old merc 20hp - merc200: fouling plugs

[jbmerc200]

Hi,

Im having problems with an old mercury 20hp merc200. I think its somewhere in the 67-72 range and I'm looking for information on this motor.

The motor starts easy and runs smooth but it is a little choppy at a low idle. On new plugs it will run a decent top end and sounds alright, but after about 10 or 15 minutes of high end and speed and throttling down, the motor will not return to full throttle. It seems to bog down at about 1/4 throttle and will not return. If I pull the plugs and check them they are usually black and filled with sludge.

What could be causing this? My local merc dealer had me put in gapless plugs and I think that's when I started having problems. Can someone confirm for me which plugs I need to put in this motor, or point me in the direction on what might be going wrong?

Thanks

 

[emckelvy]

Re: old merc 20hp - merc200: fouling plugs

If this is a motor with electronic ign, it was built to run with Mercury-designed Permagap plugs. However, if old enough it could have a magneto-style ign which doesn't have enough spark energy to run 'gapless' plugs.

'69 and earlier 20 hp motors have "Type 1" ign which is points/magneto. Points are gapped to .020" and recommended spark plug is a Champion L77JC4.

'70 to '72 use a "Type 7" electronic ign which still uses points, gapped to .020" also. Use a Champ L87V Permagap plug.

You can easily tell the diff between Type 1 and Type 7 ign 'cause Type 1 ign coils are under the flywheel. Type 7 ign coils are mounted externally, at the upper rear of the powerhead.

Since you may be having ign problems, either ign system would benefit from a points cleaning & re-gapping. You'll need a flywheel puller and some Grade 8 bolts to remove the flywheel. Torque the flywheel nut to 35 ft-lbs.

Your problem could also be caused by weak spark in general, besides points gaps (most common), other issues such as weak coils or weak electronic ign. But most of the time the electronic igns fail completely rather than just go low-voltage.

Do a spark test, crank over the motor with both plugs out, inserted into the plug wires & grounded to the block. You should get a sharp, purplish/bluish spark. If it's orange or reddish there's a problem. Another common problem area on Type 7 elex igns is the insulator block between points conn and external wiring, thru the 'breaker box'. This plastic gets old & breaks down. Then the hi-voltage leaks thru the plastic to ground and you get weak or no spark. Best to replace both points' insulators while you're in there.

If you find no other reason other than spark plugs for the fouling issue, you can always slap a set of L77JC4's or NGK equivalent in there. Good for prolonged idling periods although typically a Permagap plug has a longer service life (if not fouling out for other reasons).

A decarbonizing treatment might be in order and be sure you're using quality TCW-3 outboard oil mixed at a ratio of 50:1.

http://forums.iboats.com/showthread.php?t=158076&highlight=seafoam

HTH..............ed

 

[jbmerc200]

Re: old merc 20hp - merc200: fouling plugs

Well I'm not sure I know what the coils are just by looking, so I'm still not quite sure what I have yet. I went oreillys and asked for the champion l77jc4's and the champion l87v plugs. They only had the l77jc4's in the ngk equivalent, which are ngk b7hs 10. I am leery to put them in and try b/c they are probably 1/8" taller than the ngk buhw-2's that are currently in, given the difference in length.

I'm hoping that by taking a few more pictures of the motor and maybe you can help me narrow down the right plug for this thing...

Thanks for your help

 

[emckelvy]

Re: old merc 20hp - merc200: fouling plugs

Your motor as pictured doesn't have elex ign. You have the old mag-style ign, points-fired with coils under the flywheel. No wonder it runs bad on Permagap plugs! The old-style ign doesn't have enough spark energy to fire this kind of plug consistently.

Use the electrode-style plug, that's what the motor was designed for. It's not too long, it's supposed to be that way.

If you still have any kind of spark-related issues running a std-gap plug, it's probably time to pull the flywheel and clean/gap the points.

HTH.......ed