Piston burn pattern?

[RWN50]

Revengable - No stacks but have installed them in the last day, mechanical pump and the cowl isnt vented more than stock

Champ20B - Its premix and i often run a cap of metho every few tanks to dissolve the water. We do have a problem with water ingress slightly as we use it alot in the surf.

Here is the plug that came out of the stock head when the picture was taken, done about 15 hours. i did cut one open and it had a carbon ring very black at the bottom and about 0.8mm wide




Here is a pic of the plug that came from the center firing head, about 1.5hrs on it, looks a bit wet though as i washed the motor out after last run (in the salt) so its very wet from idling

[Revengable]

This one will suck water if all the pan openings are not sealed.....you can feel it in the buttdyno....your pistons look like they've been steam cleaned.Attachment 59716:

[Revengable]

This one will suck water if all the pan openings are not sealed.....you can feel it in the buttdyno....your pistons look like they've been steam cleaned.Attachment 59716:

[Powerabout]

maybe a combustion chamber moved over toward the inlet side?

[ice_spy]

I'm no expert but here's my suggestions.
30 psi extra compression, +98oct might be causing pre ignition. Have you changed the timing? Maybe 91 ron would be better, can you hear pinging up top, maybe the crank has twisted and timing is slightly out?

I would be running a little more oil than 50:1 if you are revving beyond factory limit.

Possible water leak into pistons from blown head gasket? maybe caused from a higher compression.
Is the little amount of added metho causing a hotter burn? Maybe best to run a water fuel separator if space available.

[Powerabout]

How can 91 prevent pre ignition if 98 cant?

[Krazy Karl]

I am not going to reveal my methods for fear of starting unruly discussion but a long time boat racer showed me how a spark plug reading can reveal timing settings. The timing on these last plugs show a variance from cylinder to cylinder and overall the timing is out of wack. I am not going to say my method is fool proof but it was pretty much in the ball park on my Yamato. I agree that there is something definitely wrong on the first set of plugs. It looks like water intrusion to me too. I am not too familiar with Tohatsu's, whether timing can be set for each individual cylinder or not. If not I would be real concerned with the crank being twisted which can happen on a Tohatsu. Has it been welded or not? I don't want to scare you but there are some issues that need to be investigated.
kk

[Revengable]

This little jewel was a dog when we got it. Three threads too much timing. After timing per Sid Bass it came alive. This semi-hemi head was made by Outboarder. 70 hp carbs on M50D2 Attachment 59718

[ice_spy]

That's a mean looking tohatsu !
Powerboat, sorry I meant 91 lower octane might be better suited.
I think it looks more like water but. What's the silicon(?) around the head ? Was it used with an old head gasket?

[RWN50]

Didnt get a chance to pull the head off this weekend but will hopefully do it before the end of the week and post photos.

As far as timing goes its slightly less than factory 20 degree.

motor did 130 hours with stock limiter on and was stock, i then rebuilt it and did some work and welded the crank. there is no vibrating comeing though the motor being out of balance of firing wrong.

Revengable, what rpm are you pulling from those motors, what jets and mixture are you running and what do you set your timing advance to?