Yamaha 40 hp 2 stroke, 80's outboard running rough.

Hello. I have a Yamaha 40hp 2 stroke (3 cyl) engine from early 80's somewhere. There are no numbers on the engine, so I'm not sure what model exactly.

Before the boat season ended the engine ran very rough. Especially on idle, but it also made some noises in higher RPM's too. Also I felt it was using more fuel, but significantly loss in power. About 6-8 knots slower on full throttle. It also seemed like it was leaking some fuel or something too. the ocean didn't look too good when the boat stood still and engine was idling.


I've taken the engine into my garage for the winter and taken out the carburetors and cleaned them.
I let them lie in diesel for a day or so and cleaned them with a little brake cleaner (it's hard to find carberator cleaner over here) I also took out the reed valves and cleaned them lightly.

I fired up the bad boy today and it still runs rough and coughing.. A lot of smoke too.


- I've checked for sparks, it seems pretty fine. Good clear sparks
- I've had a compression check on it too some hours ago. Looks very good. Even compression on all 3 cylinders.

I also noticed that on cyl 2 and 3 fuel is leaking out of the choke plate. So I feel the problem is here or something with the carburetor adjustment.

On the bright side it's VERY easy to start the engine. Therefore I don't think it's a mechanical issue inside the engine.



I'm REALLY hoping I can get some help here.

Cod season is in full bloom right now and the ocean here in norway is crawling with fish atm! It won't be long before it's over!

just fyi I'm not a marine mechanic and have not worked so very much on 2 stroke engines, but I am an automotive mechanic so I've got some skill in this area.

Hey there

There are a few things that could be going on here. Given you said you cleaned the reed valves and it starts easily I'm presuming those valves where closing well. If they don't they can cause fuel to come back out through the carburettors. Also, because you cleaned the carburettors I'm going to assume the needle and seat for the float bowl is working well and the fuel pump isn't causing them to overflow. From the smoke and fuel is sounds like it is running rich. Are the chokes opening completely? You may also need to balance the carbs with a set of vacuum gauges to smooth out that idle, but it sounds like you've got something a bit more seriously wrong with it first. I'd also be inclined to put a timing light on it and see where it is at there.

Stuart

Stuart Allen said:

Hey there

There are a few things that could be going on here. Given you said you cleaned the reed valves and it starts easily I'm presuming those valves where closing well. If they don't they can cause fuel to come back out through the carburettors. Also, because you cleaned the carburettors I'm going to assume the needle and seat for the float bowl is working well and the fuel pump isn't causing them to overflow. From the smoke and fuel is sounds like it is running rich. Are the chokes opening completely? You may also need to balance the carbs with a set of vacuum gauges to smooth out that idle, but it sounds like you've got something a bit more seriously wrong with it first. I'd also be inclined to put a timing light on it and see where it is at there.

Stuart

Click to expand...

Good morning stuart and thanks for your reply

Yes, the seat and needle for the carburetor are OK. I took them out last night to double check.
The choke is also opening completely, so it's OK too

Just for the hell of it, I'm going to take apart the reed valves and check them again, if everything is fine, I'll put it back together tonight and tomorrow I will borrow a timing light from work and see how the ignition timing is

Hello again fellow marineheads!

Just before I put the carburetor back, I noticed that the second carburetor had wrong float height. I adjusted that and it now seems the carburetor leaking problem is gone. The engine still runs rough though.

Then I did the following:

- I adjusted the carburetors to hopefully smooth out the idle and it helped somewhat.
- I put on the timing light as you said, Stuart and found that the timing is way off.
I have about 40 degrees too early igniton before TDC (sorry if I'm explaining bad here, but I'm not familiar with all the technical english words you guys are using)

I loosened the bolt and could turn the screw and move it nearer TDC mark. I couldn't move it closer than about 20 degrees before TDC. That smoothed out the engine a whole lot!! But I still feel it's too early ignition and the engine is running still kind of rough and still lot of smoke.

I also should have mentioned that I had pull start on the engine and I got a powerhead from another 40hp (I think it was some years newer) and built electrical start on the engine I have by moving the flywheel and the electrical stuff.

