Topic: CB750 Mix and Match Madness

[Mikerts_Garage]

Hey Y'all,

I've been gearing up to fix my CB750 K8, but the project has suffered from just a bit of scope creep....

Let me introduce you to the story so far.

Around 9 years ago I bought my first street bike; a CB750 K8, I drove 4 hours north to Idaho Falls to pick it up for $700. The bike didn't run at the time, and the front brake was seized up. It was not a hard bike to get going again. I fixed it up slowly and put around 11k miles on it over the years. When I picked it up It looked like this:


More recently, the bike looks like this:


The problem came when I T-boned a car who failed to yield when making a left turn. I was okay, a little bruised, but the bike didn't fair so well. I replaced the forks, triple tree, and front wheel before I realized the real problem was the frame. It seems the frame had twisted right behind the steer tube. This resulted in a bike that always wants to go left and wasn't stable at speeds higher than 50mph.

So, I need a new frame, I figured it'd probably be cheaper to buy a whole parts bike locally than source one on Ebay. This leads us to CB750 #2:


This bike is a $400 K6 roller that I picked up just a couple blocks from my house. The astute among you will be saying "the frames changed in 77' to accommodate longer tanks and different seats and a bunch of other stuff". You are correct, I did not know that at the time 

So I thought to myself, "I like the styling of the K0-K6 bikes the best, why not try to find a way to make bike #2 work" and thus bike #3 was found and purchased:



This one is a K4 roller that was sitting outside a shop most of it's life(that's why the tank is almost all primer). Surprisingly there is no rust on the inside of the tank, nor any dents. Came with original right side cover, and reproduction left cover. Engine is seized up solid and it's missing the seat and exhaust though. Picked this one up for $300

At this point I had most of what I needed to make 2 bikes, the one I wanted to keep and one that would sell as a runner(although it wouldn't have an exhaust, and would retain the twisted frame). But it just wasn't a great plan yet, lots of awkward compromises, and none of the engines were great keepers without a rebuild(not what I want to do at this time).

So that brings us to CB750 #4:


This one is a 75' F1 that I found for $550, The owner let me do a compression test on all 4 cylinders before buying, which was very nice of him. It's got all the great F parts: chrome headlight bucket, chrome chain guard, stock 4-1 header(aftermarket muffler tho), higher compression engine, and rear disc brake. However, it's missing the seat foam and cover, side covers, headlight, and left side controls.

This is the part where I could use help from all of the CB750 compatibility experts here. I have done as much research as I could think of, but I could still be wrong about what will fit what. Here are all the frame and engine numbers if that helps at all:
-K8 Frame: CB750K-2859912, Engine: CB750E-3059939
-K6 Frame: CB750-2574731, Engine: CB750E-2464388
-K4 Frame: CB750-2521875, Engine: CB750E-2402688
-F1 Frame: CB750F-2019040, Engine: CB750E-2538876

The plan is to rearrange the 4 bikes as such:

Final bike #1 - the keeper:
-frame, tank, sidecovers, forks, and everything else to preserve the styling from the K4.

-Rear swing arm, rear wheel, engine, chrome bits, and all supporting hardware from the F1. My understanding is that the F1 swing arm is 1 inch longer, but otherwise should work with the K4 frame as long as it has the matching counter shaft spacing for the sprocket; which it should since I'm also using the engine? Also from what I've read the F engines have a bit high compression than the K's?

-Exhaust and carburetors from the K8, I like the pumper carbs from the K8 the best, very punchy. Also the 4-4 megaphone exhaust has had all but one baffle in each pipe removed. which makes it sound quite mean.

-I don't yet have a seat for this bike yet, neither the K2 or the K4 even came with a seat pan. So that will have to be sourced at some point.

-The hope is to have the best of all worlds, K4 styling, F1 brakes and performance, K8 carbs and modified exhaust.


Final bike #2 - runner to sell:
-Straight frame, Carbs, and exhaust from the F1.

-Just about everything else from the K8: Tank, seat, fenders, controls, forks, wheels, swing-arm, etc. I can't find much on compatibility between these two bikes, but from what I understand everything should match up because the seat hinges are on the same side and the tank mount spacing is the same.

