Topic: Troubleshooting gear failure.

[turkey4me]

After replacing a leaking dyno and shift cover gasket I went for a 30 minute test ride at highway speed.  Stopped to check for leaks and could not downshift into third.   After 20 minute break I was able to get back in first gear and run back home through all the gears.  Reaching home same problem.   Only 4th, 5th and neutral.  Today I checked the clutch and it is disengaging.   Rolled back and forth in neutral and still don't have 1st 2nd or 3rd gear.   Any suggestions where to start troubleshooting?

76 CB 750F with Rotella 15W 40 oil.
Oil pressure at temperature 5psi at idle 65 psi at 4500rpm.   Oil level good.   Dyno leak is fixed.

[scottly]

Since the problem started after you replaced the shift cover gasket, you might want to pull the cover and see if you messed something up. Sometimes when removing the cover the shift lever shaft tries to come out, which can cause problems. I pull on the cover with my finger tips while pressing on the end of the shaft with my thumb, which keeps it seated.

[70CB750]

I had very similar problem. Mine would not shift past 3.   It was the cover, I used silicon without paper gasket and the shifter just would not move higher than 3.

Try to losen the bolt closest to the shifting lever - that's what clued me in.  Once I used paper gasket, the shifting was fixed.

[turkey4me]

Thanks, I hope it's that simple. I started tiling the shower and will need to finish that project first. I'll let you know I find later this week.

[turkey4me]

I removed the shift cover and shot a video of the gears.
I'm baffled because now it shifts easily into 1st N 2nd and 3rd.   If I roll the bike forward I can also get 4th and 5th.   Seems normal now. The only other change I made besides a new gasket was oil weight. I  dropped from 20W50 to 15W40.  Might have to go back to 20W50.

[PeWe]

CB750 is specified to use 20W-50 oil.
My owners manual specify 10W-40 if ambient temperature is below 15*C.

[scottly]

Is your rear-set linkage rubbing against the cover?

[bryanj]

If it works with cover off either the case/gasket are wrong or the external shifter is hitting something

[enwri]

Do they all extend past the edge of the case like that?

Any witness marks inside the outer case?

Could the gasket be getting wadded up under it sometimes limiting travel?

  Just checked mine, it's the angle of the camera.
I'd still be looking at the inside of the cover for marks.

It's been leaking since I looked after that "Should there be so much oil in the cover question"
may as well fix it now.

[rotortiller]

CB750 is specified to use 20W-50 oil.
My owners manual specify 10W-40 if ambient temperature is below 15*C.

No, that is not what the owners manual says.   10W40 is the general recommended oil for all temps.

[seanbarney41]

double, triple,quadruple check the assembly of your rearsets.  Looks like you have Tarrozi rearsets or at least used several of their components to build them.  What happens is if your footpeg is not assembled exactly correct, the shift lever will bind up.  If there is any binding, the shift return spring will not have enough tension to return the shift lever so it can grab the next gear.  It is easy to mis-assemble that tiny pin that clocks the shift lever.  That pin is not the same at both ends.  It MUST installed with the fatter end engaged correctly.

[70CB750]

CB750 is specified to use 20W-50 oil.
My owners manual specify 10W-40 if ambient temperature is below 15*C.

No, that is not what the owners manual says.   10W40 is the general recommended oil for all temps.

Somebody - Hondaman? - debunks it as a translation mistake.  But I would take PeWe's word for it, too. 

[newday777]

I removed the shift cover and shot a video of the gears.
I'm baffled because now it shifts easily into 1st N 2nd and 3rd.   If I roll the bike forward I can also get 4th and 5th.   Seems normal now. The only other change I made besides a new gasket was oil weight. I  dropped from 20W50 to 15W40.  Might have to go back to 20W50.

You do need to have the rear wheel moving when shifting between gears so it shifts properly. Static shifting without the wheel moving is incorrect testing. Try it again moving the rear wheel while shifting.


20w50 is better for your temps there in Yakima you are having.
The Rotella is a little lightweight for the air cooled 750 at those temps,
Rotella T4 15w40 is what I use in my 750 in temps below 70°. Summer temps above 70° I change to 20W50 Honda oil(because I have it on hand). Back in the mid 1970s when I was parts manager I learned from the techs and racers that 20W50 was far better for the air cooled bikes in temps above 70° than 10w40 if you wanted longevity of your motor and transmission.
In my wing(water cooled) in the summer temps crossing the desert southwest I use the Rotella without problems. It is good oil for wet clutches and transmissions.

[newday777]

CB750 is specified to use 20W-50 oil.
My owners manual specify 10W-40 if ambient temperature is below 15*C.

He isn't using 10w40, he's got 15w40 Shell Rotella diesel oil (available here in the states). Yes it is OK to use it in our bikes in cooler temperatures than he's experiencing right now, and yes he should be using 20W50 as he is thinking about.

[PeWe]

Yes, the CB750 shopmanual and oil sticker say 10W-40 or 20W-50 without any details

Here the manual I got with my K6 when I bought it early 1979.
Here comment when to use 20W-50.

But this is up to the owner.
A modified CB750 with hotter cam, harder springs that have been shimmed for higher force need a good oil with viscosity intact.
Mineral oil or Synth. Synth withstand more abuse as higher temp before degrading.

It is the additives that make the higher viscosity. When worn, back to basic, 10 or 20 depending on which multigrade used.

You did not get this rather small Owner's manual with your bikes? Local language and oil recommendation as here.

Mine is used by my oily fingers in 1979 and a few years forward .

[bryanj]

Perhaps that is a variation for the particular country the language is for, it does not make it a recomendation for everywhere else.
By the time the K6 came along Honda were altering things for specific countries, especially in the literature

[turkey4me]

I didn't see any witness marks inside the cover.   I reassembled the cover and now it's not not shifting correctly.   Seabarney I think you nailed the problem with the Tarozi rearset linkage.   It's not hitting anything but it does appear to be binding up. I have the link rod twisted a bit and it's moving to the left vs straight forward and back.  I will adjust the linkage and go for a ride.  I'm still going back to 20W50.

Thanks everyone for your help today.

[PeWe]

About owners manual and oil rekommendation for CB750.
Here even worse.
Manual from 1973, must be K3 see screen dump below.
https://www.manualslib.com/manual/919873/Honda-Cb750.html

It must be a history about short lived engines. Especially crank bearings, cams, rockers and guides.

From relatively thin straight oils to 20W-50.

I guess Mark Paris can be the one that has whitnessed the CB750 oil history.

Here only 20W-50 as long ss I have owned mine.
Local shop sold Quaker State straight oil early 80's.

Best mineral oil comes from Pennsylvania, right?

These relatively  thin oils might explain the short oil change intervals.

A thicker later recommended  20W-50 oil keep up the viscosity longer. 4000-5000 km  (3000 miles)  usually no problems. No oil change during long tourings of 5000km

Forum member ofreen and his bike a very good example.

Fun oil thread/info)!
https://laverdaforum.com/threads/oil-question-castrol-gtx-20w-50-vs-castrol-grand-prix-20w-50.93286/