Author Topic: 1970 cb750 2nd gear problem (Read 8832 times)

rxdc · « **on:** November 23, 2005, 03:04:40 PM »

I know I will have to strip this engine and I have rebuilt a few of these engines
Riding this bike in second gear up to 7k rpm it jumps out of 2nd gear,
only under load of acceleration only

My question if anyone can help is

What is the most likely cause?

Pinnions?

Selector forks?

Main shaft?

Selector drum?

I have allready checked behind gear lever cover and selector mechanizm is all ok

TIA
Gary UK

MetalHead550 · « **Reply #1 on:** November 23, 2005, 04:11:44 PM »

I vote the dogs or the selector forks or combo.

dusterdude · « **Reply #2 on:** November 23, 2005, 06:03:02 PM »

ditto

bryanj · « **Reply #3 on:** November 24, 2005, 08:29:32 AM »

The most common problem years ago was the dogs worn, plus the selector AND drum

rxdc · « **Reply #4 on:** November 29, 2005, 12:58:51 PM »

Thanks for the reply's, hope to get it pulled apart this weekend if I can make some space in the Garage
next to the radiator

Cheers Gary

KB02 · « **Reply #5 on:** November 30, 2005, 06:27:24 AM »

I had a simular problem with my bike before I rebuilt the engine.

When sitting at an idle, if you tilt the bike over to the left (like it was on the side stand) do you hear allot of mechincal sounds? Bopping, grinding, ect...? That's what I had with mine. Turns out the engine had been apart before and the stupid mechanic didn't put the snap ring in to hold second gear in place (well, okay, it was there... but about 3/4 of an inch away from where it should have been). This allowed the gear to slide back and forth (and is one of the gears that shouldn't do that) causing all kids of lovely noises. Fortunetly, the only damage I could find was a severely worn out shift fork. All the gears were still good.

Jim Shea · « **Reply #6 on:** November 30, 2005, 08:02:30 AM »

Gary,
The bike looks great. Is it original?
7k in 2nd, I'll have to try that and see what happens. I think I must be riding too slowly! but as I said in another post, these bikes are a bugger to stop!
Jim.

SteveD CB500F · « **Reply #7 on:** November 30, 2005, 08:12:06 AM »

I've redlined your bike in 2nd Jim - it's fine.

Jim Shea · « **Reply #8 on:** November 30, 2005, 08:14:15 AM »

Ah, now I remember that first meeting we had. Bloody great, someone else has had more fun on my bike than I have!

rxdc · « **Reply #9 on:** November 30, 2005, 12:12:01 PM »

Quote from: KB02 on November 30, 2005, 06:27:24 AM

I had a simular problem with my bike before I rebuilt the engine.

When sitting at an idle, if you tilt the bike over to the left (like it was on the side stand) do you hear allot of mechincal sounds? Bopping, grinding, ect...? That's what I had with mine. Turns out the engine had been apart before and the stupid mechanic didn't put the snap ring in to hold second gear in place (well, okay, it was there... but about 3/4 of an inch away from where it should have been). This allowed the gear to slide back and forth (and is one of the gears that shouldn't do that) causing all kids of lovely noises. Fortunetly, the only damage I could find was a severely worn out shift fork. All the gears were still good.

When I got the bike, frame engine and wheels, the top end was seized, took months to free it off with traditional methods and still ended up giving it a knock with a hammer and block, had to fit one newish liner and a set of pistons and rings. Don't think the bottom end has been apart

rxdc · « **Reply #10 on:** November 30, 2005, 12:23:25 PM »

Quote from: Jim Shea on November 30, 2005, 08:02:30 AM

Gary,
The bike looks great. Is it original?
7k in 2nd, I'll have to try that and see what happens. I think I must be riding too slowly! but as I said in another post, these bikes are a bugger to stop!
Jim.

Hi Jim

I have rebuilt this bike as original as possible, I have a full set of repainted tank, panels and airbox but held back on fitting them until it had been road tested, it has done just over 1000 miles in the past year. so this winter is time to sort the bottom end.

The only other niggle is with the carbs overflowing even after full rebuild kits, possibly a bit of muck somewhere an inline filter may help. strange thing is I get one to stop overflowing and another starts.

7k in second may be a bit misleading, it could be between 4&7k when it jumps out of gear but it allways jumps to neutral and when I have got over the shock of this bike over revving it allways looks around 7k rpm

Cheers Gary

Geeto67 · « **Reply #11 on:** November 30, 2005, 01:28:15 PM »

jumping out of second gear is a common sign that somebody abused your cb750 in a previous life. usually this is casued by speed shifting or hard shifting. One of my cb750s used to do that and it was not a bike I treated lightly. Usually it is because the gear dogs are prematurley worn or bent. Not hard to fix per se, but you do have to pull the motor and split the cases (like most work on a cb750).

rxdc · « **Reply #12 on:** December 01, 2005, 04:07:45 AM »

