70 Hp no power/thrust. help needed

Hi all, am new to this forum and need help here.
I own a super cat with 2 x 70hp Yamaha T&T 2006 model engines, the one motor has had this problem for months now.
When I run the engine on the trailer with the water earmuffs on it seems to run 100%, but when I put it in the ocean and try to accelorate there is no power and it will not pick up, it runs but thwere is no drive and it then starts missing and cuting out.
I have changed the plugs, serviced the carbs, serviced the fuel pump, checked the advance/retard lever, swapped the coils and nothing helps, so, before I give it the Smith and Wesson final service can anyone offer me some advice?

[boats.net]

Could be a fuel or ignition problem, will need to troubleshoot when it is cutting out to find problem. try using a timing light to check fire on each cylinder.

I found the problem, strange one indeed, the solution was to replace the plugs with a warmer plug, from an HB8 to an HB7 10mm gap, it now runs like a dream, I found that one motor was puting out a slightly lower voltage than the one that is running ok, this in turn managed to "blow" the new plugs each time I replaced them, by fitting the No 7's the problem has been sorted out.

I hope this can help other forum users in the future

I am having the same problem, but after fooling around with it while in the water my 70 hp two stroke (2006 model) seems to catch and run fine for a while then go back to the coughing and choking and won't get past 1500 RPMs. it almost seems like bad gas...the dealer said that it might even be the Ethanol in the gas here in the states, but I don't think so. Thing is is that the motor has about 30 hours total on it...it shouldn't be acting up at this point.

So, you just changed out the spark plugs and everything is fine...? That would be awesome if it works for me.

Worked for me, try and let me know.

Cheers MS

I'm having a similar problem with my 07 70hp. I'm curious did the plugs help? I have recently replaced my plugs but to the original HB8. I'm running circles and have replaced everyhitng I can think of trying to fix a problem no one can diagnose.

[renagade4x4]

i have an 1990 85hp that is wanting to fall on its face at full throtle any ideas would help