60 hp carbureted Yammy stumbles and stalls

[ajhand]

I have a 2004 four-stroke carbureted 60 hp Yamaha. Ever since it was new, it would stumble or stall whenever I advanced the throttle quickly. The problem sets in around 2,000 rpm give or take. If I advance the throttle slowly until I get above that range, I am good to go. If I try for a hole shot, I stall. Since the problem came with the brand new motor, I am convinced the problem is not fuel. I have tried different tanks and hoses to no avail.
I checked with a Yamaha dealer and he claims this is a chronic, incurable problem. I can't believe this is possible.

Any advice?

aj hand

 

[rodbolt]

Re: 60 hp carbureted Yammy stumbles and stalls

double check all linkage synchronization, you or your dealer will need the tech mate or equivalent manometer to do this.
if its ok then its gonna be time to inspect the accelerator pump and possible air fuel mixture screws.
the screws are located behind a brass plug that will have to be drilled out. I use a 1/8ths bit and break through the use a self tapping screw to remove them.
but no, thats not a characteristic of that engine, unless its just severly overpropped,.what is WOT RPM ?

 

[ajhand]

Re: 60 hp carbureted Yammy stumbles and stalls

I figured the guy, the owner of the shop, was putting me off. His old time mechanic sort of gave me a wink when he said all carbureted 60s act like that.
Anyway, I'm not overpropped. Tops out a bit over 5,000

aj

 

[rodbolt]

Re: 60 hp carbureted Yammy stumbles and stalls

yes you are overpropped and that engine is very sensitive to it, but I dont think its the entire problem. that engine needs to be propped to be capeable of 6000-6200 RPM with the load its carrying that day.
thats the problem with carbed 4 strokes. to many "old school" techs dont understand fuel delivery and operating theory of a 4 stroke.
as the 4 stroke only makes a power impulse every other time the piston is at top dead center the fuel delivery system is a bit different. it also makes overpropping a bit more critical.

 

[ajhand]

Re: 60 hp carbureted Yammy stumbles and stalls

Thanks Rodbolt. I'll try a different prop with less pitch, and find a guy to check into the link sync, etc.
aj