1982 Seca XJ750, ZRX1200 has company now.

[Bonz]

167 mile ride today, bike is flawless. 50 mpg. Hot as the devil's crotch coming back through Colorado Springs, low 90s and stop and go on the interstate for the short section we can't avoid. Didn't miss a beat.

[Bonz]

Will stop updating this thread, seems interest was lost when the problem was solved and problem was the tight #2 exhaust valve vs carburetors or ignition. Hopefully it was helpful for someone down the line that reads it and they investigate a tight valve.

[bikerphil]

I'm glad you got the bike running properly. I never would have never seen it coming, a valve out only a few thou causing such an issue. Is there a clearance difference because it is a turbo?

[Bonz]

Fwiw, it is not an XJ650 Seca Turbo... even though that wouldn't make a difference one way or the other. It is an XJ750 Seca as per the thread title.

Exhaust spec is .006" to .008". .004" clearance is 33% below MIN spec, and 50% below MAX spec. It's not the size of the number, it's the relative amount that it is out of spec.

Think of it like other adjustments on the bike such as fuel level/float height, jet sizes, oil level, compression in cylinders, cam chain position on sprokets, engine timing, etc. Most bikes wouldn't run or cause damage if those adjustments/numbers were 33%-50% different than what they should be.

Whether turbo or a non turbo, overly tight valve(s) would have lead to looking deeper at causes and the same remedy once all other systems check out.

Valves fully seating each firing cycle remove heat from the valve into the head. If the valve is tight, the valve never completely closes and cannot transfer heat to the head through contact. As well, if the engine doesn't make sufficient compression due to an overly tight valve, it will not fire, particularly at idle, At that point, hot exhaust gases when at higher engine speed from what combustion does take place rush by the partially open valve and burn the exhaust valve.