Gearbox giving be grief

[MikeyVeitch]

Im running into a problem with my 1127 engine. Problem is under power, the box jumps from 2nd to neutral and back to 2nd very quickly. As you could imagine, this happening whilst I’m on the throttle is not a fun experience. Through the last week, this has gotten a lot worse and is happening more frequently. I’m willing to place a bet and say that the problem is stemming from the shifter mechanism and I have had no funky sound happening when the bike is on the stand and running in second at any rpm. As you all know, coming across 1127 parts is a pain in the arse so how much of the gearbox components are interchangeable between the 1127 and the b12 motor just in the event that I have to start replacing shit?

Edit: I am also replacing the bottom half of the crank case due to a cracked engine mount. Will a 1100f bottom crank fit onto a 1100r top crank?

Edited May 17, 2023 by MikeyVeitch

[Nik]

Sorry to say, but you cannot replace only one of the cases. The upper and lower cases are machined together, with specific tolerances per crank bearing etc. You'll have to replace the cases together, or repair the engine mount on your current case.

Concerning the gearbox, I'd guess that the dogs on your 2nd/5th gears are rounded off so badly that 2nd gear is slipping out of position when you increase load. What likely happened too is that you tried to keep it in place and your gear shift forks were therefore bent too. Hence the in-and-out-of-drive action.

I'd guess you need to replace 2nd and 5th gears (they are locking into eachother) and check up on your gear shift forks (and possibly the gear drum). 

[clivegto]

As said above 2nd and 5th gear dogs are rounded off usually caused by too many wheelie's.

[MikeyVeitch]

7 hours ago, Nik said:

Sorry to say, but you cannot replace only one of the cases. The upper and lower cases are machined together, with specific tolerances per crank bearing etc. You'll have to replace the cases together, or repair the engine mount on your current case.

Concerning the gearbox, I'd guess that the dogs on your 2nd/5th gears are rounded off so badly that 2nd gear is slipping out of position when you increase load. What likely happened too is that you tried to keep it in place and your gear shift forks were therefore bent too. Hence the in-and-out-of-drive action.

I'd guess you need to replace 2nd and 5th gears (they are locking into eachother) and check up on your gear shift forks (and possibly the gear drum). 

I had to buy both crank cases but my intention was to only replace the bottom one. If I was to rebuild the engine, would it work if I was to build the 1100r components into the 1100f cases. On the subject of the gearbox, how much if it could be replaced with b12 components?

[fatblokeonbandit]

https://oldskoolsuzuki.info/archives/category/the-vault/page/3

 

its all in Frankensteins guide.

 

I was going to read it all to let you know, but cant be arsed

 

 

this one was exactly what you described, jumping in and out of 2nd...

young kid moaning that it was supposed to be buillit proof, but broke..

Talked to his dad when i picked it up, he would start it every morning and immediately wheelie up the road stone cold!!!!!!!!!!!!!

 

 

[wraith]

I think the 1100f cases have different size bearings to the gsxr cases and the b12 gear box is different as well. 

Hope I'm wrong on this one. 

[fatblokeonbandit]

lots of the part numbers are different for the gears unfortunately???

[MikeyVeitch]

5 minutes ago, fatblokeonbandit said:

https://oldskoolsuzuki.info/archives/category/the-vault/page/3

 

its all in Frankensteins guide.

 

I was going to read it all to let you know, but cant be arsed

 

 

this one was exactly what you described, jumping in and out of 2nd...

young kid moaning that it was supposed to be buillit proof, but broke..

Talked to his dad when i picked it up, he would start it every morning and immediately wheelie up the road stone cold!!!!!!!!!!!!!

 

 

I tend to let my bikes warm up and I’m not much of a stunty rider but then you never really know what life an engine has had previously. Are these selector forks damaged or are they useable?

[fatblokeonbandit]

The selector forks in the picture, they are shagged, worn, bent and have overheated..went in the bin

[fatblokeonbandit]

new second hand  ones..

 

old ones.....

 

[fatblokeonbandit]

you can do the gearbox without stripping the top end, just whip the bottom off.

 

 

 

[Gixer1460]

True but you still can't throw some random bottom case on there!

[wraith]

Think the best way forward for you is to buy new gear cogs and maybe dog bones and have your crank case welded up.

[clivegto]

Bought new gears 20 odd years ago for my brand new b12 after owning it a couple of years, seem to remember them been £120 which was a lot god knows what they will cost now. Get the cases welded up and find some good second hand gears is what I would do.

[wraith]

Robinson do them new, £80 each dog bone, 2nd gear £65 and 5th gear £99 

[MikeyVeitch]

Thanks for all the reply’s everyone. I’m going to answer some questions that you had. 
I am striping the complete engine because my bottom crank case has a cracked mount and the top has no number. Needing the number because I intend to actually declare the engine. 
I have found the bearing sizes also. Gsxr-gsf output shaft bearing is 25-62-21.6. Gsx1100f is 25-62-19. Pt no 09262-25103-000. This bearing is the only think I believe I need to replace in order to fit my complete gsxr bottom end with the bandit 1200 gearbox. 
I am feeling a bit more confident as I have removed the bottom case from my b12 donor engine and the gearbox mechanisms look simple as fuck. 

[wraith]

I haven't seen a really good 2nd hand b12 gear box, they all seem to have at least wear/pitting on the teeth.

[Gixer1460]

No OEM cases come with numbers - it was so dealers could restamp the original numbers when replacing cases! Obviously its well open for abuse ie. clone cases but there you go. Seriously a cracked engine mount is a minor fault that can be welded to 'good as new' - there have been cases on here that have massive holes in them that have been repaired. It would be a shame to replace the gearbox and lower case and then knock out the main crank bearings as they weren't matching!

[wraith]

I've had a oil cooled crankcase welded up before, when I put a Conrod through it and that was a big hole.

 

[Dezza]

Crankcase repairs aren't exactly uncommon, especially in classic racing: by far the majority of TZ750s have repaired cases. A cracked engine mount is a minor case repair that is easily sorted by someone who knows what they're doing