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If you get 9.5 and run 91 with factory ignition advance you will be fine from a detonation standpoint. I would think the flat top is better than the factory dished piston for detonation potential .

yeah these guys are probably too young to know how a points system bypasses the resistor during cranking

Depends on gas you want to use , octane Piston profile may play a role if you have a quench build. But generally 9.5 or so if you want to use pump gas. A touch higher if you have quench pistons.

Ive never had to deal with transom rot but general consensus is you can only really tell by drilling , if wood shavings come out dry just seal with 5200. transom hardware being tight is a good sign.

No difference on choke coil between a 2 or 4 bbl carb. I put a edelbrock 1409 on my previously 2 bbl carbed 5.0 merc, used the factory purple wire it works fine

My merc takes the purple to the choke, but In the drawing you posted the purple yellow is fed via the oil pressure switch as well As getting power off starter when cranking

As lou said water doesn’t compress so when you get water in a cylinder vs air and fuel, when the piston comes up on compression stroke there may not be enough room in the combustion chamber to accommodate The water volume. The compression ratio of 9 or so to one if you have a cylinder half full...

Mercruiser runs the electric choke off the purple wire going to ignition on. they say to do this so as not to lower voltage off the ignition circuit. On my old GTO i ran the electric choke off the wiper circuit upstream of the wiper switch. Easier in a car where the fuse box is. 4 ft from the...

normally when somebody runs a bead of sealant around the transom housing it is leaking either from a drive impact or rotten transom. Did you drill into the wood of the transom to verify they are sound? Are u sure that is gel coat and not a repaint of the boat? If the transom is indeed sound...

Start with a compression test. how long was it hot and how hot did it get. May have smoked something electrical. If you need to check for bent pushrods just remove the valve cover, but if it got hot a vlave is probably hung up and why the pushrod is bent. the DDIS all dogde speak of is obvious...

Not air but you will get water in the gear lube turning it milky and potentially ruining bearings and gears. What did you lube look like when you drained it ? Assuming you likely just winterized? If water gets left in ther and freezes can crack the housing

Even if the connectors are not the same you can make your own from -to cable if you go on line you can find these marine and automotive connectors and make up what you need

Yes that is a common leak point unless u know when it was resealed I would recommend doing a pressure test and checking all the seals maybe just have the entire drive resealed

Is that two speedi sleeves? one for each seal area I assume ?

Your should be min 4600-5000, i think so e of the Merc mpis go to 5200 on the same block. bottom line prop it such that you get 4900-5100 light load and the performance will be good. i would verify your tach is accurate and in good working order. what is yiur speed at 4100 rpm

just a note to OP - SAE J1171 will be on all electrical components ( maybe fuel components too) that are marine rated. Vs automotive the alternators fuel pumps distributors etc will have design features to prevent spark and fuel vapor explosions and fire . it not the part number for the component

looks like a light maybe a light for a storage compartment

That doesn’t sound like a marine fuel pump , did you get an automotive one

I have no first hand experience with the 470, but generally the speedy sleeve seems to work for folks . If you are having these repeated failures there is another issue thinking maybe misaligned timing cover or bent cam snout, wrong speedy sleeve , issue with water pump to cam alignment. Yes...

you are taking about sea water pumps not the block circulation pump tgat the op speaks of. Your 454 will have a circulation pump mounted to the front of the block over the timing cover. This circulates water through block and heads, and is feed by the crank mounted seawater or raw water pump.