1984 40hp 2 stroke Yamaha missing while cold

[cortlillard]

Hello all! I have searched the forums, and have not seemed to come up with an answer.

I have a 1984(?) 40HP Yamaha 3 cylinder 2 stoke outboard, and it seems to be mugging on one of its cylinders while warming up while in gear.

I can put it in neutral, and it seems to be fine, but while in gear, under load (in the water), it seems to not be firing on one cylinder. I have tested for spark, and that is good on all cylinders, the compression on all cylinders is good, I have cleaned the carbs thoroughly, and that does not seem to do it.

After warming up for approximately 10-15 mins, the engine very noticeably starts working properly, and the boat can get on plane.

I initially though i was burning too much oil, because i had some sludge coming out of the exhaust while testing it on land, so i reduced the oil to 75:1 from 50:1, I then tried putting premium gasoline in it to see if that would help, but that does not seem to be the issue.

I suspect that the carbs need to be tuned for proper fuel to air ratio, but i cant even tell which carb to tune, because i do not know which cylinder is having the issue.

I have no tach or any gauges for that matter. The engine is on a 1968 sea king, and I don't want to invest too much (either than time) into it.

 

[cortlillard]

Re: 1984 40hp 2 stroke Yamaha missing while cold

I will be able to get any more details once i am in front of the engine later this afternoon.

THANKS GUYS!

 

[99yam40]

Re: 1984 40hp 2 stroke Yamaha missing while cold

Model # would be good to post
Check to make sure fuel pump is not leaking fuel into crankcase through pulse port because of whole in diaphragm.

You can use a spray bottle with fuel/oil mix in it to spay into the individual carbs while problem is occurring to see if will get the missing cylinder to fire. This will tell if one is running lean.

I do not know if your motor is this way, but my 99 C40 has different size jets in middle carb than the top and bottom, hope you did not get jets mixed up or carbs in wrong position if it has different jets

Pinching off the fuel line to individual carbs might let you know if the individual ones are running too rich. It would start firing once the carb started to run out of fuel

 

[renegade15]

Re: 1984 40hp 2 stroke Yamaha missing while cold

Also sync the carbs if you've had them off. When you checked for spark, did you use a tester? Or remove plug and ground to block to test for spark? If you used a tester, are you sure your plugs are all good? Maybe try new plugs? Check timing too?
I would also go back to the 50:1 oil, would hate to toast a ring/piston!

 

[cortlillard]

Re: 1984 40hp 2 stroke Yamaha missing while cold

No, I did not use a spark plug tester, and I think that may have been my problem. I just checked again, and don't look to be getting a spark on the 3rd cylinder.

I believe that I may have thought of my issue backwards. I believe that this system stops spark to the 3rd cylinder when there is an overheating condition. I have an overheat light hooked up, but it never illuminates, so i thought that that was not an issue. It may be burnt out, or the wiring may have god bad somewhere, but i just don't suspect that that is it, because the issue occurs right when you first put it in the water cold.

I will look at my wiring this weekend, and post what i find.

Thanks again guys!

-Cort Lillard

 

[99yam40]

Re: 1984 40hp 2 stroke Yamaha missing while cold

Not sure about that motor but my 1999 C40 the overheat retards timing to limit RPMs

Start with basics, check compression, then spark, fuel pressure/vacuum, then timing while problem shows up and before.

If all that is good then probably carb problems

Fuel pump is operated by #3 crankcase pulse, so if it is leaking fuel into crankcase it will not fire properly on #3. It will also wash the oil off of cylinder walls and cause piston and cylinder damage

 

[cortlillard]

Re: 1984 40hp 2 stroke Yamaha missing while cold

I am pretty sure I found the issue.

The old owners of this engine abused it, and did not understand the electrical system at all. The also removed the automatic oil system from the engine, so you have to add oil to the gas manually.

I unraveled all of the electrical tape around the wire bundle in the engine, and found that they were caused a short in removing the old oil delivery system, which also connected to the CDI computer. The short made the engine think it was out of oil, limiting the RPMs down to 2000.

I am not sure if this is it, but hopefully I will get out on the water sometime within the next few days to test it out.

Again, I appreciate all of the help gentlemen!