Spark plug blown out

[midsized johnson]

90 model 85 Hp Yamaha 3 cylinder. My buddy was running at around 5200 rpm, everything going good until, bam! Sparkplug blows out. He took the cylinder head off, and everything looks clean, but the upper cylinder part of the head has some hairline cracks. It doesn't appear that the piston came loose, more like the head got hot, cracked, and compression shot the plug out. He says the thermostat (new) looked messed up. I'm pretty sure the thermostat wasn't working correctly because on a test run last week he pointed out that no water was coming out of the exhaust ports while running at upper end speeds. I know that on my outboard, water dumps out of the exhaust ports at anything other than idle speed. BTW, he says the cylinders look good, and the motor isn't locked up. Any advice on what to check for, other than replacing the cylinder head and, of course, the thermostat?

 

[Ray Neudecker]

Re: Spark plug blown out

Any thing from the impellor on which might cause low water pressure.

 

[rodbolt]

Re: Spark plug blown out

a test of the alarm system would be in order as well.

 

[midsized johnson]

Re: Spark plug blown out

Pressure gauge read 15 lbs. Pressure gauge outlet is on the cylinder head. The telltale (pisser) runs out of port side of the exhaust cover water jacket. What is normal water pressure at upper end for this motor? <br />Good point about the alarm. The motor had sat for a while and had lots of salt buildup inside the water jackets. He cleaned it well, new impellor, thermostat, gaskets, etc. The aluminum inside the waterjackets now has no paint (sandblasted corrosion and salt build-up off), should he try to coat it with something to prevent corrosion?

 

[Texasmark]

Re: Spark plug blown out

zinc-chromate primer (paint) or alodine (chemical acid) are two options. The alodine works best but is a bigger hassle. It actually corrodes the surface of the aluminum preventing further corrosion from the water. Most aircraft aluminum parts receive this coating before painting with zinc-chromate primer than top coat; or left bare with the alodine coating.<br /><br />Local automotive paint store would have it; follow directions.<br /><br />Mark

 

[midsized johnson]

Re: Spark plug blown out

Thanks Mark, I'll let him know.

 

[Texasmark]

Re: Spark plug blown out

10-4