Newb with a '78 CB750A question

[danielz]

I just bought a '78 CB750A a few days ago and have already started changing it to make it my own. I am not very familiar with the CB and was wondering, other than the transmission, what is the difference between the CB750A and CB750K? I ask because I have found some after market parts that I would like to use, but I don't want to have to do to much modifications to make it work. I work long hours and won't have a lot of extra time to do that. Would a seat for the K fit fit the CB750A?

 Any help is greatly appreciated.

[TOOLS1]

The A is almost the same bike as the 77-78 K. The seat should change over. The A is a great bike! I have 4 of them.
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[kandrtech]

Off the top of my head:

Carbs (venturi size and idle controller)
Carb holders/rubbers
Carb to airbox rubbers
Cam (less lift and different valve timing)
Pistons (lower compression)
Main engine cases
Deep oil pan
Oil pump
No oil tank (K has one)
Rear brake lock
Right instrument gage (no tach)
Battery
Alternator (higher output)

[TOOLS1]

The carbs are the same as the 77-78 K, and F models. As are the intake, and air box boots.
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[kandrtech]

In all due respect, my Honda literature indicates that the auto carbs have 24mm venturis, while the K/F carbs have 28mm venturis.

According the parts fiche:

77/78 K carb holders are 16211-405-000, and 16212-xxx-xxx (same), 16213, 16214
77/78 F carb holders are 16211-410-000, and 16212, 16213, 16214
77/78 A carb holders are 16211-393-020, and 16212, 16213, 16214

77/78 F/K air box to carb holders are 17253-405-000
77/78 A air box to carb holders are 17253-393-770

Upgrading the auto bike carbs to the 28mm carbs net a nice boost in HP.  The problem with that upgrade is the lack of 77/78 carb holders.  Going with the older round tops is the better choice, as you can then also buy the round top carb holders.  The down side is that the accelerator pump wasn't provided until 77.

[kandrtech]

Here's a pic of the auto and non-auto carb throats:

[TOOLS1]

Interesting. I stand corrected. However I just discovered that the Buffet Racer has had the carbs switched before I acquired it. They were laying in the trunk with several rubber intake boots. Guess I now know why. However I do know that all the internal parts are the same slides, float, accelerator pump, and linkages. I used K-F parts to rebuild the carbs on the TOOLmatic two years ago. Now excuse me while I go to compare air boxs' to see if they are different.
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[kandrtech]

The slides differ, too.  I recall that the 77/78A slides are the same as the 550 slides, but not common to the K/F bikes.

Can't speak to the airbox itself.

The rest - I expect you are entirely correct.

[shinyribs]

Thats very interesting. Thanks for that info Kandrtech

[kandrtech]

I only know this stuff because of two things:

1.  My sons CM450A (450cc Auto) blows me away off the line.

2.  I hate leaving things stock, if there's a quick way for more power.

So . . . I've tried to learn about the "A" bikes, and how to make them faster - on the cheap!  I certainly don't know all of the answers, but I know some of them . . . .

[shinyribs]

I like how you think!

My bud,Eric,has a CM540A with a side car on it. I've rode monkey a time or two in it. I'm around 220 lbs (depending on the current Holiday) and Eric is a good 260 or more. It really surprised me how will that ''little'' 450 moved us around! Neat bikes for sure.

[TOOLS1]

The Vice President of the Vintage & Custom Motor Cycle Club here in KC has a 400 matic. It goes pretty good, but his other bike is a Gold Wing, and going from 100+ HP to maybe 30 does not impress him. However I have been helping out a friend with this CB 400.
 
It will run 95 MPH.
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[750K8]

Tools: I'm pretty sure that the Cb750a seat will not fit on a CB750k, and vice versa.

[TOOLS1]

I'm not sure if the mounts are the same, however I do know that the A goes down lower on the sides, and will hit the K side covers. I do have a 77 K model, but have never paid any attention to the seat mounts; I will have to go look at them.
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