Topic: K8 Highway Racer - Finished

[JoeCooley]

Hello all! I'm a long time lurker and admirer of the sohc4. Decided to join and share my project after I finally took the dive and got my own.

So here it is! My newly purchased cb750 K8. It's had extensive work performed on it by the previous owner(s).

 I drove all the way to Irvine, CA after numerous texts and calls with the owner. It was in a bad running condition when I arrived to check it out but, I was already in love and committed to the bike. So I loaded it into the truck and hauled it back to San Diego.

The day I brought it home.


Here's the CL ad for some background info. I got it for a little less than the asking price.


It took quite a bit of cleaning and tuning but I've finally got the CR Special carbs running decently. There is still a small stumble in the mid range at certain load/rpm. I think it may the the 55 slow jets or a throttle cutaway change.
Look at those clean intakes ports!


DynaS ignition


My trick for a less messy oil change.


How the bike sits now with a new koso speedo/tach, clip on raised, add signals, led tail light, and cleaned.



My current stable of vehicles. 74 Jensen-Healey, K8, & 02 WRX Wagon (DD). Too may projects...

[madmtnmotors]

As long as you enjoy it and can ride it safely what's not to like? Good score on the CR carbs. As long as the other modifications don't create any unsafe conditions I'd say you did pretty well since the West coast prices tend to run a little higher than "back East". The funnel under the oil tank for draining is the same method I use. I also cut up a beer carton to use as a "diverter" underneath the oil filter housing to help keep oil off the pipes. It doesn't always work 100%, but sometimes it does!

Welcome aboard!

[calj737]

Seems a decent bike. I would encourage you to pull the exhaust and have it ceramic coated (inside and out) to help its finish and stall any future rot. It will also doll up the looks rather nicely. Replace those exhaust gasket copper O-rings when you do.

If you're running a 170 rear tire, likely there's offset sprockets front and rear. It would be advisable to pull the shift cover and inspect the cases in that area to insure there's no chain/case conflict or damage occurring. Too many folks install O-ring chains and they're too wide on the front sprocket which causes the chain to eat a hole in the case.

[madmtnmotors]

If you're running a 170 rear tire, likely there's offset sprockets front and rear. It would be advisable to pull the shift cover and inspect the cases in that area to insure there's no chain/case conflict or damage occurring. Too many folks install O-ring chains and they're too wide on the front sprocket which causes the chain to eat a hole in the case.

Holy cow, good eye. The high res pictures help a lot. Looks like a custom rear brake stay arm with a "dogleg" to clear the wheel. Well, mostly clear, since it looks like the scuffing on the tread aligns with the brake stay arm (or the inside of the swingarm).

Is the back side of that brake stay arm polished up shiny where it used to be painted at it's closest point to the rear wheel? 

[JoeCooley]

The bike is stable at speeds and rides quite well. It handles turns and stops very easily. The suspension geometry doesn't seem to be anything unsafe with all the part swapping. Thanks for the comments guys. I think I got a pretty good deal for all the work/time/money that has been put into it.

The bike does have offset sprockets. I haven't counted the teeth yet to see the ratio. The small sprocket cover has been drilled and clearance for the chain. I will have to pull it off to see make sure the chain isn't rubbing the case. I have a spare and need to replace it anyway. The PO did a pretty bad job of "clearancing" it with what I guess was a grinder.

No dog leg on the brake stay. It is curved a little to clear the tire. Not sure if that is factory or a modification.

The exhaust pipes are in pretty bad shape. They are leaking at the slip joints and many pin-holes that leak. The muffler baffles don't do much for sound reduction and the bike is quite loud. I will be replacing them with a MAC 4-1 canister setup soon. Just bought one used on ebay. Hopefully it shows up soon and fits the bike.

Eventually I'd like to weld in a frame hoop and put a brat seat on it. I really like the DimeCity seats with the flip up on the end. I just have to save my pennies because the are pricey.

[madmtnmotors]

No dog leg on the brake stay. It is curved a little to clear the tire. Not sure if that is factory or a modification.

The "curve" is the "dogleg" that I refer to. The stay arm should be straight due to the dynamics of the forces involved under braking. Under hard braking the rear brake panel is going to be pulling hard on that stay arm and the resulting effect is the forces exerted on the stay arm will be trying to pull it back out straight. This stay arm could easily deflect under hard braking and be rubbing the rear tire under braking. Any evidence of the tire touching the inside of the stay arm? (i.e. paint rubbed off the backside of the stay arm where it is closest to the tire?)

[JoeCooley]

Oh I understand now. I was thinking something different. I will have to check it after work. I'll report my findings.

On another note: I received my mac 4-1 today. I love when parts show up! It's like Christmas! Haha

[madmtnmotors]

I love when parts show up! It's like Christmas! Haha

Indeed! 

[SOHC4 Cafe Racer Fan]

[rickmoore24]

Nice score, I recall seeing this on the OC CL. It looked like a good bike in the ad when I saw it last. Does the head have port work done on it? It does look really clean in there. Cheers! Rick.

[JoeCooley]

Not sure on the port work. It does look really clean in there....

Small update:
Got around to installing the MAC 4-1 I got off eBay. The pipes were previously installed on a dohc bike. So a few changes were in order.
As you can see, there are major differences in the early and late exhaust clamps. I picked up a set of 77-78 clamps fromy a local moto junk yard along with some new gaskets.
Late sohc left, dohc right



How they fit. Don't mind the small dent in the muffler. It'll be replaced with a reverse cone from coneng.com and angle up a bit more to tuck a little better. It sounds good and isn't nearly as loud as the 4-4 that was on the bike.


I also have some old school Honda tank badges that I'm thinking of using. Not sure where to mount them. I have a couple options. These will be nice to break up the black & polished look and throw a little bit of color into the mix.



Overall happy with the bike. I have a quick video I'll post up also.

[calj737]

You might want to invest in a pair of rear sets for your bike. With the front end changes, you will likely find the footage changes more comfortable and better suited for your riding posture.

[JoeCooley]

You might want to invest in a pair of rear sets for your bike. With the front end changes, you will likely find the footage changes more comfortable and better suited for your riding posture.

I agree. Rear sets are in the near future. Any recommendations for an economic bolt on kit? It's really not that bad riding posture the way it is currently.

[JoeCooley]

Here's a quick video.

[rickmoore24]

I don't know about economical but....

http://www.fastfromthepast.com/rearsets/honda

[754]

 What are you doing with the old pipes ?

[JoeCooley]

What are you doing with the old pipes ?

Not sure. Probably just put them in storage for a future project. The head pipes and spigots are in great shape but the other half with the mufflers welded on are in bad shape. Leaking welds and shifty craftsmanship went into making them. I might use them to build a nice 4-2 setup.

[JoeCooley]

Took the bike for a 50mile ride today and met up with my gf for dinner. This is my first real ride with the new exhaust. I love the new sound of the 4-1 Mac setup. Loud enough to sound sporty but not obnoxiously loud and making my ears ring. Super happy with my eBay purchase.

She snapped a quick action shot...

[JoeCooley]

So maybe you guys can help me. I've installed a Koso TNT tach/speedo gauge. The tach is very jumpy. I've seen people build tach filters for this tach with other bikes but nothing CB750K specific. Does anyone have any info to fix this tach problem?

Here's a quick video.

[rickmoore24]

Check the ground and all connections, if they are good you can install a resistor but I'm not sure what size. Half watt maybe?? Inline of the tack signal of course. 


*edit: this will smooth out the signal.

[calj737]