Gear conunudrum

[mr_hippo]

This has got me stumped, can you help?
On the big ring and only on two of the mid range sprockets all drive is lost for few revolutions. Doesn't happen on the middle ring or on any other sprockets and at any speed. Usual suspects have been checked; broken, bent or missing teeth, bent rear mech, untuned gears, misaligned chain. The chain is new. I think that we can rule out sticking pawls as this would the whole range and the problem csnnot be replicated or replicated!
Any ideas?
Typo in title, can any mod please correct it? Many thanks

 

[Biker Joe]

You mention the chain is new. Perhaps it's an incompatibility issue, wrong size chain?

 

[eck]

+1 for the new chain being a possible culprit. IME a new chain can skip on some sprockets and not others. Do you have a new(er) cassette you could try? Also, is the lockring on the cassette tight enough?

 

[RecordAceFromNew]

This has got me stumped, can you help?
On the big ring and only on two of the mid range sprockets all drive is lost for few revolutions. Doesn't happen on the middle ring or on any other sprockets and at any speed. Usual suspects have been checked; broken, bent or missing teeth, bent rear mech, untuned gears, misaligned chain. The chain is new. I think that we can rule out sticking pawls as this would the whole range and the problem csnnot be replicated or replicated!
Any ideas?
Typo in title, can any mod please correct it? Many thanks

A long shot, but a worse case of worn alloy freehub splines than pic below is a possibility, if those individual sprockets are not pinned together or on a spider.

 

[02GF74]

what do you mean drive is lost?

if you have fitted a new chain then unless you have a chain wear tool at replaced cahin at 0.75% wear. it is likely the sprockets will have worn. you will find that you tend to use 2 or 3 most of the time so a new cahin will skip on them. my definition of skip is the chain engages in the sprocket, you pedal and long, and it jumps one or two teeth so there is momentary loss of drive and then within 1 or tow revoutionsthe same happens again, nasty grunchy nises happen.

loss of drive can also occur if the rear mech is not properly adjusted, in this case the side plates of the chain sit on top of the sprocket and simialr noises ensue.

finally there is a duff freehub, when the pawls are worn and/or and not engaging in the ratchet, the freehub spins so the drive is not transferred to the wheel via the hub. in this case you may end up truning the crank for much than the other too with no resistance, this can be very bad if you are pedalling hard as the sudden loss of resistnace can result in an accident.