-The hope for this bike is to have a decent runner to sell, everything should work and be complete, the engine will just be a little tired. The biggest problem is if I go with the K8 seat and tank, I don't think the K8 side covers will mount up(they are just unpainted reproductions). I have the F1 tank and seat pan(in okay condition), but no side covers and the seat foam doesn't exist online as far as i can tell.


Final bike #3 - just enough to run and sell:
-Straight frame, engine, swing arm and wheels from the K2.

-Carbs, airbox, and some other bits from the K4.

-K4 also came with a $30 ebay seat I'll throw on there, I ill probably use my old dented-up green K8 tank from the crash, it won't mount up well but it'll work good enough.

-front forks from the F1 probably?

- It won't have an exhaust which isn't good, but I think I can get it running open header and sell it as a project to some one. The K2 engine is questionable as the compression isn't great and the left side cover is unbolted for some reason. But I'm confident I can at least get it running.


Final bike #4 - a terrible parts bike: It's easier to say what is going to be bad about this one, as it's just a stockpile of the worst parts.

- Bent K8 frame, Seized K4 engine, horrible modified chopper forks from the K2, and whatever other bad parts come off the other 3 bikes.

- This one will probably sit on the side of the garage under a cover, I will snag parts off it if I ever need to.


All of this also means I will probably have a F1 tank and seat pan/ cowel to sell, I will post that here if that is the case.

I think I have enough oil tanks, hoses, cables, etc across the 4 of them to make it all work. I won't know for sure until I start digging in.

Please let me know what y'all think, and if you have any notes on compatibility

Thanks for reading -Kyle

[CB_Jeff]

Hey Kyle, a nice set of bikes you have there, too bad about the K8. 
I'll follow your thread, I have the parts to do the same thing with an early-K frame.  The tricky part is rear brake M/C and pedal, they are not fully compatible with a KO / K6 frame.  It's been done, but might require some fab work and / or welding
I will follow your progress!

[BenelliSEI]

Following. Lots to do!

[seanbarney41]

As an F and K owner and lover, I wouldn't bother with the F swingarm and rear wheel/ brake.  The F bike disc system is primitive and as the rear brake only does approx. 25% of the stopping power there is really only the penalty of the excess weight and service parts for those calipers are getting extremely scarce.  Also, the F swingarm is longer and will just make an already extremely stable bike even more so.  I think the 75/76 F countershaft sprocket lines up with 69-76 K rear sprocket making the F engine a bolt in replacement.  It's the 77/78F that have the countershaft 10 mm outboard. 

[Honda Hansel]

Well looks like you have the work cut out for a long time
I know what I would do .in this case
The CB750F bike would be my choice

Lost track of all your plans for all the models ?

For the compression and HP and camshaft changes did not change till the 1977-78 model CB750F
where the head design changed .
Larger valves and cam change to more duration/lift and combustion chamber and domed pistons

I do have a 1977 CB750F ready to go if you loos interest in your project 
Good luck
will follow your progress

Honda Hansel


     

[Mikerts_Garage]

Hey Kyle, a nice set of bikes you have there, too bad about the K8. 
I'll follow your thread, I have the parts to do the same thing with an early-K frame.  The tricky part is rear brake M/C and pedal, they are not fully compatible with a KO / K6 frame.  It's been done, but might require some fab work and / or welding
I will follow your progress!

Jeff, thank you for the info, i was a bit worried about the brake pedal swap. Can you expand on what fabrication? I will be more inclined if i can modify the pedal and not the frame.

As an F and K owner and lover, I wouldn't bother with the F swingarm and rear wheel/ brake.  The F bike disc system is primitive and as the rear brake only does approx. 25% of the stopping power there is really only the penalty of the excess weight and service parts for those calipers are getting extremely scarce.  Also, the F swingarm is longer and will just make an already extremely stable bike even more so.  I think the 75/76 F countershaft sprocket lines up with 69-76 K rear sprocket making the F engine a bolt in replacement.  It's the 77/78F that have the countershaft 10 mm outboard. 

I agree, there is no appreciable performance benefit to the rear disc. And the complexity may not be worth it either. But i just feel it has an inherent "cool factor" I love the look of the rear disc on spoked wheels, and i also think moving the rear wheel and inch back might look cool as well. If i go for the rear disc, it will purely be for intangible reasons  It also largely depends on master cylinder/ brake pedal compatability though.

