Mercury spark plugs

[Family Tradition]

I have an 1983 Mercury 70elpt. I do alot or idling ( running trotline crabbing or trolling for stripped bass) Have times when motor idles rough and was told by a mechanic to change the surface gap plugs for a regular j plug. But was told by another mechanic to only run surface gap plugs in this motor because of piston clearance. Has anybody had this motor and used regular j plugs?

 

[jimmbo]

The Plug Manufacturers would have 2 different Plug numbers if there was a Recommended Conventional Plug.
It sounds like your engine is loading up and it is getting a dose of too much fuel on occasion. It happens in carbed 2 strokes all the time when they are Idled for any length of Time. If there is a Conventional type of Plug it might not make much difference as they too can misfire when the engine loads up. In the late 60s when the Surface Gap Plugs and CD Ignitions were introduced, they were supposed to make misfires obsolete regardless of the cause. They are a very cold Plug, but were supposed to be resistant to misfire from fouling because of the Ultra Quick High Voltage Spark, which was supposed to be Invincible.
Myself, I like the Surface Gap Plugs, the ones that came in my 1984 115, were still in it 14 yrs later when I traded it in, they had about 800 hrs on them, and inline sixes load up too when Idling for extended periods, but I was not one to run at below planing speeds for very long.
Your engine's Design of 2 carbs feeding 3 cylinders doesn't help Matters either. when it comes to optimum fuel delivery.

 

[racerone]

Is the thermostat installed on this motor ?

 

[Family Tradition]

To be honest that's something I've never checked.

 

[Alumarine]

You might try synthetic oil for a tank or two.
Some people say it burns cleaner.

 

[Texasmark]

I have an 1983 Mercury 70elpt. I do alot or idling ( running trotline crabbing or trolling for stripped bass) Have times when motor idles rough and was told by a mechanic to change the surface gap plugs for a regular j plug. But was told by another mechanic to only run surface gap plugs in this motor because of piston clearance. Has anybody had this motor and used regular j plugs?

The service manual for my 90 triple looper had 2 NGK plugs listed; one surface the other J. J would make sense for trolling/idling since the insulator distance to ground is longer.

Several things you can do to help:

Run Merc fuel conditioner or Sea Foam in your fuel both in sporting goods of WW.
Do the J plug and see how it works
While at WW getting your Merc fuel conditioner, grab a quart or two of Pennzoil Marine TC-W3 Premium. It's synthetic blend and you have to try hard to get it to smoke....look hard too.
If you don't have trim and tilt (PTT) set your tilt pin in a higher position, forcing the power head to tilt forward somewhat. Merc has proven that doing that helps to prevent fuel products from pooling at the rear where the plugs are located.....Back in the 60's era their low power tiller engines all were slanted forward (like having your tilt pin in a higher hole) which helped with that and helped the lower unit slip over logs.

 

[jimmbo]

A looper 90 has a much different Piston to Head Clearance than a Direct Charge Engine. I haven't been able to find any conventional Plugs for the 83 70hp

 

[Family Tradition]

Thanks for all the help . I tried the j plugs recommended by mechanic. I turned the motor over by hand and it seemed ok but went I started it sounds like something hitting at least different so I pulled plugs back out checked Pistons and they look fine. Going to try using plug non foulers to give clearance needed just thinking about drilling the small hole in non foulers just to have full spark exposure. Ill update after I'm done to let you know if it works. But going to do other things mentioned as well.

 

[jimmbo]

Non Foulers?

 

[Texasmark]

A looper 90 has a much different Piston to Head Clearance than a Direct Charge Engine. I haven't been able to find any conventional Plugs for the 83 70hp

Yes. I had the '88 Direct charged 115 Tower and ran SGs. Never had a need to run anything else so I never checked the clearance for a J plug. That was a sweet machine....loved it! The DC piston has the Cross Flow crown and 3 rings, unlike the Looper 90 which has the flat topped piston and 2 wedge shaped rings.

 

[jimmbo]

The DC while a crossflow, uses a Piston Deflector unlike any other crossflow piston. I found the Surface Gaps to be fine plugs.
Our 1980 35 hp Evinrude came with Surface Gaps, but buried deep in the owners manual there was a conventional plug listed for motors that were used primarily at low speed for extended times

 

[Chris1956]

Gee, is the idle mixture screw set correctly? Is the fuel mix fresh? Both make a real difference in idle quality.

If it were me, I would ix up some fresh 50::1 and set the idle mixture screws precisely, using that fresh fuel. I would then plan my fuel usage, so I am idling on fresh 50::1.

Maybe buy a 6 Gal tank just for idling? Fill it just before you go...