1994 T9.9 Yamaha Choke question

[pirkfan]

How does the carburetor on this motor handle choking a cold engine? I can find no sign of an external control for a choke butterfly, there is a black "pot" on top of the carburetor (described in the Yamaha parts list as a "starter set") with two wires attached to it, but when I remove it, it has a needle valve attached. The motor starts but when any additional throttle is used, it immediately dies. When I give it additional fuel by moving the "starter rod" (again from the yamaha parts list) while it's idling, the rpm's do increase dramatically, but it tends to backfire. I think the choke butterfly is stuck in the full on position. And just what the heck does that "Starter set" do anyhow??

 

[sdsaw]

Re: 1994 T9.9 Yamaha Choke question

I'll do my best to explain how the auto choke works on your 9.8 Yam.
There isn't a butterfly choke, rather a starter plunger which is controlled by a vacuum valve which senses the temperature of your head. Richer when cold, leaner when warm. After the motor gets warm enough you should see the plunger change position, I can't remember if it drops or rises up. Manual says the thermostat must not be removed because the sensor will think the motor is cold and richen the mixture.
If you look in a factory repair manual the choke system is explained clearly.

Have you tried to monkey around with the pilot screw?
Seat it lightly and back out 3 turns as a starting point.
Sounds like you may be in need of a carb cleaning.
Don't ask why I know these things other than over the years I've gotten to know mine more than I'd like to.

Hope this helps.

Scott

 

[CatTwentyTwo]

Re: 1994 T9.9 Yamaha Choke question

I had a 95 T9.9. There was no choke plate, that black pot is called a Prime Start Syetem in my manual. It is an electro/thermal valve, when the engine is cold the needle valve is retracted and you have a rich starting mixture. After the engine starts, 12 volts is supplied to the valve and the needle extends as it warms up and closes off the jet. The manual says the resistance of the valve at 68 degrees F should be 4.8 to 7.2 ohms.

 

[pirkfan]

Re: 1994 T9.9 Yamaha Choke question

Just did a carb kit on this motor, and it was a revelation about how the darn thing works. It DOES have a choke butterfly but it's actually tied to the throttle linkage. Normal position when idling is nearly closed (full choke), the throttle linkage opens the butterfly as it gives more throttle. This works OK if the slow idle screw is adjusted properly because IT SETS THE AMOUNT OF OPENING OF THE CHOKE BUTTERFLY AT IDLE. If the slow idle adjustment screw is backed off too far (or vibrates loose to that position) then the choke butterfly is fully closed and that causes some real headaches. The carburetor has a built in fuel pump which gives a shot of fuel as the throttle is opened, if this happens with the choke butterfly fully closed (slow idle screw backed off too far), then the mix becomes WAY TOO RICH, and the motor instantly dies. Unless you have the carburetor torn down and get a peek inside, it's just not obvious what's happening.