CRGuy,
There's lots of stuff going on in a four stroke at high rpm that will cause problems. Think of everything that has to happen for a complete four stroke cycle compared to a two stroke:
two stroke:
crank rotates
piston goes up
spark occurs
piston goes down
repeat
four stroke:
crank rotates
timing chain rotates
cam is moved by timing chain
cam pushes on rockers
rockers push on valves
valves open
valves close
crank moves, piston goes back down
All this happening at quick rpm's in a TTR will cause the following to happen:
- Cam chain will "fling" against adjuster with more force. This can cause a few things to happen. 1. Like Rich sez, the chain can break 2. chain pushes out tensioner and slows down the cam rotation (like drive chain slap when you dump the clutch). Thus allowing the piston to come up faster than the valves can move out of it's way. BAM! Piston hits valve.
- Rockers will "jump" on camshaft. This is your basic valve float... The faster the cam rotates, the harder it is for the springs to hold the rocker tight to the cam. This is especially true of a camshaft with an aggressive ramp angle on the closing side of the cam. This will cause the intake valve to stay open too long, again causing piston to valve interference. It will also create big divots in the camshaft where the rocker slams back down on it (after a jump, you've got to land somewhere!).
Okay, so we install a heavier set of springs - yes they are available. No they don't completely solve the problem. If you were to build a set that were heavy enough to handle really high rpm, would that work? Not necissarily. Now the rockers are pressing harder against the camshaft, but all the time. We've created more pressure, and pressure = heat. It also equals cam wear. Oh yea, and it can yank the head off the valve too.
Let's say we solve all these problems. Okay. Now, would a TTR 125 with 2,000 more rpm actually make more power? Not unless the cam you're running was designed to work at that rpm. Otherwise, you'd be making more noise but not accomplishing anything.
Okay, so we redesign the camshaft. Hmmmm - a little more lift, and lots more duration (so the valves still stay open long enough at that faster rpm). Crud! Now the valves are hitting each other! So, we sink the valves into the head, and remove material from their edges. Okay, they're not hitting each other anymore. But we've just reduced the flow by 20%. Now it won't run well on top end because the head doesn't flow well enough, and we've lost that quick hit off the bottom because of the change in cam profile. RATS!
I hope I've shed a little light on what it's like to really design stuff like this to work correctly.
Making major modifications to a four stroke means knowing how the specific engine works, what it's weaknesses are and where it's strengths lie.
By knowing an engine's strengths, you can build towards increasing those characteristics without making the weaknesses worse.
The TTR is not a revver. It wasn't designed as such, and doesn't perform well under these conditions. It is much faster, cheaper, and reliable to build the TTR for more torque and midrange horsepower and GEAR IT RIGHT.
Powroll has tried a different ignition on the TTR, it's the same one they're using on the CRF project bike (like the Wolff system, two stage, programmable). It didn't give any increase in horsepower over the stock ignition.
I'm not saying don't try new stuff, I'm just saying that increasing rpm on these small bikes without understanding the implications is not necissarily the smartest way to spend your hard-earned cash.