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I recently got a Hayden krank Vent Plus to test the effects of a constant vacuum on the crank case. These valves are ported so the one way umbrella valve will only maintain 3 to 5 lbs. of vacuum at all RPM's in the crank case. I have got to say they work very well. Instantly I noticed the idle smoothed out a lot, and there is a lot more low and mid range power. That linked with the vacuum sync ports crossed over between 1/4 and 2/3, I believe I have gotten back everything lost on the low and mid range with the stage three jet kit and K&N pods and Vance and Hines 4 into one exhaust. That's the good news. The bad news is that the valve failed in about three days, sticking in the closed position. Hayden says that fuel in the oil is the only thing that can make the umbrella valve fail like that. While the valve was being sent back to the factory for testing and valve replacement, I immediately changed oil, checked the petcock, and did a check with a clear fuel level test gauge on each bowl watching the fuel rise to it's cut off level and a running idle test, bike on center stand and leveled. All four bowls are dead on spec. on the lower side of the bowl rim, spec. being from the bottom to the top of the rim. I captured and sealed some of the oil I changed, and it didn't have any more fuel smell to it than you would expect from oil that needs to be changed. I got four different mechanics to smell it because my sinuses are really messed up this spring. I believe the valve was a defect, but if fuel is contaminating the valve, how is it getting there? The compression is good on all four cylinders. Could it be that fuel vapor is being sucked directly passed the old valve seals into the valve cover? It may not be a concern if the valve was defective, but this Hayden valve works so well that I don't want to give it up. I got it back today and installed it where I can reach it easily for service and extended the tube down below the muffler with the float bowl overflow vents. Again the idle changed from a mild lope to sewing machine smooth, and bottom end power is way up. One thing I can say for sure, these motors love a vacuum in the crank case and if it will be reliable, it is a definite upgrade for a much improved low end. I rolled it on smoothly from 2000 RPM in fifth gear, without any of the traditional VM coughing. I didn't snap open the throttle, but I also didn't wait for the carbs to catch up either. That has never happened before.