Shifting down after 4th gear

Another problem has cropped up with the same (!) bike.

This one is being built practically from scratch with a 3/9 setup. The rear mech seems to shift down fine up until the fifth gear or so, then it needs two clicks to move it to the 4th. After that it behaves until I click into 3, when it jumps past it to the big gear.

changing up is fine´. increasing tension makes it change better from fifth, but above fifth it clicks and rattles.

Any thoughts how I can cure this? I've checked the hanger is straight and even changed it in case I missed something. Is it a worn mech?
 

figbat

Slippery scientist
How are the cables? There could be a corroded spot somewhere.
 

Ajax Bay

Guru
Location
East Devon
Check verticality of hanger. Otherwise cable outer (even when new the loop before the RD causes the greatest friction: 300 degree turn).
 

All uphill

Rolling along
Location
Somerset
If the hanger is straight and cables are good is the rear derailleur damaged?

Is the cassette sitting properly on the freehub? No wobble? The wheel aligned correctly in the frame? Frame straight?

You've reminded me my 3x9 do everything bike has been doing everything a bit less well than usual. That's this afternoon's job - thorough check over, replace cables and clean rims.
 

Ming the Merciless

There is no mercy
Photo Winner
Location
Inside my skull
Oh and check you have the cogs of the cassette spaced properly.
 
I'd go new chain then jockey wheels rather than derailleur.

Both can make a big difference IME.
New chain already fitted
Oh and check you have the cogs of the cassette spaced properly.

New cassette.

I've lost count of how many mechs I've tried.

It misses differently now but it still won't shift: either low gerar work OR high gears OR it shifts up fine OR it shifts down.

Could it be the shifters?

Seriously wondering about swapping out the freehub and rebuilding as an 8 spd bike. I'm guessing an 8 spd mech wouldn't work on a 9 spd cassette?
 
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Doubtless a dumb question, but you have checked the compatibility of mech and shifter?

I've wondered that too. I'm not sure how to check that. I've been wondering if I've done something really daft like not put the wheel in straight. I haven't done the brakes yet so I may have missed something. The thing is the wheel and cassette are from another bike and worked well on that as far as I can remember; that's why I pinched them...
 
There's compatibility charts from Shimano around - get the model numbers from the lever and mech.

Alternatively, just post them on here and I'm sure someone will be along to confirm

Thanks, I'll have a look; it's probably time for me to learn more of the tech behind bike maintenance anyway.

Mind you I tried at least four today so I'd have expected one to fit.
 
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