Paul in Oklahoma writes:
Let's see if you've had THIS one happen. Yesterday, I was riding at 60-70 mph into a strong headwind (20+ mph, I estimate). I have OEM engine guards on my '96 Royal with Aeromach highway pegs mounted to them. I also have an OEM windshield without lowers. When I put both my feet up on the highway pegs, the engine began to immediately lose power acting like it was running out of fuel. The moment I moved either foot back to the boards, full power came right back! I tried this again several times with exactly the same result. At slower speeds (< 50 mph) there was no effect. It appears that I was choking the engine with my leg position by creating a vacuum under the airbox.
Interestly enuf, same thing happened to me just the other day. Highway speeds, strong crosswind, temp at about 30F. I had JUST installed a set of National lowers and hand guards to fend off Mother Nature. First impulse when the engine started to sputter was to check the choke to see if I forgot to put it in and put the gas on reserve. Took my feet off the pegs, and ... huh?? Ran like a champ.
THE PROBLEM
This happens because of a radical difference between the fuel bowl and carb mouth pressure. When the bowl difference is greater extra fuel is pushed into the venturi .. when it is less the bike goes lean.
This is very obvious in ram air installations and is eliminated by taping the fuel bowls into the incoming air ducting to keep the pressures equalized. Leaving the hoses off in these cases is very dramatic as the bike will cease to run above certain speeds.
Apparently, installation of accessories such as lowers, fairings, highway bars ... just about anything that affects air flow over the motor, can cause the same problem.
THE SOLUTION
Relocate the bowl vent tubes from the stock position. Look for two black rubber hoses running down the back of the front cylinder from the carb bodies. Both hoses attach to a plate at the bottom of the "V" between the cylinders. To do it right, you'll need to remove the tank and air box (depending on your setup). The hoses attach to rubber "T"s between the carb bodies - you should be able to spin these around to facilitate rerouting.
Locate a convenient mounting point near the air box or filters at approximately the same height as the carb throats. Once again, position will vary based on whatever intake mods you may have done. That way you'll be sure to equalize the pressures at all intakes. I'd also suggest you point the hoses towards the back to reduce any chance of debris wandering in and blocking the vents.
<< Kudos to Paul for pointing out the problem, and special thanx go out to the gurus on the ISRA board who came up with the solution. >>