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techtalk:ref:carb01i [2019/11/14 05:53] hippysmack [Main Fuel System] |
techtalk:ref:carb01i [2024/01/10 18:28] (current) hippysmack |
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| ====== Function and Tuning of a Mikuni VM-38-9 ====== | ====== Function and Tuning of a Mikuni VM-38-9 ====== | ||
| - | {{:techtalk:ref:carb:mikuni_vm-38_parts_by_stalkie.jpg?direct&300|}} ((photo by stalkie of the XLFORUM http://xlforum.net/forums/showthread.php?t=1779524)) \\ | + | {{:techtalk:ref:carb:mikuni_vm-38_parts_by_stalkie.jpg?direct&300|}} ((photo by stalkie of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/165379-top-end-rebuild?t=1779524)) \\ |
| ====== Low-speed Fuel System (pilot system) ====== | ====== Low-speed Fuel System (pilot system) ====== | ||
| Line 28: | Line 28: | ||
| A #70 drill bit will go those 8 holes. \\ | A #70 drill bit will go those 8 holes. \\ | ||
| - | {{:techtalk:ref:carb:vm-38_pilot_jet_by_piniongear.jpg?direct&300|}} ((photo by piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?threadid=1505030)) {{:techtalk:ref:carb:vm-38_pilot_jet_location_by_piniongear.jpg?direct&300|}} ((photo by piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?threadid=1505030)) \\ | + | {{:techtalk:ref:carb:vm-38_pilot_jet_by_piniongear.jpg?direct&300|}} ((photo by piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/138683-vm-38-running-entirely-too-rich?threadid=1505030)) {{:techtalk:ref:carb:vm-38_pilot_jet_location_by_piniongear.jpg?direct&300|}} ((photo by piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/138683-vm-38-running-entirely-too-rich?threadid=1505030)) \\ |
| ====== Main Fuel System ====== | ====== Main Fuel System ====== | ||
| Line 59: | Line 58: | ||
| The clip on the needle should be in the middle notch. That is third from the top or third from the bottom. \\ | The clip on the needle should be in the middle notch. That is third from the top or third from the bottom. \\ | ||
| The needle jet is a 166 Q-2 with a single hole. \\ | The needle jet is a 166 Q-2 with a single hole. \\ | ||
| - | {{:techtalk:ref:carb:vm-38_needle_jet_by_piniongear.png?direct&300|}} ((photo by piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?t=835717)) {{:techtalk:ref:carb:vm-38_needle_jet_by_jayd241.jpg?direct&300|}} ((photo by jayd241 of the XLFORUM http://xlforum.net/forums/showthread.php?t=1675923&page=2)) {{:techtalk:ref:carb:1mikuni_vm_needle_jet_types_by_hippysmack.jpg?direct&300|}} ((drawing by Hippysmack)) \\ | + | {{:techtalk:ref:carb:vm-38_needle_jet_by_piniongear.png?direct&300|}} ((photo by piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/98281-38mm-mikuni-correct-jetting-for-an-1981-1000?t=835717)) {{:techtalk:ref:carb:vm-38_needle_jet_by_jayd241.jpg?direct&300|}} ((photo by jayd241 of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/156562-mikuni-vm38-flooding-engine/page2?t=1675923&page=2)) {{:techtalk:ref:carb:mikuni_vm_needle_jet_types_by_hippysmack.jpg?direct&300|}} ((drawing by Hippysmack)) \\ |
| Line 67: | Line 66: | ||
| It will have a .039" diameter hole through the jet. \\ | It will have a .039" diameter hole through the jet. \\ | ||
| You can stick a #61 drill bit through this small hole to check for blockage. That may be easier than removing the jet. \\ | You can stick a #61 drill bit through this small hole to check for blockage. That may be easier than removing the jet. \\ | ||
| - | {{:techtalk:ref:carb:vm-38_air_jet_by_piniongear.jpg?direct&300|}} ((photo by piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?threadid=1505030)) {{:techtalk:ref:carb:vm-38_pic_1_by_piniongear.jpg?direct&300|}} ((photo by piniongear of the XLFORUM)) \\ | + | {{:techtalk:ref:carb:vm-38_air_jet_by_piniongear.jpg?direct&300|}} ((photo by piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/138683-vm-38-running-entirely-too-rich?threadid=1505030)) {{:techtalk:ref:carb:vm-38_pic_1_by_piniongear.jpg?direct&300|}} ((photo by piniongear of the XLFORUM)) \\ |
| ====== Float ====== | ====== Float ====== | ||
| - | |||
| - | |||
| The float serves to maintain a constant level of fuel in the bowl. ((Mikuni VM Carburetor Super Tuning Manual pg 5)) \\ | The float serves to maintain a constant level of fuel in the bowl. ((Mikuni VM Carburetor Super Tuning Manual pg 5)) \\ | ||
| Fuel flows between the needle valve and the valve seat and enters the float chamber. \\ | Fuel flows between the needle valve and the valve seat and enters the float chamber. \\ | ||
| {{ :techtalk:ref:carb:vm38_needle_valve_by_norfleet79.jpg?direct&300|}} | {{ :techtalk:ref:carb:vm38_needle_valve_by_norfleet79.jpg?direct&300|}} | ||
