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Unread 01-31-2013, 09:25 PM
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Smithers Smithers is offline
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Location: San Luis Obispo, CA
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Speaking about 5 valve per cylinder heads... I bought a used 1991 FZR1000 streetbike as my first on-road bike with my nickels and dimes I had saved up working through high school. I just remember searching through the LA papers looking for good deals on the biggest bike I could afford. I had a Cycle Magazine that showed the FZR1000 as bike of the year a couple years prior so I knew it would be a power monger and it was! I blew away a couple GSXR1100's pulling above 100mph+ more than once. The 5 valve had some incredible power. Yamaha was hot with innovations such as this and they ReAlly put it on the line being the first mfg to make a motocross style 4 stroke that could hang with the 2x's. It was amazing watching crowds of 2 strokes on the supercross tracks and then to see and hear the ONE 4x bike racing in the front was awesome. I need to find the youtube video of that somewhere...

Anyhow one of the KEY ways to make the 4x's competitive and quick to rev was the addition of titanium valves. Of course 2x bikes have no valve train so any type of valve, cam or chain is a huge drain on throttle response. Here is where all this adds up in this story... the titanium valves wear down quite fast compared to steel. This is why my engine had valves that had worn down to the point where they are too tight to open up fast enough for the engine to start. I'm sure this is why the bike was left to collect dust in a barn.

The fix for the casual motocross or enduro rider? Replace the Ti with stainless valves. The down side? About 30-40% heavier valves which leads to slower response. I've ridden a few dirt bikes to know how fast the engine should rev but I'm excited to get this bike revving to see how this 4 valve engine performs. I simply swapped the shims from one valve to the other on the intake side. This got one of the valves spot on with clearance and the other one was just a tad tight still. So I took the one oversized shim to the belt sander to remove some material. Next I polished up the ground side with a knife sharpening stone and all is well. Problem solved at no cost. Time to put her back together. I'll illustrate all this out with pictures later of course.

I will probably go with the heavier and MUCH stronger SS valves when the time comes to replace them. I'm curious to see what how much of a difference it will make when the time comes. I'll bet the bike will still rev just fine and if anything help to make the bike more controllable for my girlfriend. She rides a 2x KTM 200 which has a ton of power and instant power on demand. So this 4x bike will help her focus on her dirt riding technique instead of just holding on for dear life.

Last edited by Smithers; 01-31-2013 at 09:31 PM.
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