NH 350 engine rattle

[MarkPhillips]

Hi my names Mark
Bought a 1959 Ariel Red Hunter last Autumn complete and in fairly good cosmetic order, been standing for about 3 years.
Basically unrideable due to knackered gearbox carb and brakes.
Spent a lovely winter sorting out a hundred n one bodges and getting things to work as they should including rewire, gearbox rebuild new carb and Thorspark ignition plus lots more.
Pleased to say she starts Ist prod ticks over lovely is smooth n nice to ride if not a little gutless, I would guess 12hp 55mph top end but I’m cool with that love the character.
I am however concerned with top end engine rattle, rises n falls with revs, hot or cold makes no difference is very noticeable.
Can’t find anything obvious.
Valve clearance 6 thou inlet 8 thou exhaust, spark plug colour good, no smoke and compression feels strong to me need decompresser to kick her over.
Engine was supposed to have been rebuilt 1000 miles 6 years ago have some receipts.
Loathe to tear it down when it runs so nicely.
Been told late ally head NH Red Hunters are noisy, am I worrying about nothing.
Any advice would be welcome
Cheers Mark.

[will_curry]

How are you setting the tappet clearances?

According to the User Guide the inlet is set with the exhaust valve just starting
to open and the exhaust is set with the inlet just closed bot being set with the
engine turned in the 'Driving' direction.

[pete.collings]

When setting the exhaust valve clearance, disconnect the valve lifter (by removing the cable from the lifter arm) and rotate the arm to be well clear of any engagement with the rocker arm. I once helped an experienced single owner remove an annoying clatter by doin this!

[nevhunter]

It should be Over 17 Hp Maybe 23 at best, but the Bike is not Light. The alloy head is NOISY. Every rattle comes out through the fins, amplified. If the Motor has done Say 40 K miles it would deserve a strip. Often sitting motors get corrosion in Ball and roller bearings. Big ends fail by pitting and Peeling of the Hardened surfaces Timing gears develop backlash that is Noisy Remove the Oil tank and clean it out THOROUGHLY. Also the Plate and gauze filter at the bottom of the Motor and the Centrifugal hollow bolt in the flywheel and scratch out all hard sludge. Don't round off the Hex head. Use a tight fitting socket and Tap the Head first a few times with a brass bar Propelled by an engineers Hammer A couple of sharp cracks will do wonders. IF it all seems OK put the Minimum amount of Oil in and go for a local test ride, Use a decent oil with a bit of guts in it. Keep your ears open for any Noise Increases of the Mechanical kind. Nev