Differences
This shows you the differences between two versions of the page.
| Both sides previous revision Previous revision Next revision | Previous revision Next revision Both sides next revision | ||
|
techtalk:evo:carb02 [2020/08/30 03:01] ixl2relax [The Idle Drop Method (IDM)] |
techtalk:evo:carb02 [2020/09/18 03:14] ixl2relax [Enrichener Function (Choke)] |
||
|---|---|---|---|
| Line 9: | Line 9: | ||
| **Design Info** | **Design Info** | ||
| - | From 1989-later, the carburetor installed on Sportsters was made by Keihin. It has a 40mm maximum venturi, but the design of the constant-velocity carb utilizes a vacuum-operated slide to vary the venturi opening to help maintain a good air velocity throughout the carbs operating range. | + | From 1989-later, the carburetor installed on Sportsters was made by Keihin. It has a 40mm maximum venturi, but the design of the constant-velocity carb utilizes a vacuum-operated slide to vary the venturi opening to help maintain a proper air velocity throughout the carbs operating range. |
| - | The CV40 uses a gravity-fed fuel supply through a float controlled inlet valve to fill a fuel supply bowl, from which the internal jets obtain fuel. It also has an enrichment system (mislabeled choke) that provides a pathway for fuel to bypass the carb and be supplied directly into the manifold for better starting. | + | The CV40 uses a gravity-fed fuel supply through a float controlled inlet valve to fill a fuel supply bowl, from which the internal jets obtain fuel. It also has an enrichment system, mislabeled a choke. This provides a pathway for fuel to bypass most of the carb operation and be supplied directly into the manifold, at the rear of the carb, for better starting. |
| There are a number of parts that operate in combination to control the mixing of fuel & air into a combustible fuel mixture. The vacuum slide, needle, main jet, slow jet, idle mixture screw, idle ports, tranfer ports and accelerator pump, all assist in this mixing operation over the range of operating conditions. | There are a number of parts that operate in combination to control the mixing of fuel & air into a combustible fuel mixture. The vacuum slide, needle, main jet, slow jet, idle mixture screw, idle ports, tranfer ports and accelerator pump, all assist in this mixing operation over the range of operating conditions. | ||
| - | The constant-velocity system provides a partial means of compensating for operation at different altitudes. In a fixed system, as altitude increases, the fuel mixture becomes richer due to the thinner air. But the CV system partially compensates for this fact because the vacuum-slide oparation is also affected by the thiner air, thereby supplying less fuel into the mix. | + | The purpose of the carburetor is to atomize fuel into the incoming air such that a proper air-fuel-ratio (AFR) is achieved for varying conditions. The Pilot & Main Jets each use an emulsion tube with a fuel orifice & air holes. Each emulsion tube pulls fuel from the float bowl reserve and is fed air thru small air ports (called air jets) on the face of the carb (filter side). In each case, the emulsion tube produces an air/fuel mixture that is further mixed with the air flowing into the venturi (carburetor throat). |
| + | |||
| + | The air flow is generally controlled by the Throttle Plate along with the vertically moving slide that essentially varies the venturi size to create a Constant Velocity of air (from which it gets it's name). By selecting various Pilot and Main Jets, the AFR can be altered to produce a preferred ratio. Air flowing into the venturi creates a vacuum which pulls the air/fuel mixture from the emulsion tubes into the air stream. | ||
| + | |||
| + | To create a proper idle operation, the Idle Port (on the engine side of the throttle plate) has a separate adjustable screw that allows some of the mixture from the Pilot Jet emulsion tube to be bled directly into the manifold, bypassing the throttle plate. | ||
| + | |||
| + | As the throttle plate is opened, fuel is pulled from the Transfer Ports. With more throttle, fuel is pulled thru the needle jet, in greater quantity as the tapered needle is lifted out of the needle jet opening by the vacuum-operated slide. | ||
| + | |||
| + | The constant-velocity system provides a partial means of compensating for operation at different altitudes. In a fixed system, as altitude increases, the fuel mixture becomes richer due to the thinner air. But the CV system partially compensates for this fact because the vacuum-slide oparation is also affected by the thinner air, thereby supplying less fuel into the mix. | ||
| The CV40 (1989-later) has an accelerator pump, controlled by a rod attached to the throttle cable cam, to provide additional fuel when the throttle is quickly opened for acceleration. It squirts raw fuel into the venturi throat to enhance the fuel mixture along with the quickly added air. | The CV40 (1989-later) has an accelerator pump, controlled by a rod attached to the throttle cable cam, to provide additional fuel when the throttle is quickly opened for acceleration. It squirts raw fuel into the venturi throat to enhance the fuel mixture along with the quickly added air. | ||
| - | The carb has been adjusted to control exhaust emissions. The idle mixture is set at the factory and the adjusting screw is capped because it is not to be adjusted except by factory technicians. | + | The carb has been adjusted to control exhaust emissions. The idle mixture is set at the factory and the adjusting screw is capped because it is not to be adjusted except by factory technicians. The cap can be removed and adjustments can be made. |
| - | Note from the service manual: Adjusting mixture setting by procedures other than specified in this section may be in violation of local regulations. | + | However, note this from the service manual (FYI): Adjusting mixture setting by procedures other than specified in this section may be in violation of local regulations. |
| \\ | \\ | ||
| Line 431: | Line 439: | ||
