yamaha 70hp two stroke problem

[wellman1]

hello everyone, I have just joined this forum, I am a self employed pc tech living in in northern Ireland and just recently bought a used rib with a 96 70hp 2 stroke yamaha outboard. after taking the boat out for a trial run the outboard would only run at low speed as soon as the throttle was opened up to half way the motor cut back to low speed again. I serviced the motor by checking the fuel system etc. now when I run the motor in a water barrel, it seems to run ok apart from smoke coming from underneath the forward part of the cowl, I then removed the apron around same and after starting the motor again there didnt seem to be any damage to the lower gasket, the smoke was coming from between the cowl and transom. can anybody throw any light on this, thanks.

[99yam40]

the smoke was coming from between the cowl and transom. can anybody throw any light on this, thanks.

can you not see where it is coming from?
hard for us on this side of computer screen to look for where it is coming from

[TownsendsFJR1300]

If you can snap an accurate picture and post it here would help.

[wellman1]

yamaha 70hp two stroke problem

I will try and take a picture with the motor running, but the smoke seems to be coming from a vent below the crankcase and is not as bad when the motor warms up, I put my finger into the vent which is about 3 inch long and it looks like it is a channel going towards the transom.

[wellman1]

can you not see where it is coming from?
hard for us on this side of computer screen to look for where it is coming from

Here is an attached pic with motor running.

[TownsendsFJR1300]

Blowing up the picture(double click) it appears to be simply oily exhaust exiting the water as your engine is in a tank.

If you have muffs to run the engine on, you can verify that. The exhaust smoke will pretty much just come out the prop area...

Re the RPM's cutting down, I know you mentioned you "checked the fuel system".
If its still acting up (while underway) its VERY likely that the carb jets need cleaning, especially the main jet. They MUST be removed and visually inspected.

If you can't see light thru them, it doesn't count(ie a spritz of carb cleaner in the hole may NOT fix it, pull the jets, then clean)

IME, Yamaha's RingFree works excellent at keeping the carb/fuel system clean. If the system isn't that dirty, simply running it may help it to run better. If the carbs halfways bad, dis-assembly is needed..

[wellman1]

I know its hard to tell from the pic but the smoke is blowing down from the vent. are you saying that the vent is a water vent?

[wellman1]

Blowing up the picture(double click) it appears to be simply oily exhaust exiting the water as your engine is in a tank.

If you have muffs to run the engine on, you can verify that. The exhaust smoke will pretty much just come out the prop area...

Re the RPM's cutting down, I know you mentioned you "checked the fuel system".
If its still acting up (while underway) its VERY likely that the carb jets need cleaning, especially the main jet. They MUST be removed and visually inspected.

If you can't see light thru them, it doesn't count(ie a spritz of carb cleaner in the hole may NOT fix it, pull the jets, then clean)

IME, Yamaha's RingFree works excellent at keeping the carb/fuel system clean. If the system isn't that dirty, simply running it may help it to run better. If the carbs halfways bad, dis-assembly is needed..

here is another pic, you can see the smoke better in it.

[TownsendsFJR1300]

Still hard to see...

Rodbolt or Boscoe will have to chime in.

IDK if there's an exhaust gasket by the power head that may blow out, or ??

[99yam40]

as said hard to tell where it is coming from, just a lot of smoke around the motor.

seems a gasket or corrosion has let the exhaust come out where it should not be coming from

[wellman1]

I think also that it may be the gasket between the powerhead and lower half as the exhaust is going through there. I will take the boat out for another run and see what happens. my auxilary 3.5 hp hopefully will get me back if the 70hp gives up.

[TownsendsFJR1300]

Part #17 possibly? It bolts to the top of the exhaust manifold(or the exhaust manifold corroded?):

1996 70TLRU Yamaha Outboard UPPER CASING Diagram and Parts

[wellman1]

Thanks to you all for your help. I will dismantle the motor when I get time and post what the problem is, it may help someone else with a similar fault.