How the Enrichment Circuit (Choke) Works

Maintenance how-to's and guides for the induction system including the airbox, carburetors, fuel tank, fuel pump, petcock and hoses.
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First, let's stop calling it a choke. I included that word in the title for search's sake ;). A choke controls the air through the carb venturi, affecting how much fuel is pulled through the idle or pilot jets, affecting the air fuel mixture. The XJ does not have a choke. Instead, it is an enrichment circuit.

Here's how the enrichment circuit works:

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  • 1. When you pull the knob on the side of the carbs, a plunger is moved which opens the enrichment circuit to air below the diaphragm. The enrichment plunger acts as an air valve.

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    When you pull out the enrichment plunger, there are two positions. The first position opens the air valve half way. The second opens it all the way. Here's a quick video showing how that works:

    [video][/video]

    2. This allows air flow from the diaphragm area, creating a pressure differential between the air pressure in the float bowl and the air in the enrichment circuit.

    3. This pressure differential causes the fuel from the bowl to be drawn up the enrichment circuit, through an orifice. The wider open the enrichment plunger, the higher the pressure differential, and the more fuel flows into the circuit.

    4. The fuel mixes with the air from the diaphragm area.

    5. This air/fuel mixture flows into the venturi through an orifice, mixing with the air/fuel from the pilot jet and enriching the air/fuel mixture flowing into the engine. Here's where the rich mixture flows into the venturi:

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Now that you know how the enrichment circuit works, here are some places it can fault:
  • The orifice in the fuel bowl is small, about .25mm or so in diameter. It clogs easily; much more easily that the pilot circuit. If it clogs, fuel won't be drawn into the circuit and you won't get an enrichment of the air/fuel mixture. The bike will be difficult to start or may not start at all. If it does start, the idle will be lower when cold than warm (and perhaps too low to continue running).
  • The enrichment plunger has a nitrile seal which keeps it sealed when the circuit is closed. If this is damaged, worn out, or has debris on it, it will leak, allowing fuel to flow into the enrichment circuit even when the knob is fully in. This will enrich the mixture, allowing fuel to flow and will result in a high-idle as the bike warms up. The enrichment plunger is held into the carb body with a threaded fitting. If this fitting is loose or if it leaks air, the same problem will occur;
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