Topic: new sprocket gearing for 1978 cb750K

[Wayne Grauel]

I'm in the process of working on a cb750K  '78..   (is this a K8??)

I'll have more questions in the future for other topics as I work through FAQ's etc.


Gearing.

this bike is hard to get rolling.  (compared to my older 750 Virago... just let out the clutch and go).    To me it feels Geared High --   I feel like I have to constantly gun the throttle while slipping the clutch until it actually gets rolling.  If I get it to a small roll and let the clutch out it feels like you can feel each cylinder as it thumps up and down...   No low speed maneuverability at all.. I'm not asking for a pattern bike, but it's a pain at stop lights and on hills to maneuver slowly - which adds to low speed instability. - I feel like it wants to fall over until it's rolling above 10MPH

Most riding for me is country roads - so I'm looking for good throttle performance in decent back roads with shallow turns at 50-70 MPH.  (with some torque).. maybe drop a gear...

I've looked at a lot of sprocket options and the options and the options seem limited with a 630 chain.

(NO I HAVE NOT COUNTED THE SPROCKETS YET... it's really cold out in the barn and I'm not in the tearing it apart stage until I bring it into the basement)

If I go to a 530 chain - I could go as far as  14 / 48  with JT sprockets and Chains.   

speeds at max power   

14-48 gears  are:       (1st - 5th)  0-36   16-54  20-68  24-80  28-94    This may be more like my old bushmaster so this seems a little too radical.

15-48 gears are:    0-39    17-57     22-73   16-86    30-101       

 stock 15-41 gears:    0-46   20-67  25-86  30-101   35-118

I've read the deal with a 630 vs 530 chain.  some say that the cb750 does not have the power or torque to worry about tearing the chain up...  so is this a reasonable assumption for a "casual weekend street bike"


So, the obvious questions are - Who else does (prefers) this type of riding and what works for you.  Which would bet fit the power curve of the bike (assuming perhaps a kirker goes on it).   (or if you had the option to change things right now based on these figures (if they appear correct) what would your opinions be.

I also ride open interstate roads but I'm satisfied to stay under 90-95 MPH   


Thanks
Wayne

[eldar]

You would not want to go that low on the front sprocket. Most people go minimum of 16 on the front but usually 17 or 18.
You dont want to get your gearing too high as you will run rpms like crazy on the highway.  currently with the 630 set up, you are running 2.733. You could raise it some but you would not want to go too much. Most 530 chains these days will hold up fine though. A 17/47 or 17/48 would probably be a good start.

[nteek754]

Wayne the  old 530 set ups were either 17 45 or 18 48  this set up will give you 4000 rpms in 5th gear at 60 MPH a good refrence point  if you go down one on the front it will be quicker but  the powerband is shorter  great for 1/8th mile draging but  overall I M O I like the  more stretched out power band of the stock set up and it is better for the engine good luck have fun Craig in Maine

[Wayne Grauel]

After I started thinking on this it struck me...   a 630 sprocket would have less teeth than a 530 sprocket of the same diameter -    (being the links are smaller???)

I am really not up on the two chains - only what I have been able to grasp so far.

Does this upset my ratio thinking??


So far I'm with Eldar and Craig on the logic - especially since it is easier to take advice than it is to buy a bunch of sprockets and try them out!

My spreadsheet tells me that it does not make any difference with the teeth ratio if it is a 530 or 630 chain  (which I am having a very hard time visualizing how that can be if one circle is smaller than the other)   or is it merely the ratio of the two, regardless of the size --     like a 17 / 47   set would calculate the minimum shift points at    0  20  25  30 and 35 

Thanks Again!

As soon as i make some room I'll bring the bike in and can post pics of progress ( which will be get it back in shape enough to ride and have some fun on after all these years --  and not be a total embarrassment!)


Wayne

[Snowdigger69]

I took my stock 78 cb750K to Florida with stock 15/41 630 chain 4/1 Kirker exhaust. It will run all day long at 80 about 6500 rpm. The power curve is decent for most road types. It is some noisy but what do you want. I have the 15/41 530 on my second CB750K touring style bike w/ Fairings 4/4 exhaust, it is so quiet at 80 you hear all the engine noise.. I think I prefer the wind.. Snowdigger..

[chung]

I have swapped to a 530. 18/48 is way too much for a k8. The primary drive ratio is different than the older ones. I went through this on my 1978 SS. I started with 18/48, then went to 17/48 and it was still grunting at 100 mph. I think 16/48 would be close but I sold the bike before trying that ratio.

[Wayne Grauel]

What about the starting off in 1st gear..  has anyone noticed this or could it be assumed that somone has geared it up?

Is is normal for an inline 4 to start from a stand still and have the engine put put put like a john deere tractor until it gets any speed (5-10mph) (and also gyroscopic stability) .  Almost feels it was starting in 2nd gear (but it's not!)

I just did not know if this is typical or if this is just the nature of the beast.   In comparison to my Virago 750 (even remembering from years back) you could idle along with the clutch out in 1st and have your feet on the pegs no problem..

I've ridden this one on some long trips (2 years ago) and you'd better have your feet ready to plant when you come rolling in.

Thanks again
Wayne


Here is a link to my website - I put the gearing calculator spreadsheet up there if anyone would like to have it..

Wayne

http://www.woodsglen.com/gearing_calc.xls