1990 4 hp yamaha 2 stroke

[Yamahahahaha]

Bought my 1990 4 hp yamaha 2 stroke used . It runs fine ( idle and in gear ) in the back yard on the hose and idles fine on the boat . When I throttle up in gear the motor will run up to good rpm for a short distance , then bog and stall out . It starts right back up and will repeat the issue over and over . It had a under cowl tank that I've cleaned thoroughly , I've rebuilt the Carb , checked the reeds and replaced all associated gaskets but , I'm still having the same issue . Any ideas on where to go next would be greatly appreciated !

 

[QBhoy]

Back to the carb and check diaphragm too I think. Obviously check you have the tank vent open too. Just in case.

 

[Yamahahahaha]

Thanks , I'll look into the diaphragm more closely . I actually tried running the motor slowly with the tank cap just laying on top of the tank to make sure the vent wasn't plugged but yielded the same bog/stall as before .

 

[mike_i]

What condition was the primer bulb in when the motor started to bog down? Run the motor again and when it starts to bog down pump the bulb and see what happens. If there are any fuel filters have you checked them?

 

[QBhoy]

What condition was the primer bulb in when the motor started to bog down? Run the motor again and when it starts to bog down pump the bulb and see what happens. If there are any fuel filters have you checked them?

Dont think it has a primer bulb. I’d assume it’s a built in tank.

 

[Yamahahahaha]

Correct , it's a built in tank . I'm replacing the diaphragms in the fuel pump and see where that gets me .

 

[mike_i]

My boat has a 100 gallon built in tank and a primer bulb, I assumed all outboards have a primer bulb, am I wrong?

Dont think it has a primer bulb. I’d assume it’s a built in tank.

 

[GA_Boater]

My boat has a 100 gallon built in tank and a primer bulb, I assumed all outboards have a primer bulb, am I wrong?

Primer bulb is not needed for a gravity fed tank like the OP has. Some gravity fed small OB's do have a push to prime bulb like some lawn equipment, but not all small OB's have one.

 

[QBhoy]

My boat has a 100 gallon built in tank and a primer bulb, I assumed all outboards have a primer bulb, am I wrong?

Would take a while for a 4hp to go through a 100 gallon tank. Haha. Yeah most small outboards that size have a built in gravity tank of about half a gallon or under. Sometimes an option to run a remote tank too on 4hp though.

 

[boscoe99]

My boat has a 100 gallon built in tank and a primer bulb, I assumed all outboards have a primer bulb, am I wrong?

Well, you are somewhat, sort of, wrong.

Small motors with integral fuel tanks don't need a primer bulb.

Except for if and when a VST is emptied, EFI two strokes and four strokes don't really need a primer bulb. Some folks choose to not have a primer bulb installed because it is one more thing that can go wrong. Disrupt their boating.

Then too, there are a very very few that choose to use an electric pump to send fuel to the motor. In lieu of a primer bulb.

 

[Yamahahahaha]

Thank you for your Input gentlemen . My motor is a 4hp 2 stroke and has an integrated fuel tank . It also has a 3 position petcock and an auxiliary connection for an external tank , if desired . I do not utilize the external hook up . I replaced the diaphragms for the fuel pump as suggested in an earlier post and will post results from that repair .

 

[mike_i]

You're right, I didn't catch the cowl tank, that does make a difference.

Primer bulb is not needed for a gravity fed tank like the OP has. Some gravity fed small OB's do have a push to prime bulb like some lawn equipment, but not all small OB's have one.

 

[tpenfield]

I have a Yamaha 3 HP of similar vintage . . . Single cylinder/ 2-cycle. It does not run very well no matter what I do . . . New carb, fuel system cleaner, etc. I am thinking these engines just don't run very well. At full throttle, the engine will die from fuel starvation . . . at other times it wants to run only on choke or partial choke. Other times it is fine. I had an 8hp Yammy in the past and it was pretty much bullet-proof, so this little Yammy has been a disappointment. I'm thinking that a single cylinder outboard with integrated tank is just not a good engineering feat.

I use the engine as a back-up to my other outboard, which is a Merc 7.5

 

[QBhoy]

I have a Yamaha 3 HP of similar vintage . . . Single cylinder/ 2-cycle. It does not run very well no matter what I do . . . New carb, fuel system cleaner, etc. I am thinking these engines just don't run very well. At full throttle, the engine will die from fuel starvation . . . at other times it wants to run only on choke or partial choke. Other times it is fine. I had an 8hp Yammy in the past and it was pretty much bullet-proof, so this little Yammy has been a disappointment. I'm thinking that a single cylinder outboard with integrated tank is just not a good engineering feat.

I use the engine as a back-up to my other outboard, which is a Merc 7.5

They are brilliant. Had the same issues. Carb clean twice and a diaphragm and it was all good. Also on a 3hp (called a Malta here)

 

[robert graham]

Some 3 way petcocks have small screen/filters than can stop up....check it out...