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Author Topic: CB750 gearing  (Read 3524 times)

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1198bmw

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CB750 gearing
« on: November 02, 2011, 09:17:51 AM »
Hi all,
I have a 1978 CB750 K8.

At 60mph its reving at 6500rpm- surely thats to much.
Its got 15t front sprocket and 41 rear and a 17 in rear wheel which i'm told is correct for this model. Was there a gearing problem with these models and what is the solution.

1198bmw
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Offline MCRider

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Re: CB750 gearing
« Reply #1 on: November 02, 2011, 09:20:13 AM »
Quote from: 1198bmw on November 02, 2011, 09:17:51 AM
Hi all,
I have a 1978 CB750 K8.

At 60mph its reving at 6500rpm- surely thats to much.
Its got 15t front sprocket and 41 rear and a 17 in rear wheel which i'm told is correct for this model. Was there a gearing problem with these models and what is the solution.

1198bmw
Either the clutch is slipping or the tach is wrong.

Could be something else, but that's my vote.
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1988 NT650 HawkGT;  1978 CB400 Hawk;  1975 CB750F -Free Bird; 1968 CB77 Super Hawk -Ticker;  Phaedrus 1972 CB750K2- Build Thread
"Sometimes the light's all shining on me, other times I can barely see, lately it appears to me, what a long, strange trip its been."

Offline I Zombie

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Re: CB750 gearing
« Reply #2 on: November 02, 2011, 01:43:45 PM »
I've kinda got the same issue, but after new clutch & springs & countless adjustments, i'm guessing my tach is off. My tach say's I'm idling @ 2000rpm, but many a buddy say's "it sure don't sound like 2000 to me"
« Last Edit: November 02, 2011, 02:22:50 PM by I Zombie »
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Offline TommyT

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Re: CB750 gearing
« Reply #3 on: November 02, 2011, 01:46:16 PM »
my 1978 cb750K tach reads 2100 rpm at idle, but my dwell/tach tells me it's at 950-1000.
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Offline MCRider

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Re: CB750 gearing
« Reply #4 on: November 02, 2011, 01:47:32 PM »
Hmmm...all 78Ks. Coincidence?
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Ron
1988 NT650 HawkGT;  1978 CB400 Hawk;  1975 CB750F -Free Bird; 1968 CB77 Super Hawk -Ticker;  Phaedrus 1972 CB750K2- Build Thread
"Sometimes the light's all shining on me, other times I can barely see, lately it appears to me, what a long, strange trip its been."

Offline zzpete

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Re: CB750 gearing
« Reply #5 on: November 02, 2011, 02:08:37 PM »
I read in the FAQ: http://forums.sohc4.net/index.php?topic=372.0
With the 15/41/630 setup, the bike spun in 5th at 70MPH, 5300RPM.
If memory serves correct my 1977 CB750 K was about 5100 RPM at that speed. Tire size, clutch slippage, etc. could also be factors.
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Offline Duke McDukiedook

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Re: CB750 gearing
« Reply #6 on: November 02, 2011, 02:11:22 PM »
I haven't seen a tach on these bikes yet that were accurate.
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Offline zzpete

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Re: CB750 gearing
« Reply #7 on: November 02, 2011, 02:12:13 PM »
Quote from: DukieFrankenkit on November 02, 2011, 02:11:22 PM
I haven't seen a tach on these bikes yet that were accurate.
Me neither!!!!
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Offline coma13

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Re: CB750 gearing
« Reply #8 on: November 02, 2011, 02:24:10 PM »
are aftermarket tachometers any more accurate? or is it something in the tach drive that causes inaccuracy?
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Offline I Zombie

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Re: CB750 gearing
« Reply #9 on: November 02, 2011, 02:28:32 PM »
I haven't found a spare 1 to try yet, I've just been runnin it as is for the last few years.
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Offline MCRider

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Re: CB750 gearing
« Reply #10 on: November 02, 2011, 02:33:05 PM »
Quote from: coma13 on November 02, 2011, 02:24:10 PM
are aftermarket tachometers any more accurate? or is it something in the tach drive that causes inaccuracy?
Tach drive not an issue. Its mechanical and foolproof.

An aftermarket tach would be more accurate, mostly because its new. Or electronic. If its a custom bike, I'd go that way.

Earlier K series tachs are pretty good. Being that we're talking about 4 78Ks here sounds like that year tach had some issues. But I think a refurbish would fix it. If you want to stay stock looking.

A shop I talked to would do it for $85. open it up, clean and lube it.

Buying a new one would be pricey, and you still don't know how old it is. Could have set on the shelf for a while. The lube goes bad from gravity and condensation.
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1988 NT650 HawkGT;  1978 CB400 Hawk;  1975 CB750F -Free Bird; 1968 CB77 Super Hawk -Ticker;  Phaedrus 1972 CB750K2- Build Thread
"Sometimes the light's all shining on me, other times I can barely see, lately it appears to me, what a long, strange trip its been."

Offline I Zombie

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Re: CB750 gearing
« Reply #11 on: November 02, 2011, 02:36:53 PM »
I was thinkin of the mini speedo & tach combo from Cycle X, $70. Lose the bulk & clean up my tree's a bit & hopefully get a better reading too.
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78 CB750K

Offline coma13

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Re: CB750 gearing
« Reply #12 on: November 02, 2011, 04:44:14 PM »
Quote from: MCRider on November 02, 2011, 02:33:05 PM
Quote from: coma13 on November 02, 2011, 02:24:10 PM
are aftermarket tachometers any more accurate? or is it something in the tach drive that causes inaccuracy?
Tach drive not an issue. Its mechanical and foolproof.

An aftermarket tach would be more accurate, mostly because its new. Or electronic. If its a custom bike, I'd go that way.

Earlier K series tachs are pretty good. Being that we're talking about 4 78Ks here sounds like that year tach had some issues. But I think a refurbish would fix it. If you want to stay stock looking.

A shop I talked to would do it for $85. open it up, clean and lube it.

Buying a new one would be pricey, and you still don't know how old it is. Could have set on the shelf for a while. The lube goes bad from gravity and condensation.

I've got a 77K and I already replaced the stock tach with a little aftermarket mechanical one but I haven't started the bike up yet. Hopefully I don't run into any issues with that guy!
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Offline I Zombie

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Re: CB750 gearing
« Reply #13 on: November 03, 2011, 06:20:02 AM »

I've got a 77K and I already replaced the stock tach with a little aftermarket mechanical one but I haven't started the bike up yet. Hopefully I don't run into any issues with that guy!
[/quote]
I'd love to know if it fixed the issue or is more accurate
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78 CB750K

bollingball

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Re: CB750 gearing
« Reply #14 on: November 03, 2011, 06:57:46 AM »
I just listen to the motor. ;)
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