Outboard high idle with cover on.

[SVEscapado]

Hi All
My son has a 4hp Yamaha four stroke outboard on his boat which starts OK and idles pretty well for a single cylinder four stroke but the moment the cover is put on the idle speed doubles.
Nothing is touching the throttle linkage or cable. The moment the cover is removed the idle slows to normal.
Any ideas to solve much appreciated.

 

[greeny]

If the cover is not touching anything on the engine when put in place then the only other thing is, could putting the cover on be affecting the airflow into the carb by starving it (other air paths blocked for some reason and an unusually good seal on the cover) but that wouldn't happen immediately and probably wouldn't account for 2x revs. Sounds unlikely but all I can think of.
I think a double/ triple check of what may be being touched/ moved by the cover is needed. Throttle cable, linkages, choke system, is the carb. slightly loose and the cover pressing on it creates an airpath on the gasket. Clutching at straws. I have a similar engine (Yam 4hp 4stroke) and hard to think what it could be. I'm at the boat tomorrow so I'll have a look at mine to see if any possibilities jump out at me.

 

[SVEscapado]

If the cover is not touching anything on the engine when put in place then the only other thing is, could putting the cover on be affecting the airflow into the carb by starving it (other air paths blocked for some reason and an unusually good seal on the cover) but that wouldn't happen immediately and probably wouldn't account for 2x revs. Sounds unlikely but all I can think of.
I think a double/ triple check of what may be being touched/ moved by the cover is needed. Throttle cable, linkages, choke system, is the carb. slightly loose and the cover pressing on it creates an airpath on the gasket. Clutching at straws. I have a similar engine (Yam 4hp 4stroke) and hard to think what it could be. I'm at the boat tomorrow so I'll have a look at mine to see if any possibilities jump out at me.

Thanks for the suggestions.
I've already tried removing the carb and inlet manifold and resealing them but no difference. It's a little confusing as I thought air starvation would slow the thing down not speed up. To be honest I've never had much luck with single cylinder four strokes. I have a 6hp Tohatsu on my tender which never ran smoothly from brand new.
Thanks again.

 

[VicS]

Thinking similarly to Greeny regarding air flow........... Check the air intake in the motor cover, but I think if that was restricted it would have the opposite effect

Also wondering about exhaust gases ( from a leaking gasket perhaps ) being trapped under the cover, but again I think that would have the opposite effect

 

[greeny]

Thinking similarly to Greeny regarding air flow........... Check the air intake in the motor cover, but I think if that was restricted it would have the opposite effect

Also wondering about exhaust gases ( from a leaking gasket perhaps ) being trapped under the cover, but again I think that would have the opposite effect

I agree Vic but as I said, clutching at straws. Looking for something that can happen with no physical contact. Air leak on intake may cause it though but not without physical contact somewhere from the lid to move something.

 

[mattnj]

its must be moving something, the case is flexing maybe, or the catch is pulling something just enough to nudge the throttle, would would need a tiny movement (maybe 1mm) to increase the idle speed....sounds like something isn't right on the lid fitment to me, that's where i would look...cant see it having any air restriction that speeds it up!

 

[greeny]

My Yam 4hp 4 stroke. My engine not be the same model as yours.
Had a look today and the only thing I can see is that the lid is touching / moving the throttle cable causing the revs to increase. On mine. a movement on the cable of 1/2 inch can vary the revs appreciably when in neutral and no load. Pics 1 an 2 show how close to the lid the cable is. Throttle cable is the upper / right hand cable in the bunch. But if it's not touching the lid then that is not the answer.
I'm interested to know the answer to this one.