Now I think maybe because of moving the electrical parts over from that other engine to my engine - that may be what caused the "too early ignition" (not sure what word to use to describe it) haha

On outboard engines, is it good with a little bit of early ignition, like 5-10 degrees? Or should I try getting it to TDC mark?

Hey Benji, glad your motor is running a bit better now, and don't worry, your English is fine.

I don't have the service manual for your motor hear unfortunately, but it is quite possible that it should be a bit advanced at idle. I think it should be about mid twenties BTDC at WOT. I read somewhere that if you don't know the correct position you should find the point of highest idle rate then back it off 4 degrees. Can't verify whether that is the right thing to do though! If you Google your engine model at a key phrase like "BTDC" you should find the figures you are looking for.

With regards to the smoke, could it simply be too much oil in the fuel? Do you know what the mixture is meant to be for that outboard?

Stuart

Checked the flywheel key ?

Stuart Allen said:

Hey Benji, glad your motor is running a bit better now, and don't worry, your English is fine.

I don't have the service manual for your motor hear unfortunately, but it is quite possible that it should be a bit advanced at idle. I think it should be about mid twenties BTDC at WOT. I read somewhere that if you don't know the correct position you should find the point of highest idle rate then back it off 4 degrees. Can't verify whether that is the right thing to do though! If you Google your engine model at a key phrase like "BTDC" you should find the figures you are looking for.

With regards to the smoke, could it simply be too much oil in the fuel? Do you know what the mixture is meant to be for that outboard?

Stuart

Click to expand...

Okey, I found an old workshop manual in my house, so I will try to find the specs for ignition timing.
yes, i'm using right mixture for the engine, 1 dl oil per 5 litre fuel, thats what the manual says

I'll post back here when I correct the timing - hopefully tomorrow.

racerone: Ie Yes, it looked okey when i put on the flywheel. It was the first thing I checked because I had worries that it was broken..

I'm sorry that I haven't been posting anything. The engine was kinda put on hold because of crazy much work to do
in the workshop.

GREAT NEWS!

I noticed that the engine didn't get enough advance with the timing lamp. After a lot of messing around with the screws I noticed the linkage was put on the wrong way (see picture)







Probably my own doing when I built it from pull-start to starter motor.

So the when I put the linkage on the right way I had to adjust the timing again (because I messed around with it so much) It still was alot of smoke from it, but I figured I give it a go in the ocean. I attached it to the boat today and holy mother how smooth it ran. It probably was good that I cleaned the carbs after all.

It's running great. Super quick again and almost no misfirings.


Thanks everyone for commenting and helping me with these problems. Hopefully I won't get any more problems now with the engine. I'm going fishing tomorrow for sure!!

CASE CLOSED!

Hi, Im currently having the same problem with my 40hp yamaha. Well when i start it up, it will run in idle but when i get to about half throttle, it starts to back fire and jump around all over the place. Im pretty sure its due to the advance timing of the engine but was wondering what degrees it should be before TDC. I have searched the internet and can not find anything? I also noticed that someone on this thread stated somewhere around the mid twentys? Does anyone know the precise degrees?
And does anyone have a workshop manual to these engines? Many thanks,

George

Benji91 said:

I'm sorry that I haven't been posting anything. The engine was kinda put on hold because of crazy much work to do
in the workshop.

GREAT NEWS!

I noticed that the engine didn't get enough advance with the timing lamp. After a lot of messing around with the screws I noticed the linkage was put on the wrong way (see picture)

Probably my own doing when I built it from pull-start to starter motor.

So the when I put the linkage on the right way I had to adjust the timing again (because I messed around with it so much) It still was alot of smoke from it, but I figured I give it a go in the ocean. I attached it to the boat today and holy mother how smooth it ran. It probably was good that I cleaned the carbs after all.

It's running great. Super quick again and almost no misfirings.


Thanks everyone for commenting and helping me with these problems. Hopefully I won't get any more problems now with the engine. I'm going fishing tomorrow for sure!!

CASE CLOSED!

Click to expand...

Checked the flywheel key ?

i have done already and its still in place and there.