70HP YAMAHA 1990 only runs on 1/3 cylinders!

[angusgordon16]

I have a 1990 Yamaha 2 stroke 70hp it starts fine but won’t surpass 1500rpm & it cuts out. I unplugged the coils while it was ideling to see if it would change the sound of the engine and I had 2 out of 3 coils off without a change in engine sound or power. Only 1 cylinder was producing power! . Any ideas of what could be causing the 2 cylinders to not produce power? They have spark.

 

[99yam40]

you need
1. good compression,
2. good spark at the proper time,
3. and the proper fuel to air ratio for the fuel to burn correctly
find out which of those is a problem

 

[brodmann]

That's right. Short and sweet! Go to an auto parts store. Most national chains have tool rental. Rent a good compression tester, purchase a $10 spark gap tester and set it for at least 3/8" and make sure spark is strong enough to jump that gap. I would have started by spraying starting fluid in the carbs to see if that make RPM's jump up. If it does, you're not getting gas to that cylinder. I'm guessing you have clogged jets and need a good carb rebuild. Watch a video and look up a carburetor diagram blow up for your motor so you can see where the jets are and get them cleaned out perfectly.
Also, when did it last run? What was recently done to it? When did it last run well and reach a good wide open throttle RPM rate?

 

[angusgordon16]

That's right. Short and sweet! Go to an auto parts store. Most national chains have tool rental. Rent a good compression tester, purchase a $10 spark gap tester and set it for at least 3/8" and make sure spark is strong enough to jump that gap. I would have started by spraying starting fluid in the carbs to see if that make RPM's jump up. If it does, you're not getting gas to that cylinder. I'm guessing you have clogged jets and need a good carb rebuild. Watch a video and look up a carburetor diagram blow up for your motor so you can see where the jets are and get them cleaned out perfectly.
Also, when did it last run? What was recently done to it? When did it last run well and reach a good wide open throttle RPM rate?

That's right. Short and sweet! Go to an auto parts store. Most national chains have tool rental. Rent a good compression tester, purchase a $10 spark gap tester and set it for at least 3/8" and make sure spark is strong enough to jump that gap. I would have started by spraying starting fluid in the carbs to see if that make RPM's jump up. If it does, you're not getting gas to that cylinder. I'm guessing you have clogged jets and need a good carb rebuild. Watch a video and look up a carburetor diagram blow up for your motor so you can see where the jets are and get them cleaned out perfectly.
Also, when did it last run? What was recently done to it? When did it last run well and reach a good wide open throttle RPM rate?

It has good compression and it fires just fine I swamped coils and no difference so they are all good. It bangs healthy on #2 but 1-3 seem weak the engine will run on just the #2 cylinder fine but if I try just 1 or 3 alone I have to keep the throttle to it or it’ll almost die. I sprayed gas in the carbs while all coils were plugged in and it jumped the rpm’s.. carb issue? This seems like a fuel issue to me.

 

[angusgordon16]

you need
1. good compression,
2. good spark at the proper time,
3. and the proper fuel to air ratio for the fuel to burn correctly

you need
1. good compression,
2. good spark at the proper time,
3. and the proper fuel to air ratio for the fuel to burn correctly
find out which of those is a problem

So after cleaning the carb it started banging on all 3 cylinders but 1-3 seem very weak and not firing like they should. I can run the engine on any 1 cylinder alone now but have to keep throttle to 1-3 to keep it alive on #2 it fires up and idles great alone. When I squirt gas with all 3 coils back on it jumped rpm’s on 1-3. Not enough fuel getting to the cylinders? New carb or air fuel mixture ? I’ve tried adjusting it and it didn’t seem to do anything…

 

[99yam40]

not enough fuel due to plugged/restricted passages or a vacuum leak.
something is not letting getting the proper fuel to air ratio into the 1 & 3 cylinders.
heated sonic cleaning machine helps when the carbs are taken completely apart and soaked/ cleaned properly.

 

[angusgordon16]

not enough fuel due to plugged/restricted passages or a vacuum leak.
something is not letting getting the proper fuel to air ratio into the 1 & 3 cylinders.
heated sonic cleaning machine helps when the carbs are taken completely apart and soaked/ cleaned properly.

I will try that thank you for your help and knowledge! I appreciate it a ton

 

[brodmann]

You need to order carb kits and do it correctly with all new gaskets and some time spent on the jets. Here's a video to help you out.

 

[brodmann]

Also, be sure to mark them because they are all different.