[QUOTE] I did not say all burnt valves were caused by carbon and I agree most are not. The ones that have large holes that look like they were cut with a cutting torch are the ones that indicate that the valve burn happened all at once. the valve goes from being whole to having a big hole in just a few milliseconds. How do I know this? because all burnt exhaust valves are self-limiting. They burn so far and then the cylinder can't fire and the valve will not burn any more after that. The only way a hole can get that big is for it to happen all at once. It can't happen gradually because the hole would stop getting bigger long before it got to that size.[/QUOTE] bullshit. bullshit. bullshit. gas viscosity at high temperature is not unlike treacle. if you think you're going to "leak" all your viscous burning fuel/air mix out of a tiny hole instantly, your delusional thinking is getting /way/ out of hand. for those who aren't delusional and actually have an interest in learning, valves burn comparatively slowly from a small nucleation point. it typically takes several thousand miles. [engines run for lower mileage in this condition have smaller valve holes. those run for longer periods have larger holes. go figure.] typical causes are incorrect clearance and to a lesser extent, valve defects. jre's post alludes to this because with lean mixtures or unleaded gas, combustion temperatures are higher. so, leaded valves burn if exposed to unleaded temperatures and unleaded valves can burn if mixtures run too lean. of course, you can argue this is not a "valve defect" per se, but the valve metallurgy is insufficient for operating conditions, so it amounts to the same thing. valves can also start to burn if under-worked engines accumulate excess carbon deposits preventing full closure, but that's essentially the same as a clearance issue. [QUOTE] Anyway the point I was making is not how the valve burned but that what you do can have consequences under rare circumstances that never get traced back to root causes.[/QUOTE] "never get traced"??? that speaks volumes about your knowledge level. or the lack of it. [QUOTE] You can never really no for sure what you might have done differently that could have produced a different outcome. The best you can do is play the odds.[/QUOTE] this is why you're still on the shop floor - you have no desire to learn, are too fucking stoooopid to learn even if you did, and are /certainly/ not capable of employing any form of logical thought.