Carbs and Fuel Systems Tech Tips


Can I Install S&S VFI or IST on my 2004 Bike?
The downside to making changes in gearing or wheel sizes is that it throws off the speedometer reading. If you've got a pre–'96 bike, or custom bike using a mechanical speedometer, you have to change either transmission speedometer drive gear, or front speedometer drive unit, or sometimes even have to change out the speedometer to a different ratio to correct the speedometer reading. Even then, readings may still be only within a couple of mph of being accurate. 

That's where bikes equipped with electronic speedometers are really cool. Either stock or custom bikes can be set up with a '96 and up style electronic speedometer, and in a way that allows changing the speedometer reading without the trouble of swapping out parts to get a correct reading. The 1996–up OEM electronic speedometer itself isn't adjustable. To realize the full potential of the electronic speedometer, S&S designed a nifty little unit that plugs directly into the existing wiring and requires no tools or test equipment to use. It is makes accurately calibrating 1996–up OEM electronic speedometers a snap! It fits all Big Twin and Sportster® models originally equipped with electronic speedometers. It's also great for custom builders using a '96 and up OEM style speedometer and transmission.

The calibrator allows builders freedom to create without concern for speedometer accuracy, because the unit can be adjusted so that the speedometer reads accurately with whatever combination of gearing and tire sizes has been chosen. In most cases, it can be set so that the speedometer is accurate to within .5 percent of actual speed. What does .5% mean in mph? For simplicity sake, let's use 100mph as an example. 1%t of a 100 mph is 1 mph, and half of that one percent (.5) is one half mph. So, recalibrating your speedometer to read within one half mph of actual speed is possible. It is important to note, though, that the calibrator is designed to work with OEM components, and may not be compatible with aftermarket speedometers or transmissions. 

One notable example is that the calibrator will not work with Rev-Tech® transmissions, unless you piggy back two units together. A fairly recent development is that Harley-Davidson® changed the voltage used by the speedometer system from 12 volts to 5 volts. Sharp-eyed customers have noticed this, and our Sales and Tech departments have started getting calls asking if the S&S calibrator will work with 2004 & 2005 models. The answer is yes. S&S Test and Development department has verified that the calibrator will function at five volts and the unit has been tested on 2004 & 2005 models with good results.
How Can I Tell If I Have An Intake Leak?
An often overlooked condition is an intake leak. These can often cause inconsistent problems and conditions. The symptoms vary, hesitation, lean conditions, overheating and many others. An intake leak will not be limited to just the manifold seals. It could be worn throttle shafts, cracked or hard vacuum lines (i.e. V.O.E.S. hose), loose mounts and many others. The easiest way to check for leaks is the following. Fire up the motorcycle and hold it at a steady RPM (2000). Using an aerosol lubricant or cleaner (being careful of painted parts) spray all possible leak points one at a time. If the bike revs up or dies when an area is sprayed, it has a leak and needs repaired. If the condition persists double check your work. New gaskets can be damaged when installed.
What Exhaust Should I Run On My S&S Engine And Are Drag Pipes OK?
If you have an existing 2-into-2 system that uses slip-on style mufflers, whether it is an OEM or an aftermarket system, we recommend the new S&S slip-on mufflers. 

S&S dyno tests achieve almost 8 more horsepower and 5 ft. lbs. of torque on a stock Twin Cam 88 engine using stock header pipes and S&S slip-on mufflers, and S&S Super Sidewinder engines have produced 1-1.1 horsepower per cubic inch using stock style exhaust and S&S slip-on mufflers. These mufflers will allow your engine produce more horsepower and torque than straight-through drag pipes in street and highway driving, and work equally well on both stock and large displacement engines. If you don't already have an exhaust system that accepts the S&S Slip-On mufflers, and you need to buy a complete exhaust system, the RevTech staggered dual, or Python 3 systems work well. 

Exhaust selection is critical for optimal performance. Open drag pipes work reasonably well for peak horsepower numbers on some engines but limit low speed performance on the street and can be nearly impossible to carburet. S&S has not had good results with baffled drag pipes due to the increase of exhaust gas restriction (back pressure).

Special low restriction Cycle Shack XP baffle is available through most retail suppliers, and will help with tuning in some cases. Overall, drag pipes are a poor choice for street use. Drag pipes offer nothing to break up the reversion wave pulse. This wave goes down the pipe and then the wave returns back up the pipes. When this wave is in proper time sync to hit the exhaust valve when it is open (pipe length affects the rpm that this happens) it allows the wave to enter the cylinder and when the intake is open during cam overlap it travels across the top of the piston (it is near top of the cylinder at this time) and out the intake valve. This wave continues out the intake through the carb where the next intake stroke draws it all in again. Each time this wave passes through the carburetor the venturi adds gas and makes the next incoming charge richer creating a flat spot or stumble or worse a blubber at this rpm. The longer the duration cam the worse this situation is. Also larger diameter drag pipes and fishtails seem to make this situation worse.

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