mercury 800 80 hp 4 cylinder 72' missfires

[oldglastron]

Hey there,
I'm having some issues, I have invested a lot of time and money into this outboard. A little history I have replaced the internal and external harness CDI on both.
I replaced the entire throttle assembly with a new replacement Merc control throttle assembly. Carbs have been gone through, fuel pump diaphragm update done
The bleed restrictors in the side cover replaced PSI in all four cylinders at around 140 psi. before I got this engine the distributor was replaced, I have since replaced the wires repaired the pickup under the distributor cap, new plugs fresh fuel with new Moeller fuel tank and primer. Prime bulb gets hard no fuel leaks.

My issue is I can go WOT boat planes out fine but mid throttle or full throttle the engine shakes at times like its miss firing it cleans up then does it continuously I have my timing light hooked up you can see a decrease in in engine speed then back. It does not this in test tank. I have checked top dead center timing belt location per my FSM. Running out of ideas. hope you have a few. (oh I have done a static linkage check with cables off)

 

[jimmbo]

Hey there,
My issue is I can go WOT boat planes out fine but mid throttle or full throttle the engine shakes at times like its miss firing it cleans up then does it continuously...QUOTE]

So which is it? you say WOT is fine but then at Full Throttle it starts shaking. I would be looking carefully at the wires for damaged insulation, broken conductor inside the insulation. A connector loose or grounding out as the throttle/timing is advanced

 

[oldglastron]

Jimbo,
Thanks for the reply, ya its kind of hard to explain. the boat never stalls or even really changes speed the engine shakes and then is smooth then shakes then smooth. It has new wires bulk wire from mercury new screws for the cap and plug terminals. I'm not really fond of the design since when the ignition advances the entire cap and wires move I could see where issues could develop. The new CDI harness has all new grounds block grounds have been cleaned and I'm running star washers under the grounds. I was even able to find a NOS spring and plunger for the cap I've put power run continuity tests on all the wires.and cap terminals
WOT seems when it does it the worst, but if I'm at half throttle on plane and throttle up it does it to. The strange thing is when I go WOT from neutral the boat comes right out of the hole no missing until it planes out. We have a lot of these motors here in CO I've worked on lots of them and never had this many issues with one outboard. I have a test tank with a splash back so the water recirculates but it never does it in the tank. I have used inline spark testers, timing light I have a tool that checks spark output in KV ( I'm a master auto tech for a living) I am for sure it is ignition related pinpointing it seems to be elusive..
If you or anyone else has run into anything with these old 4 cylinders it would be great to hear some of your experiences

 

[merc850]

This problem doesn't occur when in tank (static test) but happens when on water, this sounds like something is shorting or disconnecting a wire; are you using the same control box for both tests? the problem could be the ignition switch or poorly fitting harness plug; is the mercury tilt switch connected? it could be shorting out. I would put a Voltmeter on the red wire to the switch while testing on lake then check the white wire beside it looking for battery voltage.
How did you fix the pickup in the trigger assembly? Is there a ground wire from the distributor body to the block?

 

[jimmbo]

Is there a change in sound when the shaking happens? Operate the throttle and observe the linkage movement and for any contact with any wires. Merc850 might be on to something about the control box.