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Author Topic: 1976 cb750 ss gear ratio? insanely low  (Read 7983 times)

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Offline gty2k2000

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1976 cb750 ss gear ratio? insanely low
« on: January 25, 2011, 10:55:10 AM »
i just purchased a cb750F1 1976, and after riding for a while with my buddy i relized that the gearing in this bike is insane. in 5th gear going about 45-50mph my bike is turning @5000-6000 RPM im not sure my speedo is inop. IS THIS NORMAL. and can i change it so that runs on the interstate arent so extream
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Offline TwoTired

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #1 on: January 25, 2011, 11:04:36 AM »
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Offline gty2k2000

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #2 on: January 25, 2011, 11:43:30 AM »
so is this normal for my bike is it ok to run on the interstate at such high rpms
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Offline Mark M

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #3 on: January 25, 2011, 11:50:56 AM »
That s not considered high rpm on these engines. They like to be reved  ;D
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Offline dave500

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #4 on: January 25, 2011, 12:11:15 PM »
count both sprocket teeth and see if its been changed from standard.
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #5 on: January 25, 2011, 01:02:23 PM »
5th gear, 5000rpm, ~70mph with stock size tire, 17-48 sprockets
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Offline Bodi

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #6 on: January 25, 2011, 05:00:06 PM »
Check the chain sprockets. I don't think you can get sprockets to make that much of a difference though. I think your tach is wrong.
Why is the speedo inop? If someone smashed the front end and replaced the tach with one for a different bike model the ratio may be wrong and the reading bogus. Honda bikes under 500cc have a different tach ratio than over 500cc. Buggered if I can remember what they are.
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Offline Retro Rocket

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #7 on: January 25, 2011, 05:03:26 PM »
Quote from: Bodi on January 25, 2011, 05:00:06 PM
Check the chain sprockets. I don't think you can get sprockets to make that much of a difference though. I think your tach is wrong.
Why is the speedo inop? If someone smashed the front end and replaced the tach with one for a different bike model the ratio may be wrong and the reading bogus. Honda bikes under 500cc have a different tach ratio than over 500cc. Buggered if I can remember what they are.

Sprockets will definitely make that difference, like others have said, check your sprocket sizes and get back to us...

Mick
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Offline dave500

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #8 on: January 25, 2011, 11:21:48 PM »
i tried a 550 37 rear sprocket on my 500 that should be a 34 tooth,it went well but was like i lost fifth gear,they make a difference alright.
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Offline Grabcon

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #9 on: January 26, 2011, 08:31:03 PM »
Follow the link. It is a gearing web site. You will follow the instructions and list your bike. It will show MPH per gear with stock or non-stock gearing. It is pretty simple.

http://www.gearingcommander.com/
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Offline madmtnmotors

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #10 on: January 27, 2011, 06:42:37 AM »
If the sprockets teeth count are close to stock then the clutch may be slipping. How many miles?

My 78' CB750 both F and K will cruise at 70mph in 5th gear at 5000rpm with 15/41 sprockets.
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http://forums.sohc4.net/index.php/topic,139544.msg1579364.html#msg1579364
                                          
Charging system diagnosis: http://forums.sohc4.net/index.php?topic=1012.msg8345#msg8345
Get the manuals: http://manuals.sohc4.net/cb750k/
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Headlight Switch: http://forums.sohc4.net/index.php?topic=113986.msg1283236#msg1283236
Branden's leak free top end thread: http://forums.sohc4.net/index.php?topic=107040.0
Engine Lifting Made Easy: http://forums.sohc4.net/index.php/topic,58210.msg1684742.html#msg1684742
                                      http://forums.sohc4.net/index.php/topic,100352.msg1675840.html#msg1675840
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"Café" : http://forums.sohc4.net/index.php/topic,84697.msg953814.html#msg953814
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Tank Latch: http://forums.sohc4.net/index.php/topic,165975.msg1919495.html#msg1919495
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Offline Bodi

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #11 on: January 27, 2011, 08:48:23 AM »
"Check the chain sprockets. I don't think you can get sprockets to make that much of a difference"
vs
"Sprockets will definitely make that difference":

