Topic: CB500 3rd Gear Slippage

[Puffin]

Hi all.

My CB500 has started to slip in 3rd gear when under load.  It's kind of a 'jerk' as if is disengaging and then re-engaging.

Anybody know exactly what's going on?  I suspect it's a worn gear or bent forks

Is there anything I can try, test or adjust before tearing the engine apart?

Any help appreciated. 

[bryanj]

The centre selector on the 500 is known to wear excessively, I think this is the same as the 350 twin selector, new ones are very difficult if not impossible to find now and if it has been "slipping" for some time the gear dogs will be worn as well. One pair of gears have been updated and must be fitted as a pair as the tooth profile is what was altered Your safest bet is a complete good gearbox or a bottom end, The 550 bottom end can be used but you need the RH rear engine mount plate from the 550 as the clutch operation is different and the cable stop is on that plate

[Puffin]

Thanks for the advice. 

I have just rebuilt the engine and only done about 2000 kms on it since.... shame as it was running like a dream until last week. 

Fortunately I've only done 2 small rides to work on it since the fault appeared and then I parked it whilst I figured out what was going on.

I did measure the selector forks before reinstalling and the 'ends' where they connect with the cogs were within factory tolerances.  I guess however that the center fork could have been bent?

Interesting point about the gears being updated.  I'll keep an eye out for this.

 

[Shtonecb500]

my motor just lost 3rd gear, thats what yours is doing, skip past third and go to forth, pull ur motor and rebuild or swap it.

[dave500]

the drum also can float side to side,this was address with the 550,if the drum moves it takes the forks with it.

[Puffin]

Thanks Dave.  Looks like I need to check this first... had a quick look at the manual and it looks like I already have the updated Shift Drum screw and Shift Drum mechanism.

Any idea where the fault usually occurs and/or how to go about checking if it's the drum movement causing the problem?

[bryanj]

If you have totally lost 3rd gear it will be the gear dogs broken which means lower case off

[dave500]

that shift drum screw update isn't the same at all like the totally reworked 550 one,it can still float sideways,if your getting third then its slipping out the dogs and slots are rounded out causing a ramp effect,also the gear wheels can wobble on their shaft causing the same thing,its the stack up of drum float/gear wobble fork wear and rounding off that gets worse and worse.

[Puffin]

Sounds like I need to find a 550 bottom end or a low mileage 500 transmission then.

Bugger... thought all the hard work was over and I could finally enjoy riding.

[1952paul0]

Sounds like I need to find a 550 bottom end or a low mileage 500 transmission then.

Bugger... thought all the hard work was over and I could finally enjoy riding.

there real easy to build and I doubt you will need much in the way of parts you only need to split the crankcase the problem will be obvious.

[bryanj]

550 bottom end will need to be complete with all casings and you will need the RH rear engine mounting plate as well

[dave500]

third gear and its mate are the only gears shared between 500 and 550s.

[bryanj]

As dave said there is only one pair of gears that transplant and only as a running pair as the tooth profile is different

[dave500]

the mesh teeth are the same the dog shapes and sizes changed.

[HondaMan]

An old trick for this (it wasn't just in the Honda 500-4, but other bikes, too, like Yamahas) was to weld the tines of the shifter forks to make them wider. Sometimes the slip groove on a given gear was cut at the allowed max width and the fork that the engine got happened to be at the other (narrow) extreme, so the dogs did not engage very deeply. Sometimes the cheapest 'fix' for a given tranny was to widen (sometimes just one side of) the fork tips. If the dogs are rounded (and/or the companion slots tapered from the dogs), often just a "refurb" (often called a "pull-cut" or "back-cut" of these mating parts) is all that is needed. In the USA, you can get this done at APE (California) or Fast By Gast (Buffalo, NY), not terribly expensive.