Misfire / bogging down 90,s mercury

[James3402]

Have a 90's mercury 90 hp and when you open up the throttle I have a miss fire in the mid range when I am in the boat by myself I am getting 28 knots on a aluminium prop on a 5.4 searider.
but when in idle it stalls on and off.
Any ideas please I have put power tune through her today. Was going to look at timing next is that the right way to look at....
Thanks

[neil.mccrirrick]

Could be many things. I'd start with changing spark plugs first for mid range problem. Tick over could be a blocked jet or just need adjusting more detail needed to diagnose


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[James3402]

Plugs changed but rev counter is u/s so need a new one too :-( what's the best way to find out which cylinder lead off one my one and see which runs and what doesn't ? Was going to buy a spark checker is it worth the money.

[paddlers]

Just had similar symptoms on my Mariner V6. New plugs etc no change, turned out to be a magnet adrift in the flywheel. Could be a lead, I bought new last year. Try removing the leads one at a time with the engine running with some plastic/well insulated pliers and see if it changes anything.

Good luck with it

[cptsideways]

Sounds like carbs, but also had similar issues where the plug leads get knocked off when you put the cover back on!

[Turbodiesel]

Take the carbs off and get them cleaned.

[uncle al]

and when the carbs have been cleaned, set up the OBM as per the service manuall, and I have a team tip for you, when you are trimming the obm in ,only trim it in so that it is straight up and down. If you tuck it under the carbs go into an over rich running which could cause problems.I always used to set the carbs idle mixture on all three carbs at one and a half out from lightly seated.If that5 dont work, have you got the correct spark plugs?
Have you got good compressions/
Have you swopped over the spark plugs and did the problem move with the swopping of the spark plugd if so a dodgy spark plug

[James3402]

Okay so been busy but got to the boat today to see what carbs I had to order a seal kit ready for the stripping and rebuild as per everyone's helps but what model are they or can I just not see it before my eyes lol.

[James3402]

Now the pictures lol

[uncle al]

just give them the serial num ber of the OBM they can sort it from that,