90 h.p., 2 cycle fuel enrichment problems

[MISTY HARBOR]

I have a 2005 90 horse 2 cycle that has developed a cold start problem. Engine will not start cold unless manual fuel enrichment lever is used. Runs fine otherwise. The enrichment system appears to be very simple, like that used on the old snowmobiles. My understanding is that the solenoid should lift the valve plunger from the seat in the #2 carb., & then close as engine warms. I removed solenoid assy. & checked for plunger retraction while cranking. It didn't move. I checked voltage at the same time, aprox. 4 v . I applied 12 v, still no motion , unit got mildly warm, checks 3.4 ohms. It would appear that unit is not functioning properly, but I am not an expert. This is a new unit, as I initially removed original, applied 12v, it didn't move and I replaced it. I have learned a lot more since then, mostly from this forum, but still don't know enough! PLEASE ADVISE ! THANK YOU!

[boscoe99]

You appear to be confusing the Yamaha PrimeStart device with a solenoid that closes a choke valve to provide additional fuel to assist the motor in starting. The plunger actually extends with the application of electrical power. The plunger is retracted when cold and extends very slowly over time as it becomes heated based on voltage being applied to it.

PrimeStart is Yamaha's term for automatic fuel enrichment.

It might be more than you want to read about, but here is a description of the system operation.

Caution - This relates to your model and similar models only. Not to be confused with PrimeStart for V6 two stroke carbureted models, four stroke carbureted models, HPDI models, OX66 models and four stroke EFI models.

To control fuel enrichment for starting and warm-up, an electrothermal valve/enriching fuel pump assembly is mounted on the carburetor.
The enriching fuel pump is actuated by pulses from the crankcase. The electrothermal valve is heated by power from the lighting coil. The enriching pump draws fuel from the carburetor float bowl and pumps it to the electrothermal valve unit which controls the amount of enrichment fuel. This fuel is then drawn into the intake manifold with the assistance of intake vacuum to enrichen the fuel mixture entering the combustion chamber.

The electrothermal valve is normally open when the motor is cold, with the amount of valve opening determined by ambient temperature. After the engine starts, power from the engine’s lighting coil begins to heat the electrothermal valve. The valve expands as it is heated, so its valve seat starts to close, con*****ing the amount of enrichment fuel passing through it.

The electrothermal valve heater is energized as long as the engine is running. Therefore, the valve opening is continuously reduced, which restricts enrichment fuel flow once the engine has warmed to normal operating temperatures.

[MISTY HARBOR]

90 h.p., 2 cycle enrichment problems

Thank You for the education. With this in mind I will pursue carb. obstructions / diaphragm problems in this circuit. Does this make sense? In regard to proper functioning of the prime start device, are there any function tests to perform? Do you know at what temp it opens? At 75 deg. F., I measured the distance the plunger was extended & it was far enough out to be in a "closed" position when assembled to #2 carb.. Please Advise. Thank You for sharing your knowledge.

[boscoe99]

Yamaha, in their dearth of service manual information, only provides resistance values for the prime start device and advice to measure the distance of the pin with and without electrical power being applied to it.

The pin will extend outward X millimeters when cold. When 12 volts is applied to it the pin will extend. How fast and by how much is not listed in the SM. They don't even mention it extending when power is applied. Only that the distance of the pin is different with power than without power

Crappy SM data from Yamaha.

[99yam40]

Just for the heck of it, what is your starting procedure you are using on this motor when cold?
You do know the procedure is different between prime start and a choke system.

If you open the throttle it will not start properly


If you still have the old one , measure it at room temp then put in refrigerator for a while and measure again to see if it moves farther in the colder it gets.( I do not know if it does open farther the colder it gets)

Hook up to 12 volts for a few minutes and measure it again to see how far it moves out. It takes awhile to heat up the wax pellet inside so it expands to push the needle out.

may just need to clean the passages or you are not using proper procedure for cold starting