Topic: CB750K3 transmission shifting issue

[Mark K]

Haven't been on the forum for awhile, distracted by life, my '73 CB750 project got pushed to the back burner. I finally got a fuel tank installed and turn signals installed (and they work!). Rode the bike around neighborhood. Along with carburetion issues that I have not been able to resolve, I found the transmission will not shift into 4th or 5th gears.

Transmission shifts smoothly into 1st from neutral. Up shifting from 1st to 2nd to 3rd gear, all smooth shifting, no issues. Downshifting same deal, no issues. When I try to shift from 3rd into 4th, transmission acts like I just shifted into neutral and the shifter will not go up anymore (for 5th gear).

Bought bike October 2020, not running, engine seized. I have no idea if the transmission worked previously. Engine and transmission completely disassembled by me and my son.  All gears appeared to be in decent shape, no metal chips/chunks/bits in the drained oil or bottom of the case when I cleaned it out. I reassembled the transmission, per the factory service manual, re-using all gears and internals. I did install new primary and cam chain tensioners, along with new gaskets and seals on the entire engine/transmission.

I appreciate any help/advice/recommendations on what the transmission issue is and how I can fix it.

Thanks in advance!
Mark K

[bryanj]

Possibly a bent selector or damaged gear dogs

[scottly]

Does the neutral light work properly? The neutral switch on the bottom of the engine fits into a groove in the shift drum, which limits any sideways movement of the drum. If the switch isn't all the way in, the drum can move back and forth, instead of the shift forks moving back and forth. The switch needs to bottom out on the case.

[newday777]

Or possibly a loose screw at the end of the shift drum?
Hondaman Mark has found several.

[MauiK3]

Hopefully it's an external thing.

[Mark K]

Does the neutral light work properly? The neutral switch on the bottom of the engine fits into a groove in the shift drum, which limits any sideways movement of the drum. If the switch isn't all the way in, the drum can move back and forth, instead of the shift forks moving back and forth. The switch needs to bottom out on the case.

Scottly,

The neutral light works properly, but I will check to make sure the neutral switch bottoms out on the case.

Thanks,
Mark K

[Don R]

 I had one that acted up, dropping into gear while I was trying to kick start it. The stud holding the shift mechanism was loose, it didn't want to tighten so I got a helicoil set, when I attempted to drill the hole the bit snagged, there was one already in there. The stud's threads were worn so I took one out of a scrap engine and tightened it in with red loctite on it. So far, so good.
 The moral of the story is, check everything. Look for the thrust washers on the shaft, I had a broken one that came out in the oil pan, but I may have put the replacement back incorrectly or missed a resulting bent shift fork, mine won't go far enough into 5th to stay in.
 Same deal, it's a low mile K7 that we've already been into once, now it needs to come back apart. When the top case is off it's easier to check the shifting, when the bottom is off, you just need to get it right and try to shift it before you put the engine back in.
 

[Mark K]

Or possibly a loose screw at the end of the shift drum?
Hondaman Mark has found several.

I will remove the side cover and check the shift drum screw. Hondaman wrote in his book about using blue Loctite on the screw and punching in new retaining notches, not sure if he was talking about this screw or not?  I did not read the section on this in his book before reassembling, my mistake.

Thanks,
Mark K

[Mark K]

Possibly a bent selector or damaged gear dogs

Hoping it will not require an engine removal/teardown but will definitely check those items if I have to go that route.

Thanks,
Mark K

[Mark K]

Hopefully it's an external thing.

That's what I am hoping for as well!

[Mark K]

I had one that acted up, dropping into gear while I was trying to kick start it. The stud holding the shift mechanism was loose, it didn't want to tighten so I got a helicoil set, when I attempted to drill the hole the bit snagged, there was one already in there. The stud's threads were worn so I took one out of a scrap engine and tightened it in with red loctite on it. So far, so good.
 The moral of the story is, check everything. Look for the thrust washers on the shaft, I had a broken one that came out in the oil pan, but I may have put the replacement back incorrectly or missed a resulting bent shift fork, mine won't go far enough into 5th to stay in.
 Same deal, it's a low mile K7 that we've already been into once, now it needs to come back apart. When the top case is off it's easier to check the shifting, when the bottom is off, you just need to get it right and try to shift it before you put the engine back in.

