Stuck valve????

[Desburnett]

Hmmm,

BSA A7 Shooting Star 500cc

Rebuild complete...small feature in October Classic Bike. 2 miles on first run out and selected lower gear by mistake momintarialy over revving the motor. Result, running on one cylinder.

Limped home on one cylinder. Felt pipes, LH cold. Checked sparks all ok. Ran a compression test, no compression on LH side.
Removed tappet covers to find LH inlet rocker not engaged with pushrod.

99.999% sure pushrods were fully engaged when fitting rocker cover. I know its tricky but used the comb and checked all was engaged.
Subsequent setting of tappets and checked out ok with no abnormal setting on tappet adjusters. Has started virtually first kick many times and ran to the MOT test centre last week a total trip of 8 miles.

Lifted the rocker box off last night, all checks out ok, rockers run free on spindles, washers and thrust washers in correct place, no sign of damage to rockers or cups.

So suspect inlet valve stick open, cam rotated dropping pushrod out of cup, piston must have touched valve to close it leaving pushrod out of rocker cup.

Will take head off and check further. New springs were fitted but in these days of pattern parts...who knows.

Des

[BSA_54A10]

One of the reason why over head vales were invented was the rev limitations of pushrods.
Some where between 7000 & 9000 rpm the pushrod will part company with either the rocker or the cam followers.
Exactly when depends upon the grind of the cam, the pocketing of the valve, the strength of the valve springs and the contact angle between the pushrod & rocker arm .
Changing back like you did is exactly the receipe for making this happen.
A trick an old racer showed me is to make some clock springs to fit over the rocker arms to push them back down onto the pushrods. He reckons it was worth an extra 500 to 1500 rpm

[A10Boy]

Des that sounds to me like your brief "over rev" bounced a valve and the pushrod simply jumped of the rocker. If you have equal compression on both cylinders now, there wont be any damage.