| As the fuel enters the chamber, the float moves up to a pre-determined level due to the action of buoyancy. \\ | As the fuel enters the chamber, the float moves up to a pre-determined level due to the action of buoyancy. \\ | ||
| - | You can see the needle valve seat in the left pic below. ((photo by norfleet79 of the XLFORUM http://xlforum.net/forums/showthread.php?t=1561531&page=2)) \\ | + | You can see the needle valve seat in the left pic below. ((photo by norfleet79 of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/144093-mikuni-vm38-carb-help/page2?t=1561531&page=2)) \\ |
| - | When the buoyancy and the fuel pressure balance out, the needle valve (right) is pushed up into the seat to shut off the incoming fuel supply. ((photo by norfleet79 of the XLFORUM http://xlforum.net/forums/showthread.php?t=1561531&page=2)) \\ | + | When the buoyancy and the fuel pressure balance out, the needle valve (right) is pushed up into the seat to shut off the incoming fuel supply. ((photo by norfleet79 of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/144093-mikuni-vm38-carb-help/page2?t=1561531&page=2)) \\ |
| {{:techtalk:ref:carb:vm38_needle_valve_seat_by_norfleet79.jpg?direct&300 |}} | {{:techtalk:ref:carb:vm38_needle_valve_seat_by_norfleet79.jpg?direct&300 |}} | ||
| The fuel level in the bowl controls the amount of fuel in the fuel mixture. \\ | The fuel level in the bowl controls the amount of fuel in the fuel mixture. \\ | ||
| Line 85: | Line 81: | ||
| Therefore the pre-determined float level should not be changed arbitrarily. \\ | Therefore the pre-determined float level should not be changed arbitrarily. \\ | ||
| - | There is a float bowl baffle plate in the carb which is there to prevent air bubbles from forming in the float chamber. ((piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?t=765846&page=4)) \\ | + | There is a float bowl baffle plate in the carb which is there to prevent air bubbles from forming in the float chamber. ((piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/93137-mikuni-vm38-success-story/page4?t=765846&page=4)) \\ |
| Air bubbles in the float bowl would affect the fuel mixture going into the jets. \\ | Air bubbles in the float bowl would affect the fuel mixture going into the jets. \\ | ||
| Line 91: | Line 87: | ||
| On this particular set of floats, one side of the float tang is set at 21/32", the other side of the float tang is a little lower at 20/32". \\ | On this particular set of floats, one side of the float tang is set at 21/32", the other side of the float tang is a little lower at 20/32". \\ | ||
| - | {{:techtalk:ref:carb:vm38_float_1_by_norfleet79.jpg?direct&300|}} ((photo by norfleet79 of the XLFORUM http://xlforum.net/forums/showthread.php?t=1561531&page=2)) {{:techtalk:ref:carb:vm38_float_2_by_norfleet79.jpg?direct&300|}} ((photo by norfleet79 of the XLFORUM http://xlforum.net/forums/showthread.php?t=1561531&page=2)) {{:techtalk:ref:carb:vm38_float_3_by_norfleet79.jpg?direct&300|}} ((photo by norfleet79 of the XLFORUM http://xlforum.net/forums/showthread.php?t=1561531&page=2)) \\ | + | {{:techtalk:ref:carb:vm38_float_1_by_norfleet79.jpg?direct&300|}} ((photo by norfleet79 of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/144093-mikuni-vm38-carb-help/page2?t=1561531&page=2)) {{:techtalk:ref:carb:vm38_float_2_by_norfleet79.jpg?direct&300|}} ((photo by norfleet79 of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/144093-mikuni-vm38-carb-help/page2?t=1561531&page=2)) {{:techtalk:ref:carb:vm38_float_3_by_norfleet79.jpg?direct&300|}} ((photo by norfleet79 of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/144093-mikuni-vm38-carb-help/page2?t=1561531&page=2)) \\ |
| ====== Enrichment (starting) System ====== | ====== Enrichment (starting) System ====== | ||
| The Mikuni VM does not have a choke. Instead it uses a 'starter system' (more recognized as an enricher). ((Mikuni VM Carburetor Super Tuning Manual pg 5)) \\ | The Mikuni VM does not have a choke. Instead it uses a 'starter system' (more recognized as an enricher). ((Mikuni VM Carburetor Super Tuning Manual pg 5)) \\ | ||
| Line 104: | Line 101: | ||
| ====== Tuning and Adjustments ====== | ====== Tuning and Adjustments ====== | ||
| ===== Adjusting the air screw ===== | ===== Adjusting the air screw ===== | ||
| - | |||
| - | |||
| When the settings are not correct, various symptoms are noticed. ((Mikuni VM Carburetor Super Tuning Manual pg 14)) \\ | When the settings are not correct, various symptoms are noticed. ((Mikuni VM Carburetor Super Tuning Manual pg 14)) \\ | ||
| * **Running rich**: | * **Running rich**: | ||
| Line 132: | Line 127: | ||
