Yamaha 4 HP, cranks, runs, but fails to come back from idling down to shift gears

[FYC PC]

Yamaha F4LMHA
6BV L 1028317
On a Colgate 26 sailboat used in sailing class.
Cranks easy (pull rope), goes full range of rpms until you idle down to change gears, then loads up and will not come back up to rpm.
I have changed plug and added SeaFoam to tank. No better.
It usually stays bogged down for five to ten minutes, then comes roaring back, usually just as we get the sails up or we return to the slip and it is no longer needed.
When it is bogged down, you can lift the latch on the cowling, ease the cowling up a slight bit and it revs up immediately!
I have inspected the cowling and it seems to be getting plenty of air.
I do not see anything that looks like the cowling is binding.
Throttle is full open in attached video.
Next steps?
Thanks

 

[tpenfield]

I have the Yammy 3 HP, which is pretty much the same thing. Finicky motors for sure. I think the reason is that they are 1 cylinder engines and as such not really designed to go low RPM.

After much fiddling with the carburetor and using carb cleaner, I've found that I needed to shift into gear at a higher RPM . . . not the best thing to do in the world, but it seemed to be about the only way to keep the engine running and be able to accelerate.

I often run the engine at half choke, as it seems to lean itself out easily and has a runaway RPM all by itself.

My recommendation would be to get a small twin cylinder outboard (I think you have to go up to about 6 HP)

 

[brodmann]

If it runs better when you take the cowling off then it can only be because that allows more air flow. That means it's running too lean at idle.

 

[topgun3690]

Could be an exhaust leak inside the cowling.....when you crack it open it suddenly gets a shot of good air.

 

[99yam40]

Could be an exhaust leak inside the cowling.....when you crack it open it suddenly gets a shot of good air.

this would be my thought also.
cowling fills up with exhaust gases so not enough o2 for the fuel to burn properly

 

[topgun3690]

this would be my thought also.
cowling fills up with exhaust gases so not enough o2 for the fuel to burn properly

The OP was a one time poster....10 days ago....may never see this.

 

[FYC PC]

The OP was a one time poster....10 days ago....may never see this.

I am checking back in and sincerely appreciate all the input! Our part time mechanic agrees and believes he has identified a gasket he will try replacing. This weekend, we just removed the cowling and ran without it. No problems except for the increased volume.

 

[topgun3690]

I am checking back in and sincerely appreciate all the input! Our part time mechanic agrees and believes he has identified a gasket he will try replacing. This weekend, we just removed the cowling and ran without it. No problems except for the increased volume.

Glad you came back......sounds like you are on the right track.