Understanding the CV carburetor pilot circuit / idle mix circuit

[Shuriken]

I had asked before about the technical function of the idle mixture screws on the v-max carbs and had originally thought they metered air flow-only through Pilot Air Jet 1 (PAJ1), but this diagram seems to provide more clarity and a different answer. Someone please make corrections or clarifications as needed.

If our v-max carbs operate the same as depicted below, then our idle mixture screws allow adjustment of the fuel flow through the pilot fuel jet. So turning the screws in/tighter/clockwise until lightly seated will reduce fuel delivery at idle; turning the screws out/looser/counter-clockwise will increase fuel delivery at idle.

Perhaps there can be confusion because applying air pressure through PAJ1 is the technique used to eject loosened idle mix screws when cleaning, so one can conclude that there is indeed an air pathway to the idle mix screws. But as shown below, this is likely an indirect path via backpressure, instead of a direct path!

If this is the case, then the idle mixture screws on the typical CV carburetor should actually be directly metering pilot fuel flow, not air flow. The fuel/air subsequently gets mixed thereafter and delivered to the engine during higher vacuum pressure situations, such as at idle. So yes, you are controlling the mixture of fuel and air overall but you are doing it by increasing or reducing fuel.

In terms of troubleshooting the idle circuit, it can be common, especially with junk ethanol fuel, to encounter v-max carbs with plugged pilot fuel jets inside the jet block. Plugged pilot fuel jets restrict or cut off fuel to the engine during idle and low throttle situations, causing poor idle and low-RPM engine performance. Riders will then often go to the idle mix screws and try to adjust them to "fix" the idle problem and find that the idle mix screws have no effect. No matter how far the screws are loosened, the bike will not idle. And futzing with the mix screws without a proper understanding can lead to broken idle mix screw tips inside the carb body as well as stripped idle mix screw orifice threads inside the carb body, or other costly damage to the carbs. In the case of a used motorcycle I purchased, the latter was the case!

Typical idle mixture settings on an stock-configured v-max might be around 2.5 turns out as a starting point, and should produce a decent engine idle. If it takes more than 5 or 6 or 7 turns out on the screws to achieve idle, for example, something isn't normal. The pilot fuel jets may be too small, partially obstructed, or the carb floats may even be set too lean. If no amount of adjusting the screws produces combustion, the pilot fuel jets may be clogged, or the PAJ1 jet or circuit could be clogged. Conversely, if turning the screws 0.5-1 turn out causes proper idle, the pilot fuel jets may be too large for the given elevation or the pilot air jets are too small/lean for the given elevation.

I suspect the engineers designed the combination of OEM jets on the v-max carb to function well over a good range of elevation and conditions and the pilot fuel jet of 37.5, in combination with the idle mixture screws and pilot air jet size, will allow a proper idle configuration from sea level to perhaps 5 or 6,000 ft elevation.

Significant elevation extremes may also require re-jetting. Sea level to maybe 2-3000 ft may require more fuel and less air, and 6-10000 ft should require less fuel and more air. Examining spark plug color during different throttle positions under load helps tremendously.

I have a feeling many of you get to ride these bikes at lower elevations. I am curious how many have re-jetted for elevations above 5,000ft.

 

[jcox04105q]

I had asked before about the technical function of the idle mixture screws on the v-max carbs and had originally thought they metered air flow-only through Pilot Air Jet 1 (PAJ1), but this diagram seems to provide more clarity and a different answer. Someone please make corrections or clarifications as needed.

If our v-max carbs operate the same as depicted below, then our idle mixture screws allow adjustment of the fuel flow through the pilot fuel jet. So turning the screws in/tighter/clockwise until lightly seated will reduce fuel delivery at idle; turning the screws out/looser/counter-clockwise will increase fuel delivery at idle.

Perhaps there can be confusion because applying air pressure through PAJ1 is the technique used to eject loosened idle mix screws when cleaning, so one can conclude that there is indeed an air pathway to the idle mix screws. But as shown below, this is likely an indirect path via backpressure, instead of a direct path!