Thanks Don, I will check those items, if I end up having to remove/teardown engine.

Mark K

[bryanj]

Just remember when trying the shifting on the bench both shafts need to turn to get the gears

[HondaMan]

If the round cover on the shift drum is intact and tight, and it will not shift above 3rd gear, I've seen it twice in 750 engines that came to me for repair/rebuild, and I done it numerous more times myself during engine reassembly.

It happens when one is lowering the bottom case onto the top case with the top case upside-down on the cylinders or studs, thus sitting at a sharp angle - then the shifter fork for the 4-5 gear misses its gear's slot and ends up in between the gear and its neighbor instead of in the slot. You can see this from the [open] oil pan with a strong flashlight, looking up into the drips: the one fork will be trapped in a nice cozy slot between the 2 shafts - sorta like a lazy, overfed squirrel, just hangin' out, doing nothing...

[Mark K]

If the round cover on the shift drum is intact and tight, and it will not shift above 3rd gear, I've seen it twice in 750 engines that came to me for repair/rebuild, and I done it numerous more times myself during engine reassembly.

It happens when one is lowering the bottom case onto the top case with the top case upside-down on the cylinders or studs, thus sitting at a sharp angle - then the shifter fork for the 4-5 gear misses its gear's slot and ends up in between the gear and its neighbor instead of in the slot. You can see this from the [open] oil pan with a strong flashlight, looking up into the drips: the one fork will be trapped in a nice cozy slot between the 2 shafts - sorta like a lazy, overfed squirrel, just hangin' out, doing nothing...

Thanks for the reply HondaMan. I know this is probably a dumb question, but I have to ask.

If it is the 4-5 shifter fork having missed it's slot, is removal and disassembling of the engine/transmission the only way to fix the issue?

Thanks,
Mark K

[scottly]

Look for the thrust washers on the shaft, I had a broken one that came out in the oil pan, but I may have put the replacement back incorrectly or missed a resulting bent shift fork, mine won't go far enough into 5th to stay in.
 Same deal, it's a low mile K7 that we've already been into once, now it needs to come back apart.

Hey Don, Flybox had a problem like that where the trans wouldn't go into 5th, and it turned out to be the slightly convex washer that goes behind the clutch basket was installed wrong way out. Check that and the other stuff there before splitting the cases.

[HondaMan]

If the round cover on the shift drum is intact and tight, and it will not shift above 3rd gear, I've seen it twice in 750 engines that came to me for repair/rebuild, and I done it numerous more times myself during engine reassembly.

It happens when one is lowering the bottom case onto the top case with the top case upside-down on the cylinders or studs, thus sitting at a sharp angle - then the shifter fork for the 4-5 gear misses its gear's slot and ends up in between the gear and its neighbor instead of in the slot. You can see this from the [open] oil pan with a strong flashlight, looking up into the drips: the one fork will be trapped in a nice cozy slot between the 2 shafts - sorta like a lazy, overfed squirrel, just hangin' out, doing nothing...

Thanks for the reply HondaMan. I know this is probably a dumb question, but I have to ask.

If it is the 4-5 shifter fork having missed it's slot, is removal and disassembling of the engine/transmission the only way to fix the issue?

Thanks,
Mark K

While there's a bunch of other things that can mimic the issue I outlined above, my version would have happened when the engine was apart at some time before now. Do you know the history of the engine?

The cause shown above by Scottly would follow someone having changed the clutch housing (or removed it), then got that washer backward in the process of reinstalling it. Then it would have had a tough time shifting into 4 & 5 after reassembly as the mainshaft could be pulled about 1mm toward the clutch side.

I've seen 750 bikes that were ridden that way for a long time: the last one I saw like it had to be ridden for a while (10-15 minutes) before it could be shifted into 4th (and then 5th), and the poor owner bought it that way from someone whose shop had screwed it up for him. Apparently, that warmed up the mainshaft just enough to make it longer enough to let it shift!

[newday777]

If the round cover on the shift drum is intact and tight, and it will not shift above 3rd gear, I've seen it twice in 750 engines that came to me for repair/rebuild, and I done it numerous more times myself during engine reassembly.

