Honda's CR125 was the smallest of the two-stroke Grand Prix class motocross bikes. A liquid-cooled two-stroke single, the CR125 was surprisingly quick for such a small bike, capable of huge bursts of acceleration required to clear big motocross jumps. Excellent throttle response is required, especially when taking big jumps.
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While many Japanese manufacturers often used Mikuni carbs, Honda leant toward Keihin's. The 1994 CR 125 used the Keihin 36mm PJ15HA, sometimes called a "Keihin PJ 36." It's a single 36mm carburettor, which is larger than most carburettors used on 125cc bikes; it is, for example, the same size -- but not model -- carburettor used on Honda's slower-revving XR400R four-stroke.
Motocross bikes like the 1994 CR125 have a few special needs. If a motorcycle takes jumps, its orientation changes, so it has to have a carburettor sympathetic to that need. Otherwise, you may find yourself airborne with no fuel to make adjust the position of the bike by blipping the throttle or to make a smooth transition by rolling the throttle on when you land. Another consideration is that motocross bikes fall over more often the street bikes. So, they need a carburettor that can withstand a little abuse. The Keihin PJ 36 has a vertical plunger-choke, smaller and more protected than tab-chokes often found on street bikes, making less apt to get damaged if you tip the bike over, or pick up debris.
Like most carburettors, a number of adjustments had to be properly set and a number of different sized, interchangeable parts (jets and needle) needed to be properly chosen and matched to the riding style and atmosphere for the bike's optimum performance. Adjustments include the pilot screw, needle screw setting, and float level height. Parts include the slow jet, main jet, and needle.
Changes in Altitude and Atmosphere
Even if you have a stock CR125 with original Keihin PJ36 set to factory settings, it will require adjustments with changes in altitude, temperature, and humidity. Some circumstances need more fuel in the fuel/air ratio, also known as "rich" and some circumstances need less fuel in the fuel/air ratio, also known as "lean." Generally, the carburettor will require a richer mixture in cold temperatures, dry air, or both. Warm temperatures, high humidity, and high altitude require leaner mixtures.
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