Well looks like you have the work cut out for a long time
I know what I would do .in this case
The CB750F bike would be my choice

Lost track of all your plans for all the models ?

For the compression and HP and camshaft changes did not change till the 1977-78 model CB750F
where the head design changed .
Larger valves and cam change to more duration/lift and combustion chamber and domed pistons

I do have a 1977 CB750F ready to go if you loos interest in your project 
Good luck
will follow your progress

Honda Hansel

Hansel, I'm a little disappointed to hear the performance improvements came 2 years later. However, the F engine i have has the best compression by a good margin, so I'll stick with it. Any idea if the K engines from 77-78 came with the same improvements?

I personally like the early K's best, the fork gators, raised gas caps, and 4-4 exhausts are drool worthy imo. I also own a CB550F so i guess i get my 4-1 fill there.

Thank you all for your interest. It might be a minute before this project starts in full as winter in Utah drags into March. The plan currently is to get a group of friends together and try to get the bulk of the work done in a weekend, bikes 2, 3, and 4 will be more or less slapped together. Bike 1 will be done more properly over time.

[Honda Hansel]

Hi
The 750F 1975-1976 use the same cam as the 1977 K7 and 1978K8

The 1977 F2 and 1978 F3 use a different cam and head
The part numbers are for the cam 14101 410 000.The head has 410 casting number and the pistons are domed and the combustion chamber is shaped different and the valves are bigger
The 410 out of the part number is 77-78 F bike classification

You can all ways go on CMSNL.com and look it up
that's where I all ways go to check for parts compatibility

Good luck
Honda Hansel
 
 

[Stev-o]

Sounds as if you have "1st World Problem", which bike to build and keep!  Obviously lots of options but first thought is build the K bike for the keeper and trash the K8 frame as it's toasted. Not worth the time building a bike with it.

[Gurp]

how far over are the chopper forks?
i have a extra set of +8 forks but i would like another if you are willing to sell.

[Mikerts_Garage]

Hi
The 750F 1975-1976 use the same cam as the 1977 K7 and 1978K8

The 1977 F2 and 1978 F3 use a different cam and head
The part numbers are for the cam 14101 410 000.The head has 410 casting number and the pistons are domed and the combustion chamber is shaped different and the valves are bigger
The 410 out of the part number is 77-78 F bike classification

You can all ways go on CMSNL.com and look it up
that's where I all ways go to check for parts compatibility

Good luck
Honda Hansel

Hansel, Thank you for the information. I hope some day I get to ride a 77-78 F to feel the difference. Same with the K0 as I hear they are also a bit faster.

Sounds as if you have "1st World Problem", which bike to build and keep!  Obviously lots of options but first thought is build the K bike for the keeper and trash the K8 frame as it's toasted. Not worth the time building a bike with it.

a 1st world problem indeed, my first instinct is to build the F bike into a sidecar rig. I miss my 1994 Ural tourist, not for the rattly bucket of bolts that was the engine, but definitely for the side car. And the 3rd bike is the perfect candidate for a tracker build as it won't have an exhaust, side covers, seat, or tank that fits. But at the end of the day I don't need 3 CB750s when I already have 7 other bikes that desperately need my attention 

I don't know what the ultimate fate of the twisted frame and siezed engine will be. I already tried soaking the siezed engine in WD-40 and seafoam for a couple months to no avail. For now the plan is just to make a really bad roller as it'll be easier to store.

how far over are the chopper forks?
i have a extra set of +8 forks but i would like another if you are willing to sell.

They're long enough to make the center stand barley work, like teetering on one leg or the other. Right now neither fork leg has springs in them? the fork tubes are just so long that they bottom out in the fork bodies. Seems like a death wish to me. maybe you could add a spring and make them ~+8? I'm not sure, for some reason I took this photo, and only this photo, I believe that's a 12" harbor freight wrench for scale(see attachment).

If you are still interested let me know and I can get better measurements when I go to swap the forks out.

[Gurp]

When ever you take some measurements lmk I'll prob scoop them from you!