MercuryMarc wrote: Most of the Mark series and my Merc 400 require 6's "service Manual". Last spring I ran 8's and I would loose power near WOT. Ray and Merky suggest I try 6's. and my problem went away.

Good info thanks Marc! I have a new set of 6's in now as they were already in stock. An 8 would colder if remember correctly NGK's heat range is opposite. Lower number = hotter. In any case I'm thinking a combination of advance and fouled plugs is the issue. A test prop would be awesome but I think finding one is probably out the window.

Most of the Mark series and my Merc 400 require 6's "service Manual". Last spring I ran 8's and I would loose power near WOT. Ray and Merky suggest I try 6's. and my problem went away.

Locomotion wrote: Are you sure about the heat range on the plugs, those look pretty cold.
Myself, I would stay with 25:1 for a bit longer and recheck the timing again before going back out.

They're NGK B6S and yes I could probably go a bit hotter.

Are you sure about the heat range on the plugs, those look pretty cold.
Myself, I would stay with 25:1 for a bit longer and recheck the timing again before going back out.

2 hours run time at half throttle or less she ran pretty good. Once we got that under our belt I eased up to about 3/4 throttle and holy hell she stumbled all over the place. Would not get past 3/4. Even had the wife driving the boat while I played riding mechanic to tweak high speed mixture to no avail. Got home and pulled the plugs which were fouled at best. (not really a big surprise) I'm going to check timing again along with a fresh set of NGK's and fresh 40:1 mix.

RickG wrote: Moe, are you a member over at J.O.M.S.? There's quite a bit of discussion on the topic over there. It's a pay site, but well worth the annual dues.

I am not, but I did find the service manual break-in, basically run a rich mixture like 32 or 25:1 for the first two tank fulls or two hours and keep it at half throttle, then no extended full throttle for the next 8 hours of run time.

Moe, are you a member over at J.O.M.S.? There's quite a bit of discussion on the topic over there. It's a pay site, but well worth the annual dues.

The service manual talks about a break-in period too.

Well, she fired right up no problem! I didn't run her real long in the test tank as I'm curious if there's any "typical" break in procedure. IE don't run wide open for X amount of time etc. I'm just going off knowledge of automotive engine break in with the understanding this is a completely different animal. Any thoughts from the more knowledgeable people out there ? A quick search of the google gave me the following:

https://maxrules.com/fixbreakin.html ]

Thanks, we'll find out in a couple months !