Yamaha 70BETO Power issue after low idle and in full throttle

[stevin90]

Hello all,

Earlier this year I did buy an 70 hp yamaha autolube (6H3). Overall it runs good, strong engine.

But when I am sailing a couple minutes in low rpm's it sounds all normal but when I open throttle a little bit I can hear and feel that it is running on 2 cilinders only.
When I go full throttle, after a couple seconds the 3rd cilinder is kicking in and motor runs on all 3 cilinders again, untill I have been sailing in low rpm's again for a bit. Motor starts smoking more when I am sailing in idle for a while.

Sometimes when I keep sailing at full throttle, 5500 rpm, 1 cilinders starts working on and off, with intervals of about 1-2 seconds. I don't understand why.

I have done the following
- checked plugs, all okay, but replaced
- ultrasone cleaning of carbs

I am afraid that something might be wrong with electronics, dci box or ignition coil.
Could that be true or does anyone have a suggestion what to check or replace?

Many thanks in advance

[TownsendsFJR1300]

Check the fuel pump for leaks...Sounds like it's leaking and fouling a spark plug..

[stevin90]

I will do that as soon as I have time and will revert to you. Thank you
If I remember correctly, the primer ball is not pumping any more if I squeeze it, become hard as stone, so that should be good shouldn't it?

But how could a leaking pump relate to a smoking engine running on 2 legs in idle and coming back at higher rpm?

[99yam40]

if the pump leaks fuel into the crankcase, that cylinder(normaly #3) will run too rich at low RPMs but opening the throttle will let more air in letting the air to fuel ratio get close to correct for the plug to burn the fuel again

[stevin90]

Aaaah okaay, that sounds logical now. I believe it was indeed #3 plug that is wet in idle. I will check that, take out the pump and put some pressure on it. Hoping that would solve the problem, rather than having an electrical issue. Thanks, I will let it know what I find.

[stevin90]

I have checked the fuel pump and that one looks good, no leaking.

But I think I have a bigger problem now... I took out the spark plugs, engine is in winsterstore already 3 months and the plug on cil 3 is rotten... wet and rusty. 20191218_131658.jpg

[99yam40]

looks like water getting in there
not good

[TownsendsFJR1300]

Agreed 100% ^^^

Per post #1, you checked the plugs, ??



Best oil that cylinder until you can pull the head and address the issue... Probably a blown head gasket (or corroded block/head).

I'd pull the head ASAP (even if you oil the cylinder) as the lower bearing may also have water intrusion...


.

[stevin90]

Agreed 100% ^^^

Per post #1, you checked the plugs, ??



Best oil that cylinder until you can pull the head and address the issue... Probably a blown head gasket (or corroded block/head).

I'd pull the head ASAP (even if you oil the cylinder) as the lower bearing may also have water intrusion...


.

Yeah I checked the plugs 3 months ago, after that I had no time to work om the engine. This morning I came back and saw that the plug was rusty.

I already pulled the head and oiled all cilinders. Those seals looked good, I am afraid it is coming from the lower bearing. Friday I will bring the engine to a dealer for changing those lower seals and check the crankshaft bearings.

I don't like to lift the entire block, better I pay some money to have a prof do it.

[stevin90]

20191218_164908.jpg

[stevin90]

This picture shows lower 2 cilinders. I cleaned the pistons later and they still look good to me, so I hope my crankshaft bearings survived as wel...

What do you think of the cilinder liner (cil 2)? It shows little wear but I think it still should be fine like this? Compression test was good when I bought it.

[TownsendsFJR1300]

From what I can see, the cylinder looks ok...

If the lower, #3 cylinder was the one with the rusty/water wet plug, I'd be inspecting that cylinder/head closely.

The lower (guess #3) piston top looks super clean on the right side of it (3-5p area).

Did you inspect the head gasket closely and also the head for any cracks/corrosion?


If you can't get to the shop soon, I'd pull the reed assembly out and inspect the lower end for water / water damage AND oil the lower crank bearing by hand...


One other possibility is the head is warped. Perhaps the PO overheated and damaged it. Might take a straight edge to it and check it.

[stevin90]

Yes I checked as closely as I can. So far everything looks good.

Friday I will bring her to the shop and ask them to check the reed valves also, thanks for the suggestion.

If the head is warped, I should see some obvious damage on the gasket yes? That gasket looked pretty good, also the steel rings on top of the powerheads looked quite new.

I think that the previous owner experienced this problem also, tried to fix it by replacing head gaskets, but didn't solve the problem.

[TownsendsFJR1300]

The reeds aren't an issue (just a way to get to the lower end fairly easy).

Somethings obviously not sealing. As good as the engine ran, the leak must be pretty small but getting worse.

I mentioned about the "clean" piston, that's indictive of water getting in, in that area, cleaning the piston.

Some measuring and a sharp eye should find the issue..

[stevin90]

I could not do exact measuring myself, but close inspection with magnifiying glass didnt show any cracks.

She is going to shop tomorrow, hopefully it will be fixed when I get her back. Cannot live without yamaha