I plan to remove the engine and turn it upside down and remove just the lower crankcase, long time since I did this.
My question is how much of the clutch has to be stripped prior to removal or is just a case of removing the outer covers loosen all of the crankcase bolts and lift off leaving everything in the top case. Just a reminder or tips needed
TIA Gary

Hop on a Honda · « **Reply #13 on:** December 01, 2005, 01:33:47 PM »

The problem of jumping out of gear weathere it be 1st or 2nd can some times be the type of oil you use.
I have used a 20-50 Penrite whioch stopped the above problem .
I must admitt when the oil gets a little worn it can do it occassionally.
Good luck
Dave

rxdc · « **Reply #14 on:** December 01, 2005, 03:48:47 PM »

While I do not doubt oil change may possibly reduce the problem it will not fix the problem, it's coming apart.

I do appreciate your point

Thanks
Gary

Jim Shea · « **Reply #15 on:** December 02, 2005, 01:16:29 AM »

Gary,
Good luck, let us know what you find?
Jim.

MRieck · « **Reply #16 on:** December 02, 2005, 05:20:24 AM »

Quote from: rxdc on November 30, 2005, 12:23:25 PM

Quote from: Jim Shea on November 30, 2005, 08:02:30 AM
Gary,
The bike looks great. Is it original?
7k in 2nd, I'll have to try that and see what happens. I think I must be riding too slowly! but as I said in another post, these bikes are a bugger to stop!
Jim.

Hi Jim

I have rebuilt this bike as original as possible, I have a full set of repainted tank, panels and airbox but held back on fitting them until it had been road tested, it has done just over 1000 miles in the past year. so this winter is time to sort the bottom end.

The only other niggle is with the carbs overflowing even after full rebuild kits, possibly a bit of muck somewhere an inline filter may help. strange thing is I get one to stop overflowing and another starts.

7k in second may be a bit misleading, it could be between 4&7k when it jumps out of gear but it allways jumps to neutral and when I have got over the shock of this bike over revving it allways looks around 7k rpm

Cheers Gary

Aftermarket float needles or OEM replacements?

rxdc · « **Reply #17 on:** December 02, 2005, 05:42:33 AM »

Must admit I am not 100% sure if they aftermarket or OEM
Gary

MRieck · « **Reply #18 on:** December 02, 2005, 05:50:20 AM »

Aftermarket needles can have the wrong taper and internal spring. You have to replace the needle seat too as the original needles are not rubber tipped and wear the seat out.

rxdc · « **Reply #19 on:** December 03, 2005, 10:33:27 AM »

Ok it's apart now,the bad point is there no obvious signs of wear, everything looks to be where it should be.

I have compared most parts with another box I have and everything looks the same.

selector forks don't appear to be bent

selector drum has a couple of shiny areas and thats being picky

no missing teeth of pinions

Start with the micrometer tomorrow and check wear limits

life's fun init
Gary

MetalHead550 · « **Reply #20 on:** December 03, 2005, 01:23:26 PM »

Hmm...Only other thing I can think of is to check the gear selector linkage. Mabey its not rolling the selector drum as far as it should for 2nd and thats causing it to slide back into nuetral. Can you go through the gears by spining the output shaft with the cases apart? Check to see that there is a satisfying "click" as each gear engages, especially 2nd, if you havnt already..

bryanj · « **Reply #21 on:** December 04, 2005, 03:25:12 AM »

There rarly is missing teeth or pinions but if you look closely at the dogs on the gears and the holes they go into (or sometimes mating dogs) you will probably find rounded ends and a shiny worn side. That is enough to make em jump along with a small amount of wear on the appropriate "peak" in the selector drum groove. hope i explained it easy enough. Best of luck

rxdc · « **Reply #22 on:** December 04, 2005, 06:36:31 AM »

I was checking movement of selectors/drum last night all appeared well.

After removal of neutral light switch the drum moves freely, the three selectors follow the pattern of the drum.

I do not think it is possible to build up the roller and springs of the gearshaft mechanizim (inside outer cover) until both cases are put back together.

The bike has allways allowed me to engaged 2nd gear and it is not until I wind the bike up that it jumps out of gear

bryan j

Xmas shopping getting in the way now

Keep tuned in for the next sequel
Gary

rxdc · « **Reply #23 on:** December 05, 2005, 12:36:04 AM »

Bryan thanks for your input, did find wear in area's suggested but must admit I would have thought it minimal and not cause the problem, replaced them anyway.

Any recomends on gasket cement, usually use blue hylomar without any problems, any thing new to market that does a better job

rxdc · « **Reply #24 on:** December 05, 2005, 12:38:29 AM »

any idea what these letters mean

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Author Topic: 1970 cb750 2nd gear problem (Read 8832 times)

rxdc

MetalHead550

rxdc

Jim Shea

Jim Shea

rxdc

rxdc

rxdc

Hop on a Honda

rxdc

Jim Shea

rxdc

rxdc

MetalHead550

rxdc

rxdc

rxdc