No shift 3rd to 4th. Tear down?

[bikerpet]

I'm pretty sure I know the answer to this one but am hoping to shorten the time taken trouble shooting/repairing.

GasGas TXT Pro 2007.

• Stopped shifting after being dropped (not hard and in soft ground, but dropped nonetheless)
• Adjusted the eccentric until it shifted up and down (full up rotation on the eccentric), but when we went riding realised only shifts as far as third gear. Lever moves up full range but nothing happens going to fourth. 1st, neutral, 2nd and 3rd are fine both directions.
• Took out clutch and checked eccentric. Tried giving the lever a little extra move down to get the internal "click", but no dice.

My guess without any real appreciation of the internals of these things is that a shift fork is bent?

I've seen mention of the "shift pawl" being a problem, but it seems like that problem is consistent through the whole range of gears?

Cheers

Peter

[nigel dabster]

i think tear down.

I have reduced my gear lever as much as i can so it breaks first, no help to you but poi.

Edited September 23, 2018 by nigel dabster

[peterb]

If it were just a soft fall then it is possible that the shift fork has not bent. What can happen is that the cupped sleeve that the shift return spring sits on may have been damaged. This is usually referred to as the top hat, as it looks like a top hat. This is made in two parts, an inner piece that is pressed into a tube in a specific location relative to the spring tang, when the bike is dropped on the gear lever side, the force of the gear lever smacking the ground can rotate the inner piece inside the top hat. Visually it would look ok, only the inner is now out of alignment which does not allow the gears to be selected correctly. This could be what your problem is. The top hat used to be supplied with the return spring attached so it is easy to replace. Try this first before tearing the motor down. It is located behind the clutch basket. Bye, Peter B.

[d2w]

Very interesting Peter. Is your top hat and spring part #10 in the attached diagram?

[lineaway]

 10 is correct, the weld breaks away. This is always good to check before tear down. 10,11 and 12 is probably the cause. Buying the whole assembly is the easy way.

[sectionone]

The crossbar on #10 will break and spin inside the cylinder after a hard hit. My guess is #13 cracked and you have to buy the whole shift shaft assembly #28

Mine cracked after the bike slid down a rock face dragging the shifter upwards.

On my 07 the crossbar on #10 is not welded but pressed in. It is like a fuse, your bike will shift if it is good or won't at all if broken. After it breaks you can tap it out and replace it. I found a piece of deadbolt wood frame reinforcer fits perfect and I keep it as a spare.

[d2w]

I acknowledge that I'm not the original poster (bikerpet) but I'm interested in this repair. What (major) wrenching operations are required when replacing #10? When replacing the entire shaft? Can one do the work after only removing the clutch cover and clutch/basket, or does the case have to be split (after removing the engine from the frame, then removing the cylinder, et al)? Can the shaft seal (#5) be reused or does removing the shaft damage the seal?

Thanks.

[sectionone]

Replacing #10 just take off clutch cover and only tools you need are circlip pliers and allen wrenches. Older bikes like my 07 need to split cases to replace shift shaft. If your bike is old you might consider replacing all the seals and maybe main bearings while the cases are split. When I split my 07 I didn't need a case splitter tool.

[bikerpet]

Thanks everyone for the replies.

I now have everything apart and am replacing bearings seals (probably not the mains as they feel good and I'm getting shy of spending yet more $$$).

It looks to me like a couple of things might have contributed.

• The common kickstart quadrant/idler damage - both have bits missing. There is evidence of bits going into the box and damaging the boss that holds the RH end of the rear shift fork shaft (17) and slight damage to the face of the fork in that area (20). Other than that things look undamaged in the gearbox luckily.
• The fall onto the shifter.

Which is the real cause of the shift problem I don't know.

My problem lies in working out what's not right.