| When you have done the best you can with the adjustment, you are done. \\ | When you have done the best you can with the adjustment, you are done. \\ | ||
| - | * Screw the Air Screw in until it touches the seat lightly (closed) then turn it out 1 turn. ((piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?t=1357783&page=3)) \\ **Note**: The most this screw can be turned out is 3 turns. Beyond that the spring will no longer hold pressure and the screw will fall out. \\ | + | * Screw the Air Screw in until it touches the seat lightly (closed) then turn it out 1 turn. ((piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/129648-mikuni-38mm/page3?t=1357783&page=3)) \\ **Note**: The most this screw can be turned out is 3 turns. Beyond that the spring will no longer hold pressure and the screw will fall out. \\ |
| - | * Start the bike and let it warm up fully. \\ Obtain a nice smooth idle with the idle adjustment screw. ((piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?threadid=1505030)) \\ Set it 10% to 15% higher than you would like a normal idle speed to be. | + | * Start the bike and let it warm up fully. \\ Obtain a nice smooth idle with the idle adjustment screw. ((piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/138683-vm-38-running-entirely-too-rich?threadid=1505030)) \\ Set it 10% to 15% higher than you would like a normal idle speed to be. |
| * Then begin turning the Air Screw inward or outward slowly (no more than a 1/4 turn at a time). | * Then begin turning the Air Screw inward or outward slowly (no more than a 1/4 turn at a time). | ||
| * What you are looking for is to find the place where the engine speed is the greatest. | * What you are looking for is to find the place where the engine speed is the greatest. | ||
| Line 143: | Line 138: | ||
| * Now set the final idle speed if needed. \\ Unless you have other problems this adjustment should fix the carb and running problems. | * Now set the final idle speed if needed. \\ Unless you have other problems this adjustment should fix the carb and running problems. | ||
| - | The high idle should respond to turning out the idle screw. ((piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?t=1357783&page=3)) \\ | + | The high idle should respond to turning out the idle screw. ((piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/129648-mikuni-38mm/page3?t=1357783&page=3)) \\ |
| If the idle speed does not drop when you unscrew the screw, then you may have an air leak at the manifold O-rings. \\ | If the idle speed does not drop when you unscrew the screw, then you may have an air leak at the manifold O-rings. \\ | ||
| There is nothing else that could be causing the fast idle unless the slide was truly worn out, and that is not likely on a Mikuni with the heavy brass slide. \\ | There is nothing else that could be causing the fast idle unless the slide was truly worn out, and that is not likely on a Mikuni with the heavy brass slide. \\ | ||
| - | {{:techtalk:ref:carb:vm-38_air_adjustment_screw_by_piniongear.jpg?direct&300|}} ((photo by piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?t=835717)) \\ | + | {{:techtalk:ref:carb:vm-38_air_adjustment_screw_by_piniongear.jpg?direct&300|}} ((photo by piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/98281-38mm-mikuni-correct-jetting-for-an-1981-1000?t=835717)) \\ |
| ====== Troubleshooting ====== | ====== Troubleshooting ====== | ||
| ===== Can't get the engine to idle ===== | ===== Can't get the engine to idle ===== | ||
| - | + | Make one change at a time. If you do several things at once all that is going to happen is confusion. ((piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/129648-mikuni-38mm/page6?t=1357783&page=6)) \\ | |
| - | Make one change at a time. If you do several things at once all that is going to happen is confusion. ((piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?t=1357783&page=6)) \\ | + | |
| Take it one step at a time and put the fires out as they present themselves. \\ | Take it one step at a time and put the fires out as they present themselves. \\ | ||
| Line 173: | Line 167: | ||
| ===== Can't get the engine to start ===== | ===== Can't get the engine to start ===== | ||
| - | If this is a carb issue and if you are getting fuel to the cylinders and you are getting spark at the plugs, it should start. ((piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?t=1357783&page=7)) \\ | + | If this is a carb issue and if you are getting fuel to the cylinders and you are getting spark at the plugs, it should start. ((piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/129648-mikuni-38mm/page7?t=1357783&page=7)) \\ |
| * Make sure the petcock is open. | * Make sure the petcock is open. | ||
