Excess Valve Clearance Require Pulling Cylinder Head?

[BackRoadBruce]

I checking clearances on my valves in my 1982 CBX and have one intake valve on #5 that is far enough out of spec that even the largest shim will not bring it back into spec. A .310mm shim will have me at .135mm clearance. According to the shop manual, the most likely cause of the problem is carbon build up on the valve and valve seat. Fixing this requires pulling the cylinder head and cleaning the valve and valve seat. Do I have any other options here, or do I have to suck it up and pull the head?



If you're interested in my valve clearance readings across the board, you can view them in an Excel spreadsheet here.

[Mike Nixon]

Bruce -- Hope my booklet fills you in on the steps to take. Thanks for ordering it! Do a leakdown test if you have or can get the tool. Carbon buildup is very common, but I would be more inclined to think about a worn camshaft. Cam wear is another bugaboo with these engines. Too loose a clearance is also not as common as too tight, which is much oftener the case. :)

[BackRoadBruce]

Thanks Mike. Your comment about clearances being too tight is interesting. This is an issue on my other 82 CBX. This bike has 19K miles on the clock and is not yet running. I "rescued" it from a back yard where it had sat for many years. It's carb are slated for a full rebuild.



Back to the valves. Three of the valves are too tight and will require smaller shims to bring them back into spec. An old dirt bike racer friend told me it was not uncommon for valves to stretch over time and that this was an issue he dealt with many times on his machines. Care to comment further on this phenomenon?



If interested, valve readings from this bike are posted here in an Excel spreadsheet.

[Mike Nixon]

Bruce -- Well, this is a big issue, a big topic and one that is controversial with some folks. The short of it is that the stock valves are fairly susceptible to wear (they're plated soft metal), and the valve seats in the head move around slightly. The first thing means that the valves get wear grooves on their faces quickly, leading to loss of seal. The second means that even carefully assembled engines with good valves can sometimes temporarily lose compression when the engine is hot. Add to this a half dozen more factors and you have an engine that usually develops less than its target 160-170 psi cylinder pressure. :)