Cracked valve guide

[GBStu]

The pic below explains it all:




What could have caused this on the exhaust valve?

I rebuilt this engine about 4 yrs ago, new valve guides, rings, bearings etc (10K miles on engine).

I'm replacing the cylinders and pistons so repairing this isn’t a big issue....just wondering why it happened in the first place?

Thanks, Stu

[911 tweaks]

looks to me that you were running an air/fuel ratio WAY TO LEAN FOR WAY TOO LONG ! I would check EVERYTHING else out very throughly on this engine... sorry for the bad news, however, better to have found this now then when a valve dropped into your combustion chamber and poluted the entire enging & oiling system...
By the way, whas it your 79' SC and what induction running? I would guess CIS??

[GBStu]

It was my 79SC with CIS.

I've actually been running a wide band AFR meter for the past year or so. I've recorded the data on my G2X at a number of DE's, WOT was around 12.5 - 13.5.

What should I be checking? I'll be pulling the heads apart this weekend.

Thanks, Stu

[cgarr]

What was the spec on the valve stems? Were they new valves? I don't find may good used valves anymore, most have a huge amount of taper down in that area. Hard to tell from the pic but that guide looks rather thinned down right there.