It happens when one is lowering the bottom case onto the top case with the top case upside-down on the cylinders or studs, thus sitting at a sharp angle - then the shifter fork for the 4-5 gear misses its gear's slot and ends up in between the gear and its neighbor instead of in the slot. You can see this from the [open] oil pan with a strong flashlight, looking up into the drips: the one fork will be trapped in a nice cozy slot between the 2 shafts - sorta like a lazy, overfed squirrel, just hangin' out, doing nothing...

Thanks for the reply HondaMan. I know this is probably a dumb question, but I have to ask.

If it is the 4-5 shifter fork having missed it's slot, is removal and disassembling of the engine/transmission the only way to fix the issue?

Thanks,
Mark K

While there's a bunch of other things that can mimic the issue I outlined above, my version would have happened when the engine was apart at some time before now. Do you know the history of the engine?

The cause shown above by Scottly would follow someone having changed the clutch housing (or removed it), then got that washer backward in the process of reinstalling it. Then it would have had a tough time shifting into 4 & 5 after reassembly as the mainshaft could be pulled about 1mm toward the clutch side.

I've seen 750 bikes that were ridden that way for a long time: the last one I saw like it had to be ridden for a while (10-15 minutes) before it could be shifted into 4th (and then 5th), and the poor owner bought it that way from someone whose shop had screwed it up for him. Apparently, that warmed up the mainshaft just enough to make it longer enough to let it shift!

Interesting. I remember someone on an old goldwing forum mention that happened.

[Mark K]

If the round cover on the shift drum is intact and tight, and it will not shift above 3rd gear, I've seen it twice in 750 engines that came to me for repair/rebuild, and I done it numerous more times myself during engine reassembly.

It happens when one is lowering the bottom case onto the top case with the top case upside-down on the cylinders or studs, thus sitting at a sharp angle - then the shifter fork for the 4-5 gear misses its gear's slot and ends up in between the gear and its neighbor instead of in the slot. You can see this from the [open] oil pan with a strong flashlight, looking up into the drips: the one fork will be trapped in a nice cozy slot between the 2 shafts - sorta like a lazy, overfed squirrel, just hangin' out, doing nothing...

Thanks for the reply HondaMan. I know this is probably a dumb question, but I have to ask.

If it is the 4-5 shifter fork having missed it's slot, is removal and disassembling of the engine/transmission the only way to fix the issue?

Thanks,
Mark K

While there's a bunch of other things that can mimic the issue I outlined above, my version would have happened when the engine was apart at some time before now. Do you know the history of the engine?

The cause shown above by Scottly would follow someone having changed the clutch housing (or removed it), then got that washer backward in the process of reinstalling it. Then it would have had a tough time shifting into 4 & 5 after reassembly as the mainshaft could be pulled about 1mm toward the clutch side.

I've seen 750 bikes that were ridden that way for a long time: the last one I saw like it had to be ridden for a while (10-15 minutes) before it could be shifted into 4th (and then 5th), and the poor owner bought it that way from someone whose shop had screwed it up for him. Apparently, that warmed up the mainshaft just enough to make it longer enough to let it shift!

HondaMan,

I do not know the history of the bike. I was told a guy brought it to the guy I bought it from, who had been asked to try to get it running, then never came back for it. The guy I bought it from was cleaning out his shop and put bike on FB Marketplace, where I found it while searching for a new project. My son and I disassembled the entire bike, including the engine and transmission. I reassembled everything, had to buy a used cylinder and head, Larry Hayes bored and honed the used cylinder, (I bought head and cylinder from Larry), had his old engine shop do the valve job on the head, replaced all intake valves and was able to reuse the exhaust valves.

I reassembled the engine and transmission to include the clutch, so it's possible I installed the clutch washer incorrectly. With all of the great advice/suggestions here, I have a list of things to check prior to an engine removal/teardown. I'll keep checking back for more ideas here and post what I find once I can get time to start working on it.

Thanks again for all the information/suggestions!
Mark K

[HondaMan]

OK, keep us posted! We can walk you thru it.
Happy New Year!