| | When blipping the throttle, two things happen:\\ \\ 1. You are shooting some fuel in with the accelerator pump.\\ 2. When you blip the throttle, you are actually opening the throttle plate, and that larger opening causes a very [quick/temporary] minor drop in velocity through the carb and reduces the signal (vacuum, pull) across the fuel jets, so you flow less fuel for an instant. The accelerator pump is supposed to compensate for that somewhat. The problem is, with a very short stroke of the accelerator pump rod, like a blip does, not much if any fuel is actually shot, so the pump is not doing its job, in some cases. | | | When blipping the throttle, two things happen:\\ \\ 1. You are shooting some fuel in with the accelerator pump.\\ 2. When you blip the throttle, you are actually opening the throttle plate, and that larger opening causes a very [quick/temporary] minor drop in velocity through the carb and reduces the signal (vacuum, pull) across the fuel jets, so you flow less fuel for an instant. The accelerator pump is supposed to compensate for that somewhat. The problem is, with a very short stroke of the accelerator pump rod, like a blip does, not much if any fuel is actually shot, so the pump is not doing its job, in some cases. | | ||
| - | The second thing to say is that **YOU MUST** get a great tune from everything else before addressing the farts, because fixing other issues may eliminate or minimize the farts anyway. So, get your **timing** set correctly, get your **fuel flow** maximized and make certain you have no **air intake leaks.** Get your **throttle plate** set right (between idle & transfer ports), choose the right idle jet and use the Idle Drop Method to get the **Idle Mixture Screw** (IMS) set correctly. Make sure the bike is thoroughly warmed up before tuning. Each of these areas will need specific settings to match your engine build, whether stock, stage one, 1200/1250/1275 conversion, cams, etc. __BE SURE YOU READ ALL OF THE CARB TUNING INFORMATION IN THIS SECTION.__ | + | The second thing to say is that **YOU MUST** get a great tune from everything else before addressing the farts, because fixing other issues may eliminate or minimize the farts anyway. So, get your **timing** set correctly, get your **fuel flow** maximized and make certain you have no **air intake leaks.** Get your **throttle plate** set right (as much as possible between idle & transfer ports), choose the right idle jet and use the Idle Drop Method to get the **Idle Mixture Screw** (IMS) set correctly. Make sure the bike is thoroughly warmed up before tuning. Each of these areas will need specific settings to match your engine build, whether stock, stage one, 1200/1250/1275 conversion, cams, etc. __BE SURE YOU READ ALL OF THE CARB TUNING INFORMATION IN THIS SECTION.__ |
| The third thing to say is that it should be obvious that to correct a lean condition we need to add fuel to the mix (unless an air leak is the cause of the lean condition). But we need to add fuel carefully to address this quick, temporary, lean condition without drowning the entire carb operating range in extra fuel. A typical response is to increase the idle jet, but if you increase the idle jet larger than a size 45 or 46 you are flooding too much fuel into the transfer ports and the overall mix. | The third thing to say is that it should be obvious that to correct a lean condition we need to add fuel to the mix (unless an air leak is the cause of the lean condition). But we need to add fuel carefully to address this quick, temporary, lean condition without drowning the entire carb operating range in extra fuel. A typical response is to increase the idle jet, but if you increase the idle jet larger than a size 45 or 46 you are flooding too much fuel into the transfer ports and the overall mix. | ||
| Line 579: | Line 587: | ||
| {{:techtalk:evo:carb:cv40-enrichener-assy.jpg|}}((Pics from Phillober t=848307\\ & gm1ll t=1553245))((Annotated by IXL2Relax at the XLForum)) | {{:techtalk:evo:carb:cv40-enrichener-assy.jpg|}}((Pics from Phillober t=848307\\ & gm1ll t=1553245))((Annotated by IXL2Relax at the XLForum)) | ||
| - | An XLForum Thread showing the replacement of the cable is here: http://xlforum.net/forums/showthread.php?t=848307 | + | An XLForum Thread discussing the replacement of the cable is here (but pics are missing): http://xlforum.net/forums/showthread.php?t=848307 |
| \\ | \\ | ||
| Line 590: | Line 598: | ||
| Reversion is created in the engine combustion cycle by the relationship between the cams and the exhaust system. Specifically, between the harmonic waves created in the exhaust pipes and the overlap of the exhaust valve about to close while the intake valve is already opening. This overlap period, where both valves are open at the same time, is usually extended in high lift cams, causing the exhaust system to have a great affect on the cylinder fill operation. The exhaust functioning can have a positive or negative affect. | Reversion is created in the engine combustion cycle by the relationship between the cams and the exhaust system. Specifically, between the harmonic waves created in the exhaust pipes and the overlap of the exhaust valve about to close while the intake valve is already opening. This overlap period, where both valves are open at the same time, is usually extended in high lift cams, causing the exhaust system to have a great affect on the cylinder fill operation. The exhaust functioning can have a positive or negative affect. | ||
| - | If/when reversion is occuring, the intake charge ends up passing through the carb three times: pulled in, pushed out, and pulled back in again. Each time it is picking up fuel making the mixture richer. ((Based on info from Aaron Wilson from the XLForum (aswracing) - http://xlforum.net/vbportal/forums/showthread.php?t=1028905 - Post#12)) | + | If/when reversion is occuring, the intake charge ends up passing through the carb three times: pulled in, pushed out, and pulled back in again. Each time it is picking up fuel making the mixture richer. ((Based on info from Aaron Wilson from the XLForum (aswracing) - http://xlforum.net/vbportal/forums/showthread.php?t=1028905 - Post#15)) |
| You can read more in the REF Section on [[techtalk:evo:carb04#exhaust_pipes_cams_-_power_or_reversion|Exhaust Pipes & Cams - Power or Reversion.]] | You can read more in the REF Section on [[techtalk:evo:carb04#exhaust_pipes_cams_-_power_or_reversion|Exhaust Pipes & Cams - Power or Reversion.]] | ||
