yamaha 3 cylinder 40hp 1988 runs on 1 cylinder

[huck73]

hi need some help here i have a 40 hp yamaha 3 cylinder ,that only runs on 1 cylinder the top one . When i start it ,it idles when i take the plug leads of the bottem 2 plugs but when i take the top 1 of it stalls , firstly i thought cdi so i got a second hand one from the wreckers swap them over but the problem is still there ,next i checked spark and plugs all good blue spark , then i checked the carbies and fuel no probs all clean jets are fine compression is all about 120 ,checked the read valves all look ,there is no reason why my motor wont run please help

 

[SparkieBoat]

Re: yamaha 3 cylinder 40hp 1988 runs on 1 cylinder

you either have a compression, spark, or fuel issue. redo the spark test with a spark gap tester set a 7/16 gap. My money is going to be on the carbs though. just because the jets are clean does not mean they are working right. you have to check your needle valve and floats..I bet you have 2 stuck needle valves..it is a common issue with those model motors. I have no idea why they do it, but the only way I found to stop it was to drain carb bowls after each use if motor was going to sit up more than 1 week.

 

[99yam40]

Re: yamaha 3 cylinder 40hp 1988 runs on 1 cylinder

Spray a little premixed fuel into the bottom 2 carbs while it is running and see if the cylinders will fire, that should tell you if it is a lack of fuel problem and get you going in the right direction.
pilot jet and passages are small and plug easy

 

[huck73]

Re: yamaha 3 cylinder 40hp 1988 runs on 1 cylinder

thanks i will give it a go

 

[bag007]

Re: yamaha 3 cylinder 40hp 1988 runs on 1 cylinder

I had this type of problem with a Johnson..... Check to make sure you have gas going into each carb and then check the that the idle needle adjustment are ok... mine were so loose they spun out thus flooding the engine.. then they were too tight thus not allowing gas through.
One way to check is to start the engine let it idle and then adjust the idle screw... you will hear it kick in when the carb is adjusted and gas is flowing .... you an adjust it down too... also are you sure that all the carbs are getting gas...... I changed out the fuel line between the carbs to a clear tube so i could see the gas going to the other carbs.... one thing that might be happening is that the connecting fuel line from the top carb to the others is blocked thus straving the other carbs of fuel...

 

[bag007]

Re: yamaha 3 cylinder 40hp 1988 runs on 1 cylinder

I had this type of problem with a Johnson..... Check to make sure you have gas going into each carb and then check the that the idle needle adjustment are ok... mine were so loose they spun out thus flooding the engine.. then they were too tight thus not allowing gas through.
One way to check is to start the engine let it idle and then adjust the idle screw... you will hear it kick in when the carb is adjusted and gas is flowing .... you an adjust it down too... also are you sure that all the carbs are getting gas...... I changed out the fuel line between the carbs to a clear tube so i could see the gas going to the other carbs.... one thing that might be happening is that the connecting fuel line from the top carb to the others is blocked thus straving the other carbs of fuel...