• I've inspected the top hat (10x magnifying glass) and the weld is definitely intact, and the bar is also absolutely undamaged. Even if this was damaged I believe the gears would still shift, just not recentre correctly and need manual repositioning. This was confirmed by Hell Team. In my case the gears would not shift 3rd to 4th whatever I did, 1st to 3rd where fine, suggesting this isn't the culprit anyway.
• Inspected the shift drum (magnifying glass again) it appears perfect, no visible wear on tracks. Slight polish to the internal and external stars, but nothing that looks like wear. Bearings smooth and good.
• Shift drum shaft (28) has about .05 - .07 mm runout (clocked between centres in lathe) toward the shift lever end. Rolling it on a surface table shows no perceptible bend through the main length. The measurement in the lathe is a bit iffy - there was .05mm wobble right at the top-hat end of the shaft, running on a centre. I decided it wasn't worth trying to nail this down as I reckon it's probably what I'd call straight in this application. Perhaps an incorrect assumption.
• Inspected the forks - neither fork is absolutely dead flat back to back. I've had them on the surface table and both have some small deviations, but nothing that really looks significant. Again the Hell Team advised that unless they are badly damaged they would not effect the shifting. The pins show no sign of wear. The surfaces of the forks that engage the discs have differing wear patterns - RH rear fork (20) only has polish at the tips. (19) & (16) have polish along much of the surfaces. None of them look scored, just polished.
• Inspected the fork shafts (17 & 18) - straight and smooth.
• Inspected the internal rollers, carriers and springs - no sign of anything worn, bent or damaged. Both rollers sit symmetrically out from the shaft.
• Cavity the internal rollers run in has very slight wear at the corners the rollers run over as they move in and out, but it is very minimal - only slightly more than "polished".
• External roller (23) also appears fine.
• Selector discs all move freely and no signs of damage.

The most puzzling thing to me is that after disassembling and inspecting everything and checking each component's function (including moving the selector drum up and down the positions) I put it back together in the RH side case and tried changing gears. 1st to 3rd where good but then 3rd to 4th was as if the internal rollers just weren't engaging in the star, the lever just moved up with very little resistance. Obviously impossible to see what was going on in there. This felt about like it did when the bike was together. The difficulty is that after gradually removing each shift fork one at a time again, it started shifting and continues to do so if I put everything back together!

I'm waiting on the replacement bearings. Once I have them I'll put it all together in the RH case again and see what happens. I'm very reluctant to re-assemble it without actually finding the root of the problem though. I'd hate to put it back together and have the same thing re-appear!

I can't decide if the lack of movement of the drum is caused by something going on with the roller/star, or something up around the forks that's jamming the drum. I haven't yet found anything that would obviously effect either. Certainly the movement of the forks as the selector drum rotates isn't frictionless, but given the shafts and gears aren't supported both ends, there is no oil bath and the gears aren't constantly rotating it's hard to tell if it's more than should be expected or not.

Lineaway, you said "10,11 and 12 is probably the cause ". That would be top hat assembly and the internal rollers. What would you expect to be wrong with them? I'm suspicious about that shaft assembly but reluctant to spend nearly $200 on a suspicion.

Puzzled.

Peter

[lineaway]

 Usually it is pretty obvious with the rollers. Usually a broken side plate. I have seen people swear the top hat was fine, but was broken.

 

Edited October 11, 2018 by lineaway

[bikerpet]

11 hours ago, lineaway said:

 Usually it is pretty obvious with the rollers. Usually a broken side plate. I have seen people swear the top hat was fine, but was broken.

 

Thanks lineaway.

Wouldn't a broken top hat effect all gears similarly? This only effects 3rd to 4th. 1, 2 & 3 have always been perfect. And even if the top hat was completely removed the gears will still change, just the shifter won't return correctly.

I'm now wondering if a stray piece of metal from the starter gears was somehow caught in the shift drum stopping the internal roller from engaging for 4th. Clutching at straws.

I'll inspect the roller plates again.

Hopefully I'll get the bearings in today and see how it all works after a thorough clean.

[lineaway]

 Did you remove the top hat with the spring attached in place with zip ties. That is the way it comes as new. To some this is a small price just to make sure. It can be hard to install otherwise, but I have got it done before. But yes a small piece floating around can be a problem too. Good luck as I hate when you have the engine split and the problem is not jumping out at you.

[bikerpet]

1 hour ago, lineaway said:

 Did you remove the top hat with the spring attached in place with zip ties. That is the way it comes as new. To some this is a small price just to make sure. It can be hard to install otherwise, but I have got it done before. But yes a small piece floating around can be a problem too. Good luck as I hate when you have the engine split and the problem is not jumping out at you.

On the principle of "leave no stone unturned" I dismantled the whole thing - not looking forward to reassembling it, but it went together once so it will go together again!