Valve lash...food for thought..

This one's half rant, half tech-tip. It's been one of those days. Spent all morning filling out tedious paperwork and eventually convinced myself to spend the evening doing my traditional springtime valve-lash adjustment (which takes a while in my case as all 4 carbs need to come off). As I'm adjusting the last valve I fumble and drop the feeler gauge, and it lands squarely between the battery terminal and the top of my rocker arm in a shower of sparks and smoke, zapping a nice chunk out of the roller in the process. It's off to the side and it's not binding so I believe it should be ok to run for a year while I source some spares, but it's one of those times where you feel like testing how well torque wrenches can fly. 


[You must be registered and logged in to see this image.]

I did however make an interesting discovery...

For the longest time I'd always slightly fouled the #8 spark plug. On my old engine I'd attributed it to a piston ring. Then when the new engine did it, I thought it must instead be the Holley Dominator intake over-fueling that specific runner. However when the individual runner intake of the Webers also started doing it I was stumped. Or until now I think

I grew up wrenching on small block chevs (Canada always lags a bit behind in the Hotrod world, so they're still quite popular to this day), and had always adjusted valve lash as per the GM factory service manual (for 1971 Camaro Z28 in this case, where you set a specific series of lashes at TDC on Cyl #1 and the rest at 360 degrees from that, so TDC on cyl #6). When I was browsing the comp-cams website recently (as you do), I stumbled upon their video on setting valve lash and noticed they used a different method called EOIC (setting intake lash when the exhaust valve is opening, and setting exhaust lash when the intake valve is closing).

Being the skeptic that I am I decided to check lash separately with three different methods this time around: The GM method, the EOIC method, and using my own logic, when each individual piston hits its own TDC based on the firing order (18436572 on a chev, so #1 at TDC, #8 at 90deg, #4 at 180deg, #3 at 210deg, and so on).

Results: All three methods yielded identical lash for all valves except for the #3 and #5 intake valves which were one thousandth looser with the GM method, and the #8 exhaust valve which was three thousandths looser with the GM method. This would mean that my #8 exhaust valve was not opening as much as it should have using my previous method for lash adjustment, and would account for the fouling plug. I suspect that the GM method works just fine with a mild stock cam, but that the #8 exhaust lifter has started its track on the cam lobe when cyl#1 is at TDC on a high duration cam as in my case. It sometimes pays off to be critical of old habits. Those with hydraulic valves can probably just sit back and laugh, I won't blame you for it.

Hope that fixes it. The "Comp" method is what I have always used. Its the way I was taught.