Best way to get an accurate plug reading?

[imq707s]

I’ve always heard that the best way to get a reading on your spark plugs is to do a WOT run, and then chop the throttle. That way if you are running lean on the top end, you don’t darken your plugs by getting back down into the midrange and idle jetting. I’ve tried this before, but I’ve never been able to just “chop” the throttle…….doesn’t seem that smart when you are running 80+ and the boat is aired out. I’ve always backed down to around 50 or so and then killed the motor….the boat doesn’t do anything weird that way. Will slowing down a little bit and backing off the gas for a few seconds dramatically change your plug color? I had my lower unit shell on me once going 80+mph, and that wasn’t a fun experience….I don’t really want to go through the same thing on purpose by cutting the throttle when I’m flying the boat along.

How do you guys do it?

[Eagle One]

Best way is to start with a plug you can read. The surface gap models are not very good at all so you need a gap type plug OF THE SAME HEAT RANGE. NGK-BR9HS10 works well on V-6 Mercs. Gap them at .035 and install the plugs, warm up the engine and make a short run up to around 5000-6000 wide open and cut the engine OFF UNDER POWER, pull the boat to the ramp. pull the plugs and take a look. If they look brand new or are very light tan you're to lean to begin with so you need to go richer before you test again. If not put the plugs back in and make another run til the engine peaks and stops pulling, no need to run further. CUT THE ENGINE OFF WHILE UNDER POWER. PULL THE PLUGS AGAIN AND READ THEM. WITH A GAP TYPE PLUG... you should see a distinct heat line on the ground strap and the porcelin should be a chocolate brown or slightly lighter. The heat line on the ground strap should go toward the bend about 1/8 inch. if it goes to the curve you're still a little lean. This is more reliable than a pyrometer to get a real idea in most cases to tell you what's going on in the cylinder under power. YOU MUST CUT THE ENGINE OFF UNDER POWER AND NOT START IT AGAIN UNTIL YOU READ THE PLUGS!!!

[imq707s]

Thanks for the info!

Still makes me a little nervous to run at 6000rpms (75+ or so), and then just kill the motor. Maybe I can trim the motor in to keep the speed down a little?? ...or run a smaller pitch prop?

I'm running NGK BR9HS plugs if I remember right. They are a gap style (normal) they are not a surface gap type plug.

[bakerfreak]

Do you know anyone who has a dyno?

Also you could use a test prop, and run the rpms up right at the dock. They make one for Yamaha V6, which should fit a merc. I can post the part number later.

[WATERWINGS]

is 6000 good enough, or if the motor will turn 7000 or more, should I go that high?

[imq707s]

Do you know anyone who has a dyno?

Also you could use a test prop, and run the rpms up right at the dock. They make one for Yamaha V6, which should fit a merc. I can post the part number later.

Nope..I don't know anyone with a dyno. Motor is an OMC 3.0 Looper.

[bakerfreak]

I have a chart I got from ebay that lists test prop p/n 436080 for that motor. I can't vouch for its accuracy. Maybe one of the OMC guys can chime in.

[perfmarine1]

Here's a merc test wheel made to merc spec's a cut235764 down allum. 13p prop. will work on any motor, I don't think pitch matters. I had a prop shop make me this one cheap from a beat up old prop.



235763

[Mr. Demeanor]

Here's a merc test wheel made to merc spec's a cut235764 down allum. 13p prop. will work on any motor, I don't think pitch matters. I had a prop shop make me this one cheap from a beat up old prop.



235763

But load effects how the engine runs and will effect how the plug reads.

[perfmarine1]

Dont they tune motors on a dyno? I prefer to do it in the real world to on the water.

[racer]

Needs to be done at the rpm you run the engine to be safe.

[imq707s]

But load effects how the engine runs and will effect how the plug reads.

Yep...that's what I was thinking. Spinning 6000rpm with a special prop that puts very little load on the engine is going to be a lot different that running 6000rpms with a "normal" prop that will put a normal load on the engine.

[Mr. Demeanor]

Needs to be done at the rpm you run the engine to be safe.


And load. 6k rpm in neutral isnt going to tell you squat.

[racer]

I would assume people understand while underway with the prop they normally run.

[SS-201]

With the differences of additives in gasoline from manufactures make it harder to read plugs. Put a little octane boost in and it will burn a reddish color on the porclain, the darker the more richer.

[mrcrsr]

piston readings(a small wet spot about the size of a nickle/dime) by the transfer ports is the most accurate way to determine if the motor is rich lean, along w/ the egt's. never had good luck w/ plug readings, and found this to be true on the dyno as well, the plug is saying the motor is lean, yet the pistons showed rich, and the fumes behind the engine on the dyno confirmed it.

[perfmarine1]

You also must consider air temp. and humidity will affect jetting. That is where the injected motors excell with the ability to adjust!

[perfmarine1]

Yep...that's what I was thinking. Spinning 6000rpm with a special prop that puts very little load on the engine is going to be a lot different that running 6000rpms with a "normal" prop that will put a normal load on the engine.

That special prop is made to run motor on the trailor and it is spec'ed too simulate the load of a normal prop at movement.It is designed for timing and diog running problems. It does put a heavy load on the motor not a light load because motor is statonary.

[Markus]

With the differences of additives in gasoline from manufactures make it harder to read plugs. Put a little octane boost in and it will burn a reddish color on the porclain, the darker the more richer.

Don't look at the color at all. It is likely only to confuse you. Only look at the band at the bottom of the insulator (the one that Noah talks about), and watch carefully for aluminum specks on the electrode.

Procedure is as follows:
1. Get to the place where you want to make your test run
2. Change to a set of brand new plugs on the water
3. Make your test run
4. Inspect the plugs immediately after the test run


There is a great article by Gordon Jennings on my links page that everyone trying to read plugs should read.

[Markus]

is 6000 good enough, or if the motor will turn 7000 or more, should I go that high?

You have to be at WOT.

If WOT with you alone in the boat is 7000 rpm make a 7000 rpm WOT run.

If WOT with you, your buddy Fat Fred and your big cooler in the boat is 6000 rpm, you should get hold of Fat Fred (or trim down the bow) and make a 6000 rpm WOT run as well.

[Jay Smith]

IF you can chop the throttle down and kill the motor at WOT SAFELY I've found that to be the best reading of pistons ...

Chuck Goodman ( IMO the best fuel guy on the planet ! ) taught me long ago and I have shared the old coin test many time to interested parties..

After making the WOT pass long enough to color the piston top I roll#5 piston on a Mercury to BDC I use a dark towel and cover the power head to block all light , I then use a "bend-o-light" to look at the intake wash side of the piston, if its wet the size of a dime , its about correct, a quarter is totally safe , silver dollar a bit rich and wet half way across it pig fat and is surely 4 stroking.A piston and its color and or wet ness is the way I get a real world check of the happening in that bore...

Hope thats helped ya !
Jay