primer / injector system

Briwil

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Apr 19, 2013
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Can anyone explain how the manual primer/ injector system works on a Walbro WMC carburetor on a 1989 25 hp 2 stroke Mercury (i.e. from the time that the primer knob is pulled out to the time that it leaves the primer system housing). I have a new primer / injector kit to replace the old one and I have a Seloc manual that tells me how to do it but it doesn't tell me how it works. I never like to try to repair something unless I understand how it works. Your response would be greatly appreciated.
 

Texasmark

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Re: primer / injector system

Modern Mercs of which I have and am aware of, do not use a mechanical butterfly type choke any longer. Today it's called "fuel enrichment". On the remote control the key is rotated to the ON position and pushed in prior to starting....the next notch over.

When pushing the key in, voltage is fed to a switch in the engine that allows pressurized fuel (from the pressure the squeeze bulb generates) to flow into all carbs at the rear so that it basically goes into the intake manifold. This gives an enriched fuel charge to the intake manifold and assists in engine starting.

Years ago you had a mechanical lever that you pulled up and it had a butterfly over the air intake of each carb that, when the engine was rolled over in starting, it caused a very rich fuel mixture to enter the intake manifold.

Years later, with remote electronics and all, the activity was controlled with a solenoid operated from the control box. Still mechanical and old fashioned.

Then comes fuel enrichment which still uses a solenoid controlled by the ignition switch, but this time it injects fuel into the rear of the carb....aka intake manifold rather than controlling a rod with a series of butterflies on it.
 

Briwil

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Re: primer / injector system

I would assume that I have the old mechanical type of system and I have no key or starter motor. How is the primer knob at the front of the engine connected to the diaphragm etc. in the primer system housing attached to the side f the float bowl on the carb. Ie: how do they work together?
 

Texasmark

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Re: primer / injector system

The old system used 2 butterflies on the carb. One was the running throttle control and was controlled by the twist grip throttle linkage.

The other was a second butterfly that was mechanically moved across the front of the carb to force a reduction in the air part of the fuel/air mixture to produce a rich mixture.

Later on, when they removed the external mechanical linkage and had some kind of 12v power they installed an electrical solenoid in the engine connected to mechanical linkage again that worked as before putting an additional butterfly across the air intake.

Later on they came up with the system used before fuel injection systems whereby they still had the electrical solenoid, but it was connected to the fuel line and allowed raw fuel to enter the intake manifold thus providing for an enriched fuel supply.

HTH,
Mark
 

schematic

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Re: primer / injector system

Can anyone explain how the manual primer/ injector system works on a Walbro WMC carburetor on a 1989 25 hp 2 stroke Mercury (i.e. from the time that the primer knob is pulled out to the time that it leaves the primer system housing). I have a new primer / injector kit to replace the old one and I have a Seloc manual that tells me how to do it but it doesn't tell me how it works. I never like to try to repair something unless I understand how it works. Your response would be greatly appreciated.


With your setup, the knob actually moves a lever connected to a spring loaded diaphragm. (much like a accelerator pump of olden day)

Each cycle of the primer knob squirts fuel into the throttle body. You can see the lever move when you pump the primer.

On some designs, "turning" the primer knob also provides extra throttle opening for fast idle or troll speed adjustment
 

Briwil

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Apr 19, 2013
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Re: primer / injector system

Thanks for your response. The only lever that I see is at the port side of the carb right above the bowl and in front of the primer system housing. However, it doesn't appear to move at all. Am I looking in the correct area?
 

Briwil

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Apr 19, 2013
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Re: primer / injector system

Schematic, thanks for your explanation regarding the knob moving the lever that is connected to the spring loaded diaphragm. After taking the primer knob, bezel and cam block off of the front of the carb, I noted that the previous owner or mechanic had not connected the primer lever properly to the cam block and as a result, the primer lever was not moving and thus the primer system wasn't getting primed. This is why I was having such a difficult time with cold starts. I had already bought a primer/injector repair kit that I didn't need. However, it will come in handy at a later date. Thanks again for leading me in the right direction.
 
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