cam replacement
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"TDC" isn't good enough, it needs to be a very specific TDC.
A four stroke motor requires two full rotations of the crank to go through all four strokes. So the piston passes through TDC twice during that process. On one of those TDC's, both valves are slightly open - so slightly that you really can't reliably tell by just looking. That's called "Overlap TDC" and that's not the one you want. On the other TDC, both valves are completely closed. That's called "Compression TDC" and that's where you need to be.
Here's how to get to Compression TDC and install the rocker box:
I'm going to do a tech tips video on this whole process soon. It's really really important that this is done right and a video would be helpful to show how it's done.
A four stroke motor requires two full rotations of the crank to go through all four strokes. So the piston passes through TDC twice during that process. On one of those TDC's, both valves are slightly open - so slightly that you really can't reliably tell by just looking. That's called "Overlap TDC" and that's not the one you want. On the other TDC, both valves are completely closed. That's called "Compression TDC" and that's where you need to be.
Here's how to get to Compression TDC and install the rocker box:
- Elevate the rear wheel, remove the spark plugs, and put the trans in 5th gear
- Using the rear wheel, rotate the motor forward while watching the intake pushrod (the one closest to the carb) of the cylinder you're positioning. When you see it move down, you're on the compression stroke.
- On your bike, there's no timing window through which to see a TDC mark, which isn't there anyway. So to find compression TDC from this point, stick a plastic straw through the spark plug hole. Continue rotating the motor forward using the rear wheel. When the straw is all the way up and starts going down, you just passed compression TDC. Back it up to somewhere close to compression TDC. For the purpose of installing the rocker box, it doesn't have to be exact. Both valves are completely closed from about halfway up the compression stroke to about halfway down the following power stroke, and anywhere between those two points will work. Compression TDC is just about halfway in between those two points so it makes a good reference.
- Install that rocker box. Put in the four big bolts first and tighten them down gradually as you'll be opening the valves as you go. Install the three on the floor of the rocker box and the two allen headed screws in the corners. Torque everything to spec.
- Then put the rocker box top on and tighten the four screws to spec. A little Loctite blue on those screws is a good idea, they love to loosen up. The factory screws have some thread locker on them but it's generally only effective the first time the screws are used, so put some Loctite blue on them.
- You need about ten minutes between when you first tightened down the four big rocker box bolts and when you proceed from here. It probably took you that long to torque all the bolts and install the top anyway, but just in case you're really fast, make sure it's been ten minutes. What's happening is your lifters are "bleeding" down. Oil is escaping the lifters slowly, and as it does, the valves are closing. When the valves land on their seats, the bleeding process stops. If you proceed to the next step without this bleed-down process completed, you risk bending valves.
- Once you're sure bleed down time has elapsed, turn your wheel forward while watching the other cylinder's intake pushrod and repeat the whole process on that cylinder. Note that you'll be fighting the assembled side's valve spring pressure as you turn the rear wheel this time, that's normal.
I'm going to do a tech tips video on this whole process soon. It's really really important that this is done right and a video would be helpful to show how it's done.
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gabarnes28
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Jul 21, 2013 10:41 AM
