Pat McPherson

I modified my 7.4MPI during the off season and recently got it running again.
The modifications include: a bigger cam, ported heads, better exhaust, and re-calibrated ECU.
I have been struggling a bit as she won't idle below 1000rpm.
We did take the boat for a blast on Saturday, and she ran quite well.
I replaced one plug while out on the water and ran hard for about 20 minutes.

Can anyone tell me how the mixture looks?

The darker plug was in the engine since I got it running, maybe 90 min at varying rpm.
The lighter plug has 20 min between 3800 and 4650rpm.

Thanks for the help...

Pat McPherson

Another Shot.

Reed Jensen

I can't tell exactly from the photos... but it looks good to me... On the lighter plug that only ran for a few minutes... is there a slight "ash" ring down near the bottom of the ceramic insulator?... if so... mixture is perfect. If there isn't a ash ring ( light soot ) then it is too lean... good luck...

PatriYacht

I would say a little lean but its hard to tell with today's gas. Some leaves little color. How were your oil and water temps? What are the cam specs? You should be able to idle at 7-800 unless the cam is really radical.

rmbuilder

Pat,
This is the best reference I have seen on the topic. Larry did a great job with this, photos and text.
http://www.maxracesoftware.com/spark_plugs.htm
Bob

KNOT-RIGHT

Nice Info Bob.

Reed Jensen

Everyone has different ways of reading plugs... depending on combustion chamber design.... how long the plug has ran... dyno output... etc...nice info at the site that was posted...but not the way I learned to read plugs.....

KNOT-RIGHT

I agree Reed all my life I have been reading the insulator


I will be soon checking the plugs in my newly blown motors. I will apply some of the info on that site.

Pat McPherson

Thanks for the help guys!
I ran into the engine builder that reworked my heads at lunch time. He told me they looked OK for a recreational engine, maybe a tad rich for a race engine.
When we were out running hard, all systems were go, so I guess AZ Speed & Marine got the calibration right for the top end.
Just need to get her to idle. I swapped the L-29 cam with one only sightly larger, 7-800rpm should be a reasonable target. I picked up a new IAC so I'm hopeful to solve the problem without another calibration change.

HARRISONMIRAGE

Do you have a proper plug magnifier.I just bought on last year and couldn't believe the difference.