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Found this on a different site, thought it was interesting....

What four stroke engine from one of the Big Four (Honda, Yamaha, Suzuki, Kawasaki) used intake reeds like those commonly found on two strokes?

The answer is the 81-82 Honda XR500R. I consider it a real oddity as it's the only four stroke design I'm aware of that uses intake reeds just like those found on a two stroke.





This is a 4 valve, rocker operated, single overhead cam design (basically similar to a 400EX). The idea behind using intake reeds in this application was to prevent the carburetor from "double dipping" due to intake reversion at certain RPM.

Intake reversion is when a pulse of pressure actually goes in the reverse direction--up through the intake and carburator. It's caused by the particulars of valve timing events. Intake air picks up fuel as it passes through the carburetor in the normal direction. If that same "fuel loaded" intake air gets pushed back through the carburetor in the opposite direction then it's going to get loaded with fuel again. Carburetors will meter fuel in both directions. They're optimized for metering fuel in only one direction but they will work both ways. So when intake reversion occurs you can wind up with an overly rich mixture--some of the intake air has gone through the carb more than once. This kills throttle response and power.

Part of the problem is that big carburetors weren't as good back then. Getting a carb to work well on great big single cylinder engines was a challenge. Add in some intake reversion and the fueling really gets screwed up. After Honda abandon the 81-82 XR500R cylinder head design they started using dual carb settups (two smaller carbs) for many years. That came with it's own issues and they eventually went back to sinlge carbs on their big singles. But the carbs had gotten better by then as did an understanding of how to mitagate reversion.

Intake reversion of varying degrees is somewhat common in all types of engines (AFAIK). Aggressive cam specifications (or port timing in the case of two strokes) tends to amplify it's strength. It can be a somewhat complicated phenomenon. I'm still trying to learn the ins-and-outs of it all so I'll not try and explain reversion any further--I don't want to get it all wrong and mislead anyone.

Ultimately this idea of intake reeds in a four stroke head didn't pan out for Honda. They used this design on the XR500R for two years before introducing the RVFC (Radial Four Valve Chamber) head that's sill alive and kicking today in the TRX400EX and XR650L.
 

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interesting and kinda neat, :hat:
 
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