How to read plugs???

[dmcomp]

I have a 1978 70HP Merc (700) with compression as follows #1=125, #2=120, #3=95. My spark plug appearance is as follows: #1=Dry and slightly brown, #2=Wet and black oily deposits, #3=wet and clean (Like New). Would these indications be related to compression, All plugs fire with a clean blue spark out of the cylinder. The outboard starts without any problem but bogs down very easily and is reluctant to get on the plane. Maximum revs I can get is 2100rpm. Have tried lowering prop pitch from 15" to 13" with no improvement.Could these problems stem from the bottom carby? - 3 Cylinder x 2 Carbies. The Boat is a 15' Fibreglass.<br /><br />Regards Daryl

 

[rodbolt]

Re: How to read plugs???

its blown up.<br /> 125 to 95 is to large a spread. try a decarb but I doubt its gonna help.<br /> after the decarb have a leakdown test performed. musch more than 10% and its rebuild time.<br />next will be a careful fuel system inspection/cleaning along with a cooling system inspection and maint.<br /> you have to find the cause of the compression loss. if a leakdown test says the ring seal is ok do the fuel and cooling system maint.

 

[JB]

Re: How to read plugs???

G'Day, Daryl.<br /><br />The low compression and clean spark plug in #3 suggest that water is getting in during running. Inspect both #2 and #3. <br /><br />If there is a problem with #2 it may be unrelated to the problem with #3, but in any case it becomes relevant only after the problem with #3 is resolved.<br /><br />Rodbolt's advice is good.<br /><br />Good luck.

 

[dolluper]

Re: How to read plugs???

that sounds more like it jb cracked head gasket leaking

 

[Clams Canino]

Re: How to read plugs???

They don't have a head gasket. The seals in the lower end-cap are suspect though.<br /><br />-W

 

[rodbolt]

Re: How to read plugs???

as are the exhaust divider plate and gasket. normally if there is enough water to clean the plug you can tap the electrode end on your palm and it will leave droplets or the plug will have a grayish milk on it. in saltwater it will leave tiny green flecks on the ground electrode.

 

[dolluper]

Re: How to read plugs???

ok don't call it a head gasket call it an exhaust cover gasket [tech] there is two and a inner cover<br />or like the seals behind them any leakage here ends up showing in #3

 

[Keith Buchanan]

Re: How to read plugs???

Hi C.C.<br /><br />Lower end cap seals ?<br /><br />Not trying to hijack this thread but have had a similar problem which turned out to be the baffle plate gaskets. I am interested as to where the end cap seals are.<br /><br />Regards all Keith B...

 

[Clams Canino]

Re: How to read plugs???

The reason we make the clear distiction that it's not a head gasket is that too many people try to pull off that water jacket cover at the spark plug end thinking it's a real cylinder head. <br /><br />We don't want people thinking it's a head, or that a water leak at that cover can find any way into the motor.<br /><br />We don't want people taking that cover off at all unless it's sparaying water all over the inside of the cowling.<br /><br />That's whay the distinction was made - not just for the sake of parsing tech terms.<br /><br />-W

 

[Clams Canino]

Re: How to read plugs???

The lower end cap (at the bottom of the powerhead) is held on by three bolts. It contains 2 seals designed to seal the bottom of the crank. Use the correct puller to pull that cap though.<br /><br />-W<br /><br /><br />

Originally posted by Keithy B...:<br /> Hi C.C.<br /><br />Lower end cap seals ?<br /><br />Not trying to hijack this thread but have had a similar problem which turned out to be the baffle plate gaskets. I am interested as to where the end cap seals are.<br /><br />Regards all Keith B...

 

[Keith Buchanan]

Re: How to read plugs???

Thanks C.C. have found it on the parts breakdown now.<br /><br />Regards Keith B...