According to the posts, he has the RPM at 50mph that should mean 70mph.
That's 1.4x the stock drive ratio. With a 15 tooth engine sprocket (smallest I could find) you need a 60 tooth rear sprocket, not available except by custom order, and it would be obviously oversize.
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Offline gty2k2000

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #12 on: January 27, 2011, 09:42:52 AM »
i think the tach is way off and having a 4 to 1 kreager header is intimidating to the ear so the off tachseeems normal
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Offline madmtnmotors

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #13 on: January 27, 2011, 10:08:38 AM »
You could assume that the tach is off. If you are wrong then that would likely mean that the clutch is slipping.
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Projects:
"Evolution": http://forums.sohc4.net/index.php?topic=100352.0
"P.O. Debacle": http://forums.sohc4.net/index.php/topic,126692.msg1441661.html#msg1441661
F2/F3 O-rings: http://forums.sohc4.net/index.php?topic=113672.msg1300721#msg1300721
Cam Tower Studs: https://www.mcmaster.com/#93210a017/=t19sgp
Clean up that nasty harness: http://forums.sohc4.net/index.php?topic=137351.msg1549191#msg1549191
http://forums.sohc4.net/index.php/topic,148188.msg1688494.html#msg1688494
http://forums.sohc4.net/index.php/topic,139544.msg1579364.html#msg1579364
                                          
Charging system diagnosis: http://forums.sohc4.net/index.php?topic=1012.msg8345#msg8345
Get the manuals: http://manuals.sohc4.net/cb750k/
The Dragon: http://forums.sohc4.net/index.php?topic=45183.msg1571675#msg1571675
Headlight Switch: http://forums.sohc4.net/index.php?topic=113986.msg1283236#msg1283236
Branden's leak free top end thread: http://forums.sohc4.net/index.php?topic=107040.0
Engine Lifting Made Easy: http://forums.sohc4.net/index.php/topic,58210.msg1684742.html#msg1684742
                                      http://forums.sohc4.net/index.php/topic,100352.msg1675840.html#msg1675840
Static and Dynamic Timing: http://www.hondachopper.com/garage/carb_info/timing/timing1.html
Airbox Gasket Replacement: http://forums.sohc4.net/index.php/topic,114485.msg1290000.html#msg1290000
"Café" : http://forums.sohc4.net/index.php/topic,84697.msg953814.html#msg953814
PD Carb Choke Linkage: http://forums.sohc4.net/index.php/topic,100352.msg1669248.html#msg1669248
                                    http://forums.sohc4.net/index.php/topic,110931.msg1248354.html#msg1248354
                                    http://forums.sohc4.net/index.php/topic,48858.msg515204.html#msg515204
Follow up on your damn posts: http://forums.sohc4.net/index.php/topic,144305.msg1791605.html#msg1791605
Taiwanese Cam Chain Tensioners:  http://forums.sohc4.net/index.php/topic,155043.msg1774841.html#msg1774841
Gumtwo Seat Cover: http://forums.sohc4.net/index.php/topic,164440.msg1897366.html#msg1897366
Primary Drive: http://forums.sohc4.net/index.php/topic,166063.msg1919278.html#msg1919278
Tank Latch: http://forums.sohc4.net/index.php/topic,165975.msg1919495.html#msg1919495
Shorten your forks: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-shorten-forks-td4042465.html DO NOT CUT THE SPRINGS!
Clutch How To: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-change-and-adjust-a-clutch-SOHC-td4040391.html
Late model K7/K8/F2/F3 front sprocket cover removal: http://forums.sohc4.net/index.php/topic,178428.msg2072279.html#msg2072279
630 to 530 conversion: http://forums.sohc4.net/index.php/topic,180710.msg2094423.html#msg2094423

Sent from my Tandy TRS-80!

bollingball

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #14 on: January 27, 2011, 10:46:05 AM »
Check the clutch with the old brick wall method in 5th gear.
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Offline mlinder

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #15 on: January 27, 2011, 11:24:19 AM »
HOW MANY TEETH ON YOUR SPROCKETS?
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Offline dave500

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Re: 1976 cb750 ss gear ratio? insanely low
« Reply #16 on: January 28, 2011, 03:04:00 AM »
if the clutch was slipping that much i think youd feel it all over,like it would "flare"under accelleration,most of these old tachs speak with forked tonque anyhow,were all still waiting on the tooth count.
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