Agreed. Just the first graph doesnt state that. Which is fine. Is what it is. I just don't understand how hei vs points changes total timing by 10⁰. Engines shouldn't care what or where the spark comes from. As long as it sparks when it should. Im likely overthinking it. But i dont want to cause damage

 

[Lou C]

Been trying to figure this out for years. OMC used the same engine & same points distributor yet initial timing was set at 6* and centrifugal advance was 12* for a total of only 18*. It never made sense to me that the V6 should have so much less advance than the V8.

 

[Lou C]

Also that info about the 4.3 being an uneven firing engine was confusing to me. Yes the first GM V6s were odd firing due to the 90* angle between the cyls whereas 60* would have been perfect for a V6. Supposedly the “even fire” crankshaft with the split journals corrected this but maybe not 100%? I have a spare Prestolite in the garage I’m going to pull it out & take a look at the cam lobes.

 

[kbreks]

I can't tell from my picture. But this also isn't even the distributor that I've found that should be in a 4.3l omc. This is a prestolite 7018. Ive talked to a few marine mechanics and no one can seem to answer why i cant run 18-22⁰ total timing, it just doesn't run right. But advancing it well beyond makes it work great. Generally that would cause major issues.

 

[Scott Danforth]

remember, the 4.3 is on the raged edge of detonation compared to the V8's My guess is the advance curve was shortened to keep the motor from eating itself.

 

[kbreks]

remember, the 4.3 is on the raged edge of detonation compared to the V8's My guess is the advance curve was shortened to keep the motor from eating itself.

But shortened from 32 to 22. Thats a huge loss of power. I cant see how any boats like mine but with the thunderbolt even function. Guess i need to find someone with a similar boat and check their timing to compare.

 

[Lou C]

Interesting that Mercruiser’s “own” inboard V6 really looks like a warmed over old GM 4.3. Same 90* block, same split journal crankshaft, same style balance shaft etc if you look at Merc parts diagrams. Makes you wonder if they just bought the tooling from GM & then made some updates & refinements and there you have it!
Didn’t the production of the old cast iron 4.3 end around 2014?

 

[kbreks]

Interesting that Mercruiser’s “own” inboard V6 really looks like a warmed over old GM 4.3. Same 90* block, same split journal crankshaft, same style balance shaft etc if you look at Merc parts diagrams. Makes you wonder if they just bought the tooling from GM & then made some updates & refinements and there you have it!
Didn’t the production of the old cast iron 4.3 end around 2014?

i thought mercruiser just used Gm blocks. im not really up to date on who made what block. im actually fairly new to any of this marine stuff. i just learn as things start to break.

 

[Lou C]

Nope the Mercruiser 4.5 V6 is “theirs”
but look at the parts diagrams there’s an awful lot of similarities! Only the cyl heads look a bit different and of course the ignition and fuel system.

 

[kbreks]

Nope the Mercruiser 4.5 V6 is “theirs”
but look at the parts diagrams there’s an awful lot of similarities! Only the cyl heads look a bit different and of course the ignition and fuel system.

interesting. i could have sworn my block had a gm stamp on it. who knows.

 

[Scott Danforth]

interesting. i could have sworn my block had a gm stamp on it. who knows.

dont confuse your GM 4.3 with the 2015 and later mercruiser 4.5

 

[Scott Danforth]

Nope the Mercruiser 4.5 V6 is “theirs”
but look at the parts diagrams there’s an awful lot of similarities! Only the cyl heads look a bit different and of course the ignition and fuel system.

its basically a stroked 4.3. the 6.2 is a GMPP rotating assembly in a Mercruiser built SBC-ish block using SBC-ish heads. the 4.5 uses the same rods as the old 3.0 and pistons from the 6.2 in a mercruiser built block with a mercruiser only crank 101.6 (4.00") bore and 91.4 (3.60") stroke

yes the similarity is there because Mercruiser uses GM bits

 

[kbreks]

dont confuse your GM 4.3 with the 2015 and later mercruiser 4.5

definitely wont be confusing the 2 as mines a 1985 and im never looking at anything from newer than 1993. i wasnt even aware that there was a 4.5L

 

[Scott06]

But shortened from 32 to 22. Thats a huge loss of power. I cant see how any boats like mine but with the thunderbolt even function. Guess i need to find someone with a similar boat and check their timing to compare.

Have you verified the Val vet has not slipped and tdc on balancer is actually tdc ?

What do the spark plugs look like ?

 

[kbreks]

Have you verified the Val vet has not slipped and tdc on balancer is actually tdc ?

What do the spark plugs look like ?

I have verified this, yes. Didn't pull all the plugs. But they were replaced last season.

 

[Scott06]

I have verified this, yes. Didn't pull all the plugs. But they were replaced last season.

Was wondering if you are running overly rich and this need more timing. Would at least watch the plugs for signs of detonation if you are bringing in more total timing than stock. Would keep an eye on this as you work through things

I know when I bought my new base engine warranty stipulated no more than like 26 or 28 degrees total timing. this was on a 305 which seems to tolerate more timing than a 4.3 but less than a 5.7.

 

[kbreks]

Was wondering if you are running overly rich and this need more timing. Would at least watch the plugs for signs of detonation if you are bringing in more total timing than stock. Would keep an eye on this as you work through things

I know when I bought my new base engine warranty stipulated no more than like 26 or 28 degrees total timing. this was on a 305 which seems to tolerate more timing than a 4.3 but less than a 5.7.

i did just have my carb redone last year, im not sure if it was running rich or not. i would have never guessed that i was as far advanced as i am. engine sounds great and doesnt get hot. But ive also been told its very hard to hear engine knock or detonation on marine engines.

The part that gets me is that at 25-27 degrees, im still lacking major power and i struggle to plane. runs just fine once im on plane though. but up near 27 degrees with the EST, i started getting popping and backfiring when the timing would drop. not sure if it dropped because of the backfiring or if it was the EST module causing it. ill be swapping modules and doing more testing. just confused as to how i havent melted my engine yet running so much advance. keep in mind i have been running this boat for almost 5 years now with 32-36 degrees advance.

 

[Scott Danforth]

as mines a 1985

you know your motor is a one-year wonder. its a 4.3 with a 2-piece RMS