pcm 351w backfires out carb

Wow this is a long story ,Boat is a ski centurion tru trac 1982 pwc 351w , i replaced the raw water pump but did it incorectly and ran my boat for 2 min at 3 grand from the dock and the rubber exhaust started blowing smoke out the back and the temp guage creeped too only about 210 , so i shut the boat off and realized my mistake and fixed it , after a hour i tried too re fire but no spark.....* I noticed the key was left on and must have killed the coil or igniter kit in my distributor, so next morning new coil , no spark , so the igniter unit must have died , but no luck on back order..... so i went and bought a msd pro billit unit and installed it ..... boat idles wicked and sounds strond , set timing too 8 deg initial at 800 , removed vac advance and locked mechanical out at 25 deg total for a total of 33 deg ..... used the heavy springs like the stock one used... on the water it idels wicked , anything over idle it surges like 350hp 150hp 350hp 150hp and then over 2000-2500 pop pop pop out the carb ???*So i pulled the heads and figured head gaskets , maybe cause there was no water in block motor got hotter than i thought , looked fine..... planed heads and re assembled motor new head gaskets intake ect fires up runs great so went too lake and less backfire maybe only 3-4 but over 3000 rpm and surges the same like 200hp 350hp 200 hp 350 hp .....*i am very very mechanically inclined and checked the timing chain play cause i figured maybe it jumped a tooth but only 1/8 inch of play on the ballencer so thats great , re checked firing order 10 times cause dam it seems like 2 wires re crossed but no , when i did the initial timing i had a good backfire so i thought it might be the carb , maybe power valve or low fuel pressure ????? I lost all my tools in a fire so im limited on what i can do , should i buy a compression tester? If i cant figure this out im having a boat fire
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also i put gt-40 heads on it because when i bought the boat 2 years ago it had a bent valve and i didnt lake test the boat as it was winter

fuel pump seems too have good pres and fills a coke can in 15-20 sec cranking with plugs in , maybe i can find a vac guage cheep

opened up the carb , 4160 holley marine , look's clean and power valve is good so im soaking it over night and will blow it out in the morning and re assemble , even changed the advance springs to light ect but it runs worse , even changed the lockout too 18 total mech advance but no diff , i put my friends timing gun on the motor and it doesn't jump around and i cant see a little heat messing the stainless steel valves up , im going to vac test and report back

msd recomends 16-18 deg initial and lighter springs ???

First, if your distributor has a vacuum advance on it, it isn't marine and you have introduced a safety and liability issue.

I'd start with the PCM default timing curve unless you understand what too much timing can do to a marine engine.

With the surge and the backfire, a lean condition is quite likely......I'd start with the vacuum gauge. If that shows 'normal' readings, I'd be inclined to go to the compression test and then a leakdown test, if cause is still undetermined.

Most 'big box' auto part stores will loan (or rent) a vac and compression gauge...never hurts to ask.

Did you check the valves' sealing after the heads were planed?

planed heads cause i thought i might have blown head gaskets , heads ran fine before and were new 6 months ago

I cant get any info on the timing curve for a pcm 351w just initial

distributor came with a vac advance delete wich i used , i dont need i liability lecture

compression test shows 155-160 across the board

no liability lecture was intended....just wanted to make sure you were not amongst the clueless that become statistics. I don't have any old PCM factory literature available. Do have access to some old Merc manuals.....will see what is in them for a curve. Their last Ford based product was mid-to-late 70's so it should be pretty close and will be a starting point.

I'd think the MSD settings were more auto based than marine based. marine engines, compared to auto engines, have a significant load, nearly 100% of their in gear time. I'd go with strong springs to slow the advance, not the lighter ones.

I take it the compression numbers are very recent; any chance of vacuum readings soon?

I just rx'd email....I'll have access to the old merc manual tomorrow early am.

Here's the info from the merc manual on how they had the distributors setup for the sb fords, thru the late 70's. They are dual sloped advance curves, same for 302 and 351W. They have a min and max value. the chart covers only the advance provided by the distributor; ie add whatever initial timing to get TAT.

MAX border on the curve: starts with 0 deg at 1000 up to 20 deg at 1600, then to 26 deg at 4000.
MIN border on the curve: starts with 0 deg at 1200 up to 17deg at 1900, then to 22 deg at 4000.

These two borders form the acceptable timing advance values; ie anywhere on or between the borders is an acceptable value.

Super Super cool , thats the most helpfull info i have got yet !!! I will take my gun and match that curve or close as i can and see what happens .. If it still happens i will assume a vacuum leak and lean condition , Im trying to buy tools as i lost every tool i had in a fire ..I will post results

makomark said:

no liability lecture was intended....just wanted to make sure you were not amongst the clueless that become

Click to expand...

I took one of the little screens out of the original pretolight dissy and glued it into the vent cap of the msd , besides that its cnc and sealed tight so i figure it should be ok , msd doesn't make a marine dissy for the 351w just 460/351c

Well it turns out i had a cracked spark plug!!!! WTF after all that , im sure the springs were part of it but with a constant miss it never had a chance