If this is the case, then the idle mixture screws on the typical CV carburetor should actually be directly metering pilot fuel flow, not air flow. The fuel/air subsequently gets mixed thereafter and delivered to the engine during higher vacuum pressure situations, such as at idle. So yes, you are controlling the mixture of fuel and air overall but you are doing it by increasing or reducing fuel.

In terms of troubleshooting the idle circuit, it can be common, especially with junk ethanol fuel, to encounter v-max carbs with plugged pilot fuel jets inside the jet block. Plugged pilot fuel jets restrict or cut off fuel to the engine during idle and low throttle situations, causing poor idle and low-RPM engine performance. Riders will then often go to the idle mix screws and try to adjust them to "fix" the idle problem and find that the idle mix screws have no effect. No matter how far the screws are loosened, the bike will not idle. And futzing with the mix screws without a proper understanding can lead to broken idle mix screw tips inside the carb body as well as stripped idle mix screw orifice threads inside the carb body, or other costly damage to the carbs. In the case of a used motorcycle I purchased, the latter was the case!

Typical idle mixture settings on an stock-configured v-max might be around 2.5 turns out as a starting point, and should produce a decent engine idle. If it takes more than 5 or 6 or 7 turns out on the screws to achieve idle, for example, something isn't normal. The pilot fuel jets may be too small, partially obstructed, or the carb floats may even be set too lean. If no amount of adjusting the screws produces combustion, the pilot fuel jets may be clogged, or the PAJ1 jet or circuit could be clogged. Conversely, if turning the screws 0.5-1 turn out causes proper idle, the pilot fuel jets may be too large for the given elevation or the pilot air jets are too small/lean for the given elevation.

I suspect the engineers designed the combination of OEM jets on the v-max carb to function well over a good range of elevation and conditions and the pilot fuel jet of 37.5, in combination with the idle mixture screws and pilot air jet size, will allow a proper idle configuration from sea level to perhaps 5 or 6,000 ft elevation.

Significant elevation extremes may also require re-jetting. Sea level to maybe 2-3000 ft may require more fuel and less air, and 6-10000 ft should require less fuel and more air. Examining spark plug color during different throttle positions under load helps tremendously.

I have a feeling many of you get to ride these bikes at lower elevations. I am curious how many have re-jetted for elevations above 5,000ft.

 

[jcox04105q]

I have a 2003 with 10,000 miles, original owner. Clogged pilot jets have been a recurring problem. For the last ten years the bike has been ridden infrequently and every time it sits for several months the pilot jets are clogged resulting in the common idles ok but almost impossible to increase rpms.

The only solution I can think of is to shut off the fuel pump and run engine until carbs drain. I can think of 2 solutions, squeeze off the fuel feed line or connect a make or brake fuel pump wire continuity switch. Any thoughts?

 

[Radioguylogs]

Draining the carb bowls in the Vmax is easier than any bike around.

Look on the side of the carbs where the black hoses stick out. Just open the drain screw and the gas comes out the hose. It couldn't be easier.

However, I prefer to leave my carb bowls full. The gaskets don't dry out and there is less condensation.

 

[MaxMidnight]

You must have seen this sectional diagram which depicts what goes where, but just in case you haven't...

 

[gcurtis102]

I bought my v max new in 89 and have been fiddling with the pilot circuit since about 93.

Didn't really make much headway until internet forums for the bike became commonplace.

Currently have the carbs tuned and it is running better than ever. I live at 5000 feet and often ride over 10,000 foot mountain passes. Since new the bike had always ran rich, you could smell it when stopped.

Did a thorough clean in ultrasonic cleaner. After some trial and error settled on a 147.5 main jet and stock pilot jets. Pilot screw is set 2 1/4 turns out. Bike has great throttle response from idle and no hesitation at launch. The gassy smell is also gone.

One ritual I perform at the end of the day is to drain the float bowls. I use a gasoline rated sirenge that slips on the carb drain hoses and sucks the float bowl. This setup is clean enough that I can put the fuel back in the tank.