Engine rough at high rpms: resolution

[webbslinger]

Back in March I asked you all for opinions on why my newly rebuilt twin cam was running rough starting at about 5800 rpms. The cause has turned out to be a broken outer valve spring on the #2 intake valve. My suspicion is that the spring was defective and taken to the breaking point during the dyno test as suggested by Rohan. I think it failed later the first time it ran in the car even though the rev limiter was at 6500.
Fortunately, both a leak-down test and inspection with a bore scope confirms that the valve is straight and it didn't contact the piston. Bean's (where I got the cams and springs), didn't charge me for replacement springs and we can get them in without taking the head off so luckily repairs shouldn't be too painful.
Actually, I suspected a broken spring awhile ago but neither the builder nor the dyno operator believed that was possible so I continued looking for "car issues". After changing component after component and still having the problem occur at the exact same rpm I figured it had to be the either the ignition module or mechanical.
A new ignition module made no difference so I gave up and took the car to a mechanic who has a chassis dyno. After about one second listening to the engine rev up he said we had to take off the valve cover to see what the hell was going on under there. We pushed down on the tappets to see if they all had the same tension and bingo. Seriously, he was on it so fast a bloodhound couldn't have beat him to it.
Thanks to you all for your help and if there is a sequel to this, knock on wood, I'll let you know. Victor

[gjz30075]

Great job and thanks for the update.

[adigra]

I've been away for a couple of weeks, so only saw this post now, but am really glad to see a resolution. It's amazing what a really good mechanic/dyno man can do in short order and save weeks of aggravation. The dyno man I take my car to did, in an hour, what three separate specialists couldn't in two months.