Mercury 850 spark plugs

[Dhoopty]

Hello all - I am new to boating and the Iboat community, but the information here is amazing. I have been revewing threads for hours now (just hope the boss doesnt find out) and really appreciate the support of all here I have a 20' 91 Fisher pontoon with a 77 Mercury 850 on it (s/n 4722933). I was wondering what are the correct spark plugs for this engine. I want to change them out - as the engine is difficult to start. Thought this would be a good first step to problem solving. I pulled them and they do look pretty carbonized. I also noticed, these plugs dont look like a normal car spark plug as I didnt notice any side electrode on them (the part you usually bend in or out to adjust the plug gap). Is this normal for outboard plugs? The current plugs are Autolite 2852.

Any/all help is much appreciated!

 

[Chris1956]

Re: Mercury 850 spark plugs

Most Spark plug application charts will list your motor. The original plug was Champion L-76V or L-78V. These are gapless plugs. Newer Mercs changed over to NGK BUHA or BUHA2, also gapless. I never really found a big difference.

BTW - you should be able to get these plugs at a locak auto parts store. They should have the application chart as well.

 

[Dhoopty]

Re: Mercury 850 spark plugs

Thanks for the reply and info Chris1956! I will give them a look - also, do you know what the redline is for the engine and where it should be hitting at WOT? I replaced the prop last year with the exact same that came off of it (our lake here is pretty tuff on them after bad storms....debris!!). I seem to only run about 3800-4100 at WOT (again, have a 20' Fisher Pontoon -Freedom 200). I do not know the pitch of the prop...thinking it may have been 14(??). Would like to have an idea where I should be running with the engine so I can consider another prop if need be.

I also seem to be slipping once I hit 3500rpm - i read a few other posts that suggests it may be the prop hub slipping (the prop i installed was used) - so am gonna check that also. It seems to run pretty well up to that point or sometimes around 3200 it seems to want to 'break free'. Would you have any other suggestions?

One other thing - sorry..but it is a used boat right =) when i engage in reverse, the motor seems to 'hop' - again, may be the prop hub slipping(?)

Thanks again!!