Author Topic: CB750 popping out of 4th gear ******** SOLVED (Read 11606 times)

brandEn · « **on:** March 01, 2014, 09:07:11 PM »

CB750 K4
So my fresh rebuild has an issue with popping out of fourth gear under load. When I rebuilt the lower end I was very meticulous and inspected all the gears and associated parts and chose what I believed to be the best condition parts I had on hand. Apparently I missed something. Any ideas of anything it could be that is not a gear or worn dog? It would be cool to fix without splitting the engine. Any advice or words of wisdom is welcome.

ekpent · « **Reply #1 on:** March 02, 2014, 05:25:01 AM »

Hate to say it but sounds like you will be diving back in to re-check those parts that you mentioned. Sure sounds like the dreaded shift drum/fork/dog conundrum.

brandEn · « **Reply #2 on:** March 02, 2014, 08:18:14 AM »

I was hoping there was a solution I could look into that I hadn't thought of that didn't include opening the cases. Doesn't seem that's the case.

Well maybe somebody can help me with exactly which gear, dog and shifter fork needs to be looked at? I have no idea which one is which. That would be a big help. Plus I could look at the parts I already have to see if I need to source anything.

tweakin · « **Reply #3 on:** March 02, 2014, 08:24:27 AM »

Let me know when your ready B. I can take a day off and we will fix it up.

brandEn · « **Reply #4 on:** March 02, 2014, 08:46:56 AM »

Right on homie!

bjbuchanan · « **Reply #5 on:** March 02, 2014, 08:58:39 PM »

I have had a "weak" 4th gear that would really pop under load when my clutch needed to be adjusted properly after a while. After I got a chance to set it on a hot engine it went away except for when I rev it to 10500, that invites all other types of problems anyway tho. Since it is a fresh build maybe that could help?

brandEn · « **Reply #6 on:** March 02, 2014, 09:11:05 PM »

Quote from: bjbuchanan on March 02, 2014, 08:58:39 PM

I have had a "weak" 4th gear that would really pop under load when my clutch needed to be adjusted properly after a while. After I got a chance to set it on a hot engine it went away except for when I rev it to 10500, that invites all other types of problems anyway tho. Since it is a fresh build maybe that could help?

Clutch adjustment huh? I will look at that next time I get the engine warmed up. Worth a shot anyways!

brandEn · « **Reply #7 on:** June 30, 2014, 05:39:54 PM »

Ok so guess what. Engine pulled and cases are apart. I can't find anything wrong. Forks and drum look good as well as all the gear dogs. I am kinda freaking out here. Anyone able to point me in the right direction?

Nic · « **Reply #8 on:** June 30, 2014, 06:24:20 PM »

I'm in exactly the same position, mine has jumped out of 4th 3 times while on full yee haa, I have mine due to go back together in a few days, I have replaced all the trans bearings including the two needle rollers in the primary hub because my trans had a pretty decent whine. I don't reckon there was a thing wrong with any of the ball bearings that came out but the two needle rollers weren't great.
Like you I can't find a single thing, I even bought a used set of forks with the drum n shaft but mine measure up a tad better, so I'm just hoping new bearings will help with the gear jump??? Maybe, maybe not but I can't see anything else that could be a problem.

scottly · « **Reply #9 on:** June 30, 2014, 06:25:14 PM »

I see wear on the dogs; it doesn't take much...

Nic · « **Reply #10 on:** June 30, 2014, 06:36:14 PM »

On the second pic, yeah.

brandEn · « **Reply #11 on:** June 30, 2014, 07:33:53 PM »

All the bearings were replaced even the needle bearings. Scottly when I look at the gears I do see some shiny spots but all the corners at the tops of the dogs look square. Am I overlooking something?

scottly · « **Reply #12 on:** June 30, 2014, 08:07:03 PM »

The shiny spots are the only points of contact, and it only takes a slight amount of wear to allow the gear to get kicked out under load.

