1978 70hp 3cyl - spark plug and timing questions

[agrazela]

I have had a 1978 70hp 3-cylinder Mercury outboard for almost a year now. In that time, I have:<br />-Replaced all fuel lines<br />-Replaced fuel pump diaphragm<br />-Rebuilt carbs<br />-Decarbed<br />-Replaced plugs (Champion L76V surface gap)<br />-Performed all tests from SELOC manual (compression, spark tests, stator, trigger, coils, etc.--all passed)<br /><br />I use regular 87 octane fuel (usually Chevron or Exxon), and Pennzoil Premium Plus full synthetic at 50:1. My normal usage pattern on this engine is a bit of idling, a bit of low-speed trolling, and alot of 5-minute speed bursts between fishing spots...I rarely go WOT for any length of time. I notice that my engine never gets warm enough to even register on the temperature gauge (I have tested the sender, wiring and gauge with a pot of warm water and a thermometer and they do work).<br /><br />My problem is that the motor can bog down a bit on acceleration, and tends to wet foul the plugs. The longer I am out, the more pronounced these problems become, as well as becoming increasingly more difficult to re-start the engine.<br /><br />I have found the idle mixture adjustments that minimize these problems, but they persist. I feel certain that this is not a fuel delivery or electrical problem, but rather an incomplete ignition/fuel burning issue.<br /><br />Now for my questions:<br /><br />I have seen threads on this board suggesting that surface gap plugs are best for extended WOT running, while standard, "gappable" plugs are best for my usage pattern...presumably due to the ability to better fully burn fuel/reduce fouling. Is my understanding correct? If so, what gappable plug (and gap setting) is recommended as a replacement for the L76V?<br /><br />I have also seen conflicting information regarding setting the timing on this engine...given my usage, might it help if I tried to advance or retard (and by how much), or should I just leave it at the setting stated in the manual?

 

[agrazela]

Re: 1978 70hp 3cyl - spark plug and timing questions

^^bump^^

 

[petryshyn]

Re: 1978 70hp 3cyl - spark plug and timing questions

Try running without the engine cover on. If the symtoms improve, its possible the motor mounts have sagged allowing exhaust into the engine compartment. That engine has no thermostat, so fowling can occur on multiple short runs. Try a L77V (one heat range higher)<br />Because of the fowling tendency, I've run 100:1 mix on that engine with Opti II oil for the last three years. The engine has never run better.<br /><br />The ignition pack gave trouble on these engines as well. They often fail intermitently and missfire under load....<br /><br />Good luck!

 

[Clams Canino]

Re: 1978 70hp 3cyl - spark plug and timing questions

ONLY use a surface gap plug in there. <br /><br />Use the boat ramp to set the idle needles.<br /><br />Assuming all is as perfect as you say... this one could be tired reeds or reedblocks.<br /><br />I don't understand the exhaust part? The powerhead bolts to the leg. If the mounts were to sag, the motor and the exhaust would move down a bit - together.<br /><br />-W

 

[petryshyn]

Re: 1978 70hp 3cyl - spark plug and timing questions

I don't understand the exhaust part? The powerhead bolts to the leg. If the mounts were to sag, the motor and the exhaust would move down a bit - together.<br />

The mounts in question reside inside the driveshaft housing on the topside. The bolts and brackets pass through the housing. The bracket maintains a seal to the housing. If the mount sags, the bellows type seal cannot take up much deflection....exhaust gases can collect under the engine cowling resulting in poor idle quality after a run.<br /><br />This old style mount and seal assmy. was substituted for a different design which you may be confused with...I can't remember what year it switched.