1 super clean plug

[rastoga]

I've got a 1989, 3 cyl, oil injected, 40hp yamaha. It runs just perfect. I was going to change the plugs when I noticed number 3 (the bottom plug) was as clean as a whistle. The other 2 look like they had been used normally, perhaps a little on the oily side. <br /><br />I was thinking bad head gasket or some sort of water leak.<br /><br />I checked the compression cold, 1 was 95, 2 was 100, and 3 was 100. I ran the engine for a couple minutes and checked again, one was 90, 2 was 95, and 3 was 90. I don't know what is normal for this engine, but the good thing is that they are really close to each other.<br /><br />I don't know how I could test for a water leak. The compression seems fine and the motor runs like a swiss watch. Should I be concerned about the super clean number 3 spark plug?

 

[DJ]

Re: 1 super clean plug

rastoga,<br /><br />You can just about bet you have water ingestion in that cyl. with the clean plug. Probably a head gasket. Did it ever overheat?<br /><br />The compression look OK. They should be within 10% of each other.<br /><br />Replacing the gasket is not that bad. Spend $25.00 on a manual though or go to the library and see if you can find one and copy the pages needed. You'll thank yourself later.<br /><br />Remember, the head bolt torque spec's. are INCH pounds, not foot pounds. <br /><br />Best to retourque after a warm up.<br /><br />Since you're "tearing" into it, I would replace the thermostat and water pump impeller.

 

[rastoga]

Re: 1 super clean plug

I've only had it for about a year. It never overheated with me. <br /><br />If the gasket is allowing water into the cylinder then shouldn't the compression reading be off?<br /><br />What all is involved with changing the head gasket? Is there more than one gasket that will need to be changed? Will I have to remove the powerhead? I'll get the clymers manual from the library and check into it. But I think I'd be pretty nervous to do it myself.

 

[Hooty]

Re: 1 super clean plug

rastoga,<br />I'm like you, I'd think the leaking head gasket would show up with a compression test but the only other thing I can think of that would clean the plug is if it wasn't firing at all but you say the engine runs great so like djohns19 said, it's gotta be the head gasket.<br /><br />c/6<br /><br />Hooty

 

[alcan]

Re: 1 super clean plug

Hi rastoga<br /> The situation you describe defenatly needs attention. I agree with the others that it my be a head gasket. But do to the fact that the compression is up, I would suspect the exhaust plate gasket. This plate is located on the side of the cylenders. It is a two part afair, an inner and outer plate, It is water cooled, basicly a water cooled exhaust manifold. If it were me I would replace both. Good luck.

 

[rastoga]

Re: 1 super clean plug

Thanks alcan for suggesting the exhaust plate gaskets.<br /><br />Does anyone know what are some potential places where water could get into the #3 cylinder? I looked at a diagram, there seems to be whole lot of gaskets in this thing!<br /><br />Do you think I could try retorque-ing some of these places as a quick fix?<br /><br />What about air? Could an air leak be causing a clean plug?

 

[12Footer]

Re: 1 super clean plug

Alcan is dead-on. Exaust port plate gaskets are the most vulnerable part of the 90-125HP Forces and some Mercs. Now, I don't know bout no Yammies, but the principles are the same. I have seen water jackets leak from the head gasket,into the combustion chamber,and never effect compression.<br />But really, those three areas are suspect<br /> Head, cracked case,or exaust port plates.<br /><br />If your motor has a thermostat, and the tstat has been removed for longer than a 20minute test, the plates warp,crankcases heat unevenly, and heads contract too rapidly. Same can happen if the tstat "sticks" closed,runs the motor hot, then pops open...Or other form of water starvation,like sand in the pickup.<br />The sudden temp change when it again pick-up water, can warp/crack stuff.<br /><br />You may have a lean condition on that plug,but if the carb/injectors supply more than one jug each, you can discount them as suspect.<br />And good job on perhaps saving your engine by your thourough maintainence! Bravo !

 

[Forktail]

Re: 1 super clean plug

It's a simple job pulling the head on this outboard rastoga. 14 bolts using a 12mm socket and only one temp sensor to unplug. The head will be two-piece so you'll need two new gaskets. No need to remove the entire powerhead. You'll find the torque sequence (1-14) and the torque spec stamped right on the head itself for convenience. The torque is 3.0 KGM or 21.7 foot pounds (not inch pounds). I would guess under a half hour to remove, clean, and reinstall. You'd certainly be able to see the cylinder walls better by doing this.<br /><br />Before you tear down anything try to get a look at the piston. You can use a flashlight. Look for piston wash (clean shiny area) on the intake side of the piston. You should have some wash. A completely black piston will mean it's running too lean.<br /><br />This outboard has 3 carbs, one to each jug. So don't ignore a carburetor or lean intake problem.<br /><br />Good luck.