| - | * Check the manifold O-rings for leaks: ((piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?t=1357783&page=7)) | + | * Check the manifold O-rings for leaks: ((piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/129648-mikuni-38mm/page7?t=1357783&page=7)) |
| * Remove the carb and feel inside the manifold with using your finger. | * Remove the carb and feel inside the manifold with using your finger. | ||
| * The surface of each O-ring must be smooth and the same all the way around the surface. | * The surface of each O-ring must be smooth and the same all the way around the surface. | ||
| Line 197: | Line 191: | ||
| ===== Flooding ===== | ===== Flooding ===== | ||
| - | |||
| When you have a flooded engine, you want to take the enricher off and open the throttle wide open. \\ | When you have a flooded engine, you want to take the enricher off and open the throttle wide open. \\ | ||
| Then kick or hit the starter button to see if it will fire off. \\ | Then kick or hit the starter button to see if it will fire off. \\ | ||
| Line 207: | Line 200: | ||
| If this persists: \\ | If this persists: \\ | ||
| * Check you petcock and line to the carb. \\ If you don't have a strainer then you'll need to run an inline filter. \\ | * Check you petcock and line to the carb. \\ If you don't have a strainer then you'll need to run an inline filter. \\ | ||
| - | * Also make sure you turn the petcock off every time you shut the engine down. \\ Else gas will continue to drain from the tank through the carb / rings and into the crankcase. \\ Then you may end up with gas and oil blowing out the crankcase breather tube. ((Angell57 of the XLFORUM http://xlforum.net/forums/showthread.php?t=1675923&page=2)) \\ | + | * Also make sure you turn the petcock off every time you shut the engine down. \\ Else gas will continue to drain from the tank through the carb / rings and into the crankcase. \\ Then you may end up with gas and oil blowing out the crankcase breather tube. ((Angell57 of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/156562-mikuni-vm38-flooding-engine/page2?t=1675923&page=2)) \\ |
| * Check the sir screw adjustment. | * Check the sir screw adjustment. | ||
| * Pull the float bowl off and see if there is anything stopping up the inlet seat or sitting in the bottom of the bowl. \\ | * Pull the float bowl off and see if there is anything stopping up the inlet seat or sitting in the bottom of the bowl. \\ | ||
| - | * Never open the enricher while after the engine is at operating temp and idling. It'll kill the engine and flood the overflow hose. ((piniongear of the XLFORUM http://xlforum.net/forums/showthread.php?t=872908&goto=nextnewest)) \\ It's not a choke and it's technically called a 'Starter System' and is just that..... for starting the engine. \\ The starter System uses an enriched mixture and a flood of fuel is the result. \\ The starter system works with a vacuum, so that is the reason you are to give it absolutely no throttle when you start the engine. Use the lever each time you start the engine, regardless if the engine is warmed up, or not warmed up. \\ KEEP YOUR HAND OFF THE THROTTLE WHEN STARTING. After the engine fires up, move the lever to the off position. | + | * Never open the enricher while after the engine is at operating temp and idling. It'll kill the engine and flood the overflow hose. ((piniongear of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/101016-my-1969-xlch-restoration-thread-lots-of-progess-pics?t=872908&goto=nextnewest)) \\ It's not a choke and it's technically called a 'Starter System' and is just that..... for starting the engine. \\ The starter System uses an enriched mixture and a flood of fuel is the result. \\ The starter system works with a vacuum, so that is the reason you are to give it absolutely no throttle when you start the engine. Use the lever each time you start the engine, regardless if the engine is warmed up, or not warmed up. \\ KEEP YOUR HAND OFF THE THROTTLE WHEN STARTING. After the engine fires up, move the lever to the off position. |
| Here, a piece of fuel hose was found hung up into the float valve causing flooding. \\ | Here, a piece of fuel hose was found hung up into the float valve causing flooding. \\ | ||
| - | {{techtalk:ref:carb:vm38_needle_valve_by_norfleet79.jpg?direct&300|}} ((photo by norfleet79 of the XLFORUM http://xlforum.net/forums/showthread.php?t=1561531&page=2)) \\ | + | {{techtalk:ref:carb:vm38_needle_valve_by_norfleet79.jpg?direct&300|}} ((photo by norfleet79 of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/144093-mikuni-vm38-carb-help/page2?t=1561531&page=2)) \\ |
| - | + | ||
| - | + | ||
| \\ | \\ | ||
