Yamaha 90 Primestart Fuel Enrichment on Starting

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Author Topic:   Yamaha 90 Primestart Fuel Enrichment on Starting

jstarzy posted 06-25-2009 06:40 PM ET (US)
[A duplicate article on this exact topic was posted to another discussion area. That article and thread have been deleted. It would be greatly appreciated if authors avoid posting all of their articles in duplicate.--jimh.]
I own a [1989--please use four digits to represent years] Montauk with a 2003 [Yamaha--please use the proper name of the manufacturer] 90-HP carburetor two-cycle motor. Starting the engine cold usually requires about five seconds of cranking. At that point the motor starts, it revs at 1,200-RPM or so, and drops to idle after 20 seconds. I understand the increased idle is [due to] "Primestart" enriching the mixture until the engine comes to temperature. All things considered, this is not bad. No chokes, no pushing the key or lifting a toggle. Just prime the bulb and turn the key; five seconds or so and it kicks.
However, I picked up for my father-in-law this winter a Bayliner 160 with a 2003 [Mercury--please use the full name of the manufacturer] 90-HP two-cycle carburetor motor. That engine requires a push of the key to inject fuel into the carburetors or manifold prior to the cold start. Once this is done, turn the key and it starts instantly! You pretty much don't even hear the starter engage and it's running. I don't recall if it fast idles on its own; I don't think it does. But--instantaneous cold start every time.
Is the [Yamaha motor] working properly? Is the [Mercury fuel enrichment for staring] just a different approach that seems more effective?
It seems [Yamaha] Primestart raises the idle RPM for at least five to ten seconds even if the engine is warm, making for some rough shifts if you're in a hurry. Is this normal? [Give me your] thoughts [about the normalcy of the Yamaha starting with enriched fuel when warm causing rough shifts]?
Thanks--Jim.
jimh posted 06-25-2009 10:08 PM ET (US)
Most carburetor engines use some technique to enrich the fuel-air mixture during start up. I am not familiar with the Yamaha Primestart technique, but from your description I infer that Yamaha Primestart uses fuel enrichment to increase idle speed after start up until the engine reaches operating temperature. Perhaps someone can who is familiar with Primestart can confirm that.
seahorse posted 06-26-2009 08:36 AM ET (US)
The PrimeStart system consists of a small diaphragm fuel pump which delivers fuel from #2 carb bowl through a needle valve and up to the intake manifold where passageways direct the fuel to each cylinder.
A thermostatic valve uses a heater coil powered by the AC alternator current and a wax pellet that expands to close off a needle valve.
When the motor is cold, the valve is open and cranking the motor pulses the special fuel pump and the gas delivered to the intake manifold is the starting enrichment. It is not quick starting and may take up to 10 seconds of cranking to get the motor going.
After the motor starts, the ingition system advances spark timing for a higher idle speed to keep the motor from stalling.
As the motor runs, the AC current heats the valve coil and wax causing expansion which in turn closes off the needle valve and stops the extra fuel flow to the motor.
jimh posted 06-26-2009 09:38 AM ET (US)
Great explanation of the underlying mechanism of the Yamaha PrimeStart fuel enrichment system and engine warm-up process. Thank you.
jstarzy posted 06-27-2009 09:40 AM ET (US)
Sorry...JimH....I placed this question in "performance" by accident.
Seahorse...thanks for the description of Primestart....you've taken the mystery out of a system that seems to have little official documentation on theory of operation...
Seems like the 90HP is just fine per your explanation....the Merc does start faster, though LOL!
Jim.

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Author	Topic: Yamaha 90 Primestart Fuel Enrichment on Starting
jstarzy	posted 06-25-2009 06:40 PM ET (US) [A duplicate article on this exact topic was posted to another discussion area. That article and thread have been deleted. It would be greatly appreciated if authors avoid posting all of their articles in duplicate.--jimh.] I own a [1989--please use four digits to represent years] Montauk with a 2003 [Yamaha--please use the proper name of the manufacturer] 90-HP carburetor two-cycle motor. Starting the engine cold usually requires about five seconds of cranking. At that point the motor starts, it revs at 1,200-RPM or so, and drops to idle after 20 seconds. I understand the increased idle is [due to] "Primestart" enriching the mixture until the engine comes to temperature. All things considered, this is not bad. No chokes, no pushing the key or lifting a toggle. Just prime the bulb and turn the key; five seconds or so and it kicks. However, I picked up for my father-in-law this winter a Bayliner 160 with a 2003 [Mercury--please use the full name of the manufacturer] 90-HP two-cycle carburetor motor. That engine requires a push of the key to inject fuel into the carburetors or manifold prior to the cold start. Once this is done, turn the key and it starts instantly! You pretty much don't even hear the starter engage and it's running. I don't recall if it fast idles on its own; I don't think it does. But--instantaneous cold start every time. Is the [Yamaha motor] working properly? Is the [Mercury fuel enrichment for staring] just a different approach that seems more effective? It seems [Yamaha] Primestart raises the idle RPM for at least five to ten seconds even if the engine is warm, making for some rough shifts if you're in a hurry. Is this normal? [Give me your] thoughts [about the normalcy of the Yamaha starting with enriched fuel when warm causing rough shifts]? Thanks--Jim.
jimh	posted 06-25-2009 10:08 PM ET (US) Most carburetor engines use some technique to enrich the fuel-air mixture during start up. I am not familiar with the Yamaha Primestart technique, but from your description I infer that Yamaha Primestart uses fuel enrichment to increase idle speed after start up until the engine reaches operating temperature. Perhaps someone can who is familiar with Primestart can confirm that.
seahorse	posted 06-26-2009 08:36 AM ET (US) The PrimeStart system consists of a small diaphragm fuel pump which delivers fuel from #2 carb bowl through a needle valve and up to the intake manifold where passageways direct the fuel to each cylinder. A thermostatic valve uses a heater coil powered by the AC alternator current and a wax pellet that expands to close off a needle valve. When the motor is cold, the valve is open and cranking the motor pulses the special fuel pump and the gas delivered to the intake manifold is the starting enrichment. It is not quick starting and may take up to 10 seconds of cranking to get the motor going. After the motor starts, the ingition system advances spark timing for a higher idle speed to keep the motor from stalling. As the motor runs, the AC current heats the valve coil and wax causing expansion which in turn closes off the needle valve and stops the extra fuel flow to the motor.
jimh	posted 06-26-2009 09:38 AM ET (US) Great explanation of the underlying mechanism of the Yamaha PrimeStart fuel enrichment system and engine warm-up process. Thank you.
jstarzy	posted 06-27-2009 09:40 AM ET (US) Sorry...JimH....I placed this question in "performance" by accident. Seahorse...thanks for the description of Primestart....you've taken the mystery out of a system that seems to have little official documentation on theory of operation... Seems like the 90HP is just fine per your explanation....the Merc does start faster, though LOL! Jim.