Spark Plug Analysis

[Crownline 186]

I have a 1993 Mercruiser 3.0 XL with Digital Electronic Ignition. Pulled the plugs last evening and found the following: Plug 1 was black and sooty with some small deposits around the base of the plug and the adjustable electrode was clean but black. Plug 2 was not as sooty, but insulator around center electrode was black. Plug 3 looked much like you would expect the plugs in a well running engine to look, no deposits and a light tan color on the insulator around the center electrode. Plug 4 was about like Plug 2. These plugs probably have less than 10 hours on them. Most of the hours were after the carburator was rebuilt by the shop, but installed by me. Last weekend the engine was started for the first time for the season, and started pretty easily. Idle speed per the boat tach was around 1000 rpm. I backed out the idle adjustment screw about 3/4th turn and idle rpm dropped to about 800 rpm and I left it there. I think that my adjusting the idle screw has made the mixture too rich, but the boat performed well accelerating well and cruising well up to rated rpm of 4,800.

Can anyone give me a clue as to what the condition of the plugs is telling me? Do I have carburator issues, ignition issues, or possibly both? This engine has a miss in one cylinder on cold start-up, but after a minute or two of running it goes away, and the engine runs well thereafter, even at idle.

Any guidance would be much appreciated.....

 

[Don S]

Re: Spark Plug Analysis

You need to get the idle down to 650 to 700 rpm to adjust the mixture, and to set timing. Anything over that will probably cause problems with dieseling when you try to shut down.

 

[Crownline 186]

Re: Spark Plug Analysis

Don S ..... thanks..... had some dieseling problems when the engine was new, but have not had that problem for the last couple of years. Went away on its own. How do you adjust the timing on this engine? There is what looks similar to a distributor on the starboard side of the engine, but I think it is all electronic. Am I wrong in thinking that this distributor looking thing is not mechanical, but electronic in operation? Also, should I be looking at making a throttle cable adjustment to adjust idle rpm or just the idle screw on the carb? And lastly, should I be trusting the boat tach. If I were to find that the boat tach reads high, I would not be surprised. Shifting to forward or reverse with the boat tach reading 800 rpm is clean and smooth.

 

[Don S]

Re: Spark Plug Analysis

I would never want to trust the boats tach for idle adjustments.
The sensor that is where the distributor is, is where you adjust the timing. Should be 8? BTDC @ 650 to 700 rpm.
Then do your idle mixture adjustment. As far as idle speed, you may have to adjust the cable, if it doesn't pull it all the way to idle. You should have it adjusted so there is a slight preload on the cable when the idle screw is against the stop.

 

[Crownline 186]

Re: Spark Plug Analysis

Don S... again thanks for the advice ....will try to figure out how to adjust the timing since it did not appear obvious when I was looking at the engine last evening, will come up with a separate tach, and will check the throttle cable adjustment prior to adjusting the idle screw again........ will post the results.... BTW ..... can all of this be done successfully on muffs, or would you recommend that it only be done in the water?

 

[JCF350]

Re: Spark Plug Analysis

Read your manual.
650 - 700 in gear, means with a load on the prop.

 

[Don S]

Re: Spark Plug Analysis

You can do all your timing and idle mixture adjustments on muffs by setting the idle rpm between 650 and 700 rpm. When you put it in the water, you may need to adjust the rpm up just a bit.

 

[Crownline 186]

Re: Spark Plug Analysis

Update ... Started the boat on muffs Sunday afternoon. Idle was about 800 rpm per boat tach since when I went to my local Pep Boys and asked about getting a dwell/tach meter I was met with a "deer in the headlights" expression not only from the salesperson but the manager as well ..... I need to pursue the purchase of a tach, but certainly struck out Saturday ....

There seemed to be a little, very little, play in the throttle cable so I did adjust it one turn to tighten it which put a slight preload on the cable. After starting the engine I closed the idle screw the 3/4 turn that I had opened it and saw no improvement. Backed the screw back out 1-1/2 turns in 1/4 turn increments and ultimately found that idle performance became rougher, and idle rpm did not really change. Then closed the idle screw 1-1/2 turns and from there closed it 1/2 turn in 1/4 turn increments. This final adjustment gave me the smoothest idle and the boat tach was still reading about 800 rpm.

Put the boat in the water. It started easily (still somewhat warm from running on muffs). Idle was smooth and engine was not missing. Cruised for a couple of hours at different rpms and did some hard accelerating as in pulling a skier. Engine ran well. Put it away until next weekend and hopefully it will run as well then as it did this past Sunday. I do need to check timing, but do not know how to adjust it if it is off, and need to get an independent tach. Even though the engine seems to be running quite well, having good data to verify that it is operating as designed provides a lot of peace of mind.

Thanks to all who have helped me address this matter.