Shift, Rattle and Stuck... how bad is it?

[thewheel69]

Hi all, I have a 74 CB750 and this weekend was passing a car and wound her up in 4th with a very rough and noisy rattle shift into 5th. When I later slowed down and tried to downshift... nothing... in either direction.
The clutch works but its definitely stuck in 5th.

So... how bad is this? I'm going to drain the oil and take of the side covers to see what I can see but in looking at the shop manual, it looks like to gain access to the transmission shift fork, I'll have to take the engine out and apart?

Thanks for any advice on how to proceed...

John

[LukeM]

I've read in this forum and others, that full throttle shifts that aren't done right can result in a bent transmission fork.  Sounds like when you're trying to shift out of 5th, nothing happens?  Can't feel anything moving inside?  If so, then yeah, you may be looking at pulling the engine and disassembling it to look at the trans.  

I have not worked on the SOHC bikes, so I readily defer to those who have more intimate knowledge. Wait a little bit and see what some bigger brains suggest.

Good luck, and keep us informed.  We'll help if we can.
Luke M

[thewheel69]

Uuuhhhggg!!! Luke, your questions are affirmative... someone please tell me there is an easy fix to this problem...

Should I even crack the cases or just take it to my local metric moto guy and have him do the fixin'?

[LukeM]

Armed with a good service manual, be it the factory one, Haynes, or Clymer, it's possible for the mechanically inclined person to tear apart and rebuild the motor.  You'll need a GOOD ability with tools, attention to detail, a big clean well-lit place to work on it, patience, some specialized tools, and a decent amount of money for replacement parts. It's good to have someone who's good with bikes/motors to bounce questions off of in case you get stuck.

Something to think about when tearing the motor apart is replacing stuff that's getting old: bearings, seals, rings, valve guides, valve seals, timing chains, etc.  My mechanic referred to it as "While I'm here..."  which put the rebuild cost of my DOHC motor/trans to about $2000 for parts and labor.  That was essentially an overhaul on the engine: the trans was fine, but he recommended new seals and primary chain and stuff.

So, if you can afford to have someone else do it for you, it'll take less time, and cost quite a bit more. If you have a mechanic who's already volunteered to do it for you, so much the better.  Maybe you can work with him to pick up some skills, and maybe defray some of the labor cost.  That's between you and him.

I hope this helps.
Luke M

[Peterwh]

Actually, I hate to disagree, but I think this is not too bad if you are determined.  From memory i dont think you need much in the way of special tools.  Again, just working from memory, you will need to drop the engine out and get it on a bench.  Remove the  clutch, stator, gear change and ignition covers and the chain guard.  Remove the clutch, gear change mechanism and ignition assembly.  You can leave the stator in place.  Turn the engine upside down and split the cases.  Voilà, you now have access to the transmission.  It's scary if you haven't done it before but follow a good manual and you will be ok.  If you have someone to help, and get organized in advance with gaskets etc, my guess is you could do it in a weekend.  The only issue will be what parts you find damaged when you get it open.

[thewheel69]

Thanks for the encouragement. I think this project is within my capabilities and yes, a pre-plan, good manual and a bit of patience is always insurance toward finishing with success! Thanks and I'll let you know what I find out this weekend. (sure wish Honda had done a bit more of their great engineering on this aspect of a fantastic motor!)

Johnny-fit-it

[Peterwh]

Luke, sorry if I disparaged your reply by starting by saying I disagreed.  The only part I disagreed with was the special tools, but then I got carried away with my reply.

[Peterwh]

Personally, I think the hardest part of this repair will be re-assembling the gear change mechanism, it's a pig.  None of the manuals do a good job of explaining it with words or photographs.  Just make sure you keep all the small parts and springs together and in the correct sequence for re-assembly.

The transmission itself is actually quite straightforward

[LukeM]

No worries, Peter. I have no RL experience with the SOHC engines: just the DOHC ones. There are cases where you need access to a special tool or two to get something done.  The Haynes manual does a good job of making substitutions where possible, but it doesn't work for everything.

I agree: putting the transmission back together is probably the hardest part, with all the cogs and forks and gears and such.  That's why I'm a fan of taking pictures with camera or smartphone for a point of reference.

Luke M

[thewheel69]

Well guys, I really appreciate all your input and encouragement! Awesome group here and I look forward to adding my tiny bits of wisdom as they come... so, here are a couple bits I found that I think may help: First is a really cool video on YouTube of the CB750 motor cut-away in operation.

http://www.youtube.com/watch?v=2dTNNmQx5CY

and then attached photo of the engine diagram. I think this may show that the gear shifter mechanisms that are bent can be accessed by taking off the left cover in front of the chain gaurd? I will find out tomorrow!

Fingers crossed.

JF

[TOOLS1]

Hey! All TOOLS are special
TOOLS

[Peterwh]

I'm 98 percent certain you cannot get to the transmission without splitting the cases

[thewheel69]

It looks like I can see the gearshift selection pieces that are suspect just outside of the main transmission case. This shot is focused on the primary gear obviously but just to the lower left you can see what I'm talking about on the outside wall of the main transmission. (of course I could be wrong)

[Peterwh]

What you can see here is the gear change mechanism.  I think the indication is it's the selector forks that are bent.  They are inside the case.  You can see in this photo why you cannot get tomthe transmission by simplynremoving them primary drive guard and the gear change cover.  The two transmission shafts and the selector forks are immediately behind the crankcase wall that you can see in the Lower left of the photo.

[Peterwh]

The video is great. Thanks for posting it.  From second 59 to 1 minute 26 you can see why you need to split the crank case to get to the transmission

[thewheel69]

Peter, I think I now understand where our perspectives differ... I have all along been assuming that the issue is the "gear change mechanism" assembly on the outside of the transmission case... NOT the gear selection forks (left center and right).

So, all thats left here for me to do is drain the oil take off the left cover and inspect if the gear selector is intact... if it is... then we know where the problem is.

Again... thanks so much for all the input folks.

[thewheel69]

SHE'S RUNNING LIKE A TOP!!!

Soooo... here is my contribution to the forum.

After taking the left cover off (you have to drain the engine oil) I immediately found the problem... the retainer spring had popped off of the gear shift spindle!!! (see picture)

Looked around to make sure if anything else was broken/bent, wiped everything down with a clean rag and put the spring back in place and she shifts just fine!

WHEW!!!

Thanks for all your help and insights Luke, Peter, Tools (what the heck did you do!?) ;-)

[TOOLS1]

Now you need to go out, and buy a lottery ticket
TOOLS

[thewheel69]

No kidding!!!

[LukeM]

Congratulations! That was a nice find. Kudos to all involved. This bike thing makes you part detective. I'd be careful fort three near future, to make sure everything is ok.

Luke M