bjbuchanan · « **Reply #13 on:** June 30, 2014, 09:21:18 PM »

That second pic definitely shows wear on that dog

I have the same problem with my bike, only 4th. I have resorted to shifting by 6500 in 4th. Problem (not) solved

Nic · « **Reply #14 on:** June 30, 2014, 09:25:50 PM »

Quote from: bjbuchanan on June 30, 2014, 09:21:18 PM

I have the same problem with my bike, only 4th. I have resorted to shifting by 6500 in 4th. Problem (not) solved

lol, I have resorted to no full throttle in 4th.

brandEn · « **Reply #15 on:** June 30, 2014, 10:08:45 PM »

Poached these pics from the Cycle X website. Here is what fourth gear looks like. My fourth gear is the last pic posted above. Correct me if I am wrong but since my 4th gear is the one popping out, 4th is the one that should be replaced right? The condition of the the dogs on the other gears don't effect 4th?

dave500 · « **Reply #16 on:** July 01, 2014, 12:14:16 AM »

if the shaft or the centre bush of the gear allow wobble and any shim/spacers are worn it can be the start of gears jumping out,once they jump a few times hard the dogs are starting to round out,possibly also bending forks slightly.

brandEn · « **Reply #17 on:** July 01, 2014, 10:00:55 AM »

Thanks Dave, I will give it a look over here in a bit

flybox1 · « **Reply #18 on:** July 01, 2014, 10:15:35 AM »

such small tolerances...trans issues suck

especially with the nice weather.
At least you've got other wheels

i'm along for the knowledge....

brandEn · « **Reply #19 on:** July 01, 2014, 10:31:22 AM »

Here is a video of my fourth gear. Got some slop for sure.

SLOPPY 4TH GEAR VIDEO

flybox1 · « **Reply #20 on:** July 01, 2014, 10:53:44 AM »

Nice vid, B.
Hopefully you'll get some good responses. I'm hoping HM chimes in on what is 'allowable' for each, even with the side to side play in your 5th. Your 4th certainly wiggles a bunch

fendersrule · « **Reply #21 on:** July 01, 2014, 12:17:29 PM »

ALL gears should have some very small side-to-side play.

But an actual "WIGGLE" (where it pivots) has to be abnormal.

scottly · « **Reply #22 on:** July 01, 2014, 06:53:04 PM »

When I first bought my bike in '79, it popped out of 4th. I replaced the sliding gear, the one at the top in the Cycle x pics, with a brand new one from Honda. This pretty much fixed the problem for a few years, but it reappeared, as well as other gears starting to pop out. I bought the K7 with the snap-ring issue as a parts bike, but couldn't bear the idea of going back to a stock motor. After the bike had been a basket case for about ten years, I read about under-cutting the dogs, and figured what the hell? I split the K1 cases and took a dremel to the dogs, and so far it's worked well.

Nic · « **Reply #23 on:** July 01, 2014, 06:59:30 PM »

Quote from: scottly on July 01, 2014, 06:53:04 PM

When I first bought my bike in '79, it popped out of 4th. I replaced the sliding gear, the one at the top in the Cycle x pics, with a brand new one from Honda. This pretty much fixed the problem for a few years, but it reappeared, as well as other gears starting to pop out. I bought the K7 with the snap-ring issue as a parts bike, but couldn't bear the idea of going back to a stock motor. After the bike had been a basket case for about ten years, I read about under-cutting the dogs, and figured what the hell? I split the K1 cases and took a dremel to the dogs, and so far it's worked well.

Scott, I like the idea of that but I would think it would be kind of tricky to do, if you don't get it exact on all the dogs on both gears you end up with any number of dogs bearing more or less load???

dave500 · « **Reply #24 on:** July 01, 2014, 07:06:39 PM »

I can see another use for an electric bench mounted chain saw sharpener with a jig made up for the job so they index together?

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Author Topic: CB750 popping out of 4th gear ******** SOLVED (Read 11606 times)

Nic

Nic

Nic

fendersrule

Nic