yamaha 70 piston/engine condition.

[ziggystardust]

i took some photos of my yamaha 70 pistons cause i am installing a new head gasket.

can anyone tell me anything about the operation of this engine by looking at the pistons?
note that all surfaced are just sprayed with a light oil before taking the pictures, so everything has a yellow glance. and i know the pitcures are not too good, i am just curious.

it seems they are all burned in the same spots in all cylinders, i found some
small vertical marks in cyl nr 1, as showed here:

should i dissasemble completely and hone the cylinder or is this a to small scratch to bother? i can barely feel its there with my fingernail. if i did not see it i would not have noticed it.

the engine was operating flawlessly before i disasembled it.

i am just learning this stuff

 

[robert graham]

Re: yamaha 70 piston/engine condition.

Any particular reason for having the head off motor? May as well scrape/decarbon piston crowns while it's open, cylinders look pretty good from photos. If motor is running good, then button it up and run with heavy(deposit removal dosage) Ringfree for several tankfuls, then drop to (maintain/preventive) dosage to be sure no carbon on or behind rings causing ring sticking in grooves. My motor is a 1999 and have run Ringfree always and piston crowns a shiny clean aluminum. Good Luck!

 

[ziggystardust]

Re: yamaha 70 piston/engine condition.

well the main reason that i pulled apart the head was because i noticed large salt deposits around the head gasket, so i decided to get a new gasket set.
this engine have been owned by careless people, so i want to know the state of the engine. and because i noticed a coughing and rpm drop in idle revs. but compression checked out fine on all cyls and it is looking good

but i dont care to spend so mutch money on the engine, can i hone cylinders and re use the rings?

yes i think i am going to dremel away any carbon deposits on the exhaust port too
do i gain anything from polishing the exhaust port when i dont clean the rest of the exhaust?

thanks for the advice

 

[robert graham]

Re: yamaha 70 piston/engine condition.

You did good to check it out and replace the head gasket. The carbon on piston crowns is not near as much of a potential problem as the carbon on the piston rings and grooves but you may as well clean the piston crowns while you have the head off. I don't think you need to hone the cylinder or try to clean the exhast ports, but use the Ringfree from now on and your motor should be fine from a cylinder/piston stand-point. Good Luck!

 

[robert graham]

Re: yamaha 70 piston/engine condition.

Regarding your coughing/RPM drops/etc., those are most likely fuel delivery/carburetor related issues. Probably need to disassemble and clean carbs. Try draining each carb onto a clean white rag to see what's in it and that may give you some idea of their condition. Check and clean every part of fuel system from the tank to carbs. Install a fuel/water separator(Walmart $27), will save you a lot of fuel/water related problems. Once you get it all squared away that motor should run like a top! Good Luck!

 

[ziggystardust]

Re: yamaha 70 piston/engine condition.

yes i think i will service the carbs, as its running perfect on high revs, and use ring free.
now i am just waiting for the gaskets to arrive and i will have a complete engine by the end of the weekend. then its time for the interior of the boat to be rebuilt as i want it.

i bought an english boat last year, a spirit daycrouiser 18.5" 1987, and i wrecked it by the time summer was over. the wood in the boat was not water proof or solid so the interior wall shattered in a little storm one day. spirit daycruisers are not made to survive a beating.
but thats a good excuse to build it like i want it. its a good boat in the water


thanks for the tips