Are these REALLY the right plugs to use?

[chuck in WA]

I'm replacing the spark plugs on my 1985 Mercruiser 140. My SELOC manual says to use one of the following:<br /><br />ACDelco MR43T (what the old ones are)<br />Champion RBL8<br />Autolite 144<br /><br />When I went to the marine store today to get them, the guy sold me some "better" ones to use - Champion RV15YC4. Anyone know anything about them? By comparison with the old, the new will extend about 1/8 to 3/16" farther into the cylinder than the old. I hesitate to install them before hearing your expert opinions. Thanks in advance!<br /><br />PS - My engine diesels a bit if I shut it off right after running at high speed. A short cool down period avoids the problem, which I hear is a common one with the 140. I figured a slightly colder plug would help the situation, but when I asked the question at the store today, I was told that a high idle is the only thing that causes dieseling. I'm not sure I agree, but then I'm just a shadetree mechanic. Thoughts?

 

[achris]

Re: Are these REALLY the right plugs to use?

Chuck,<br /><br />My opinion? Float test the Champions and put in either the AC Delco MR43T or NGK BR6FS.<br /><br />Higher idle? to stop run-on? Don't think so! Slower idle, maybe. I have a 4.3LX that does the same thing. Only way to stop it is the cool down for 30 seconds.<br /><br />Chris.....

 

[rabidfish]

Re: Are these REALLY the right plugs to use?

The RV15YC4 is correct for some applications. The NGK BR6FS is a better choice. Your dieseling can indeed be caused by other factors. High idle is one. High heat is another. Before you change the plugs, do a "decarb". Removing any excessive carbon from the combustion chamber may relieve the problem. Then change the plugs and give it a run. <br /><br />How is the engine temp running?

 

[rodbolt]

Re: Are these REALLY the right plugs to use?

look on the engine decal. it will tell you the correct plug. the RV8 and the rv15yc4 were both used as well as a few others here and there. try to use the ones the valve cover decal states.

 

[marinemech1]

Re: Are these REALLY the right plugs to use?

i didn't think merc recommended rv15yc4 plugs we use rv9yc in most mercs but be careful with the 3.0 litre 140 there were several changes to spark plugs that were recommended. i would check with another dealer to be sure you put in the correct plug for your motor<br />if all else fails install the same number plug that you removed until you are sure which plug is correct.

 

[trog100]

Re: Are these REALLY the right plugs to use?

they are extended nose plugs.. they are suppposed to extend the heat range of the plug.. the theory goes thus... cos the electrode pokes further into the cylinder its in a better position to start the fire.. at idle the electrode runs hotter (better it dosnt soot up and stays cleaner} cos its more in the middle of the flames.. at hight revs the electrode runs cooler (better cos it dosnt burn away) cos its more in the airflow from cool incoming mixture..<br /><br />in theory u have the best of both worlds.. they are almost universally accepted as "better" than the standard nose plug.. <br /><br />i have them in my 3.0L engine.. i think they are "better".. assuming there is piston and valve clearance i cant see any logical reason for not useing them..<br /><br />trog100<br /><br />ps.. i aint saying champions are "better" just extended nose plugs which come in all makes..<br /><br />ps..2.. extended nose plugs should not be confused with long reach plugs.. reach is the depth of the thread.. again this comes in long and standard reach.. and both standard and long reach plugs come in standard and extended nose versions..

 

[chuck in WA]

Re: Are these REALLY the right plugs to use?

Thanks for the input guys - I called a few other local shops as well, and no one seemed entirely sure if the RV15YC4's would work properly or not. I ended up taking them back and got some of the NGKs. Maybe I'll experiment with others next season, as I'm pretty busy until mid-summer. For now though.....peace of mind.....priceless.

 

[John Carpenter]

Re: Are these REALLY the right plugs to use?

I had the same concerns a few years ago when my mechanic made the same recommendation regarding plugs for the 305 in my boat. In that engine at least, they did seem to make an improvement. Pretty sure it was the same plug.