1/4" ON the pistons?! Holy powerloss bigJ!! haha But really, you're probably meaning your valves and yeah they'll get pretty dirty but they shouldn't be THAT bad. I've seen the intake valves on a '79 400cid from a trans am, THOSE were nasty. Uhmm but I had 109k on my '93 SSEi and still only had fairly minor amounts of carbon buildup. Might wanna take a look at how rich you're running and why. On a fuel injected computer controlled car I don't think you should have that much. It'd be time consuming but maybe you can insure the valve is closed and spray *** loads of carb cleaner right on top of the valve and let it soak for a while. Then maybe use a shopvac to suck the remainder out so it doesn't fall into your cylinder and find itself a new home. Once you're done with the closed valves bump the car or turn it over using a socket to ensure the next valve is closed and repeat. You'll never get it all off, I pulled mine and let them sit in industrial cleaner for almost a week and still couldn't get about 1/8" or less off but it'll make a difference. Think about it, people spend sizeable amounts of time and money getting 0.02" larger valves (Jeff haha
) for that "performance gain", imagine what that sludge is doing.