sparkplugs on a merc 500.

[rolmops]

Hello Guys.<br />The merc I was given is #2160244 which makes it a 1967.The plugs I found in there were champion L78V and my manual says it should be L4J.Is L78V a factory adviced replacement or is it a mistake by the previous owner?<br />Thanks in advance,Rolmops

 

[emckelvy]

Re: sparkplugs on a merc 500.

Depends on whether you have the standard ign model or the "SS" model. The magneto-ignition model would take conventional plugs, while the electronic-ignition "SS" model takes surface gaps. However, L78V is too hot of a heat range for a 50HP and instead you should be using L76V's or equivalent.<br /><br />BTW you can use conventional plugs in a CDI model, but don't use surface-gap plugs with magneto ign as the mag doesn't develop enough voltage to fire them properly.<br /><br />You can tell if yours is a magneto if it's stand-alone with no other coil or electronic components connected. The electronic ign model will have a separately-mounted ign coil, and a switchbox (power pack or CDI unit) mounted on the front cowling support.<br /><br />BTW you'll probably get more response posting your Q's over in the Mercury forum!<br /><br />HTH...........ed

 

[rolmops]

Re: sparkplugs on a merc 500.

Thanks for your reply.<br />I just saw ,that I posted in the wrong forum.My apologies for that JB.

 

[JB]

Re: sparkplugs on a merc 500.

To Merc Troubles

 

[Chris1956]

Re: sparkplugs on a merc 500.

Ed Mc. - I saw your reply to Rolmops. I have a 1977 Merc 1500 which uses surface gap plugs. I would like to use conventional gap plugs, but have been unable to find a cross reference. Do you have a suggestion of a conventional plug to replace the NGK BUHW I am using now?

 

[emckelvy]

Re: sparkplugs on a merc 500.

I wonder if standard-gap plugs would run too hot in a 1500.<br /><br />It's funny, I was looking in my Selog Merc book and in earlier 6's they used L78's or L77's, then about 1975 they went to the colder L76's. I've always used L76's in the inlines that would take them (i.e., long-reach and CDI ign) and never had any detonation problems.<br /><br />Maybe they reconsidered their choice of plugs, such as the way they backed off their timing recommendations to 21 deg, back when the quality of gasoline was going down.<br /><br />I'd definitely be running a cooler plug in a 1500, they're the highest-stressed of all the production inlines. <br /><br />The older non-electronic ign inlines took a Champ L4J plug, I suppose you could use this and feel safe about a rough equivalent. I have a feeling that the L4J may have been superceded, if so it may be equivalent to the L77JC4 or QL77JC4. The "Q" model is spec'd for OMC CDI ignitions due to its special internal resistor. I don't know if Merc CDI's even care about that stuff.<br /><br />But you should check the spark plug conversion charts to be sure it's the correct heat range. You can also check for proper tuning by making a wide-open-throttle run then turning off the ign. and reading the plugs. Better lash the motor down if you do this because it'll tilt up like mad!!!<br /><br /><br />Are you having problems with surface gap plugs? Usually the only thing they don't like is prolonged periods of idling, and I wouldn't think you'd be using your 1500 for a trolling motor!<br /><br />And in looking further on the 50 HP, the strange thing is, some later years use L78's while others use L76's?? Go figure!<br /><br />Here's an Champion/NGK crossover chart I found:<br /><br /> http://www.spark-plugs.co.uk/pages/technical/champion860-n288.htm <br /><br />And more sites which show the L19V crossing over to BUH. I guess a little more research would show the diff between a BUH and BUHW. I suspect not much!<br /><br /> http://bayshorecad.com/Sleds/id80.htm <br /><br /> http://www.enduro-challenge.com/zuendkerzen-champion-ngk.html <br /><br /> http://home.online.no/~stigbye/snowmobile/teknisk_informasjon/NGK/kryssreferanse/Champion.htm <br /><br /><br />You'll also note that both L4J and L77J appear to cross over to B7HS. Hmmmmm.............

 

[Chris1956]

Re: sparkplugs on a merc 500.

Ed Mc. Thanks for the response. I find that the surface gap plugs tend to foul easily. I do not know why, since they were developed to be fouling-resistant. <br /><br />Specifically, when I was recently breaking in my rebuilt 1500 powerhead, I had all kinds of fouling trouble while using the 24::1 break in fuel mixture. Fouling was reduced when I "Graduated" to 32::1 fuel mixture and got better still when I switched to 50::1.<br /><br />I still think I have intermittent fouling, but since I have a light boat, it is hard to tell, unless you are running WOT, and can check the tach.<br /><br />Ironically, my 1959 Johnson OB never fouled a plug, eventhough it ran 24::1 and had points/condenser ignition. It used Champion J4J conventional plugs. I would like to try some conventional plugs in the Merc 1500, and see if it makes a difference. Of course, I do not want to damage the motor in the process!

 

[phatmanmike]

Re: sparkplugs on a merc 500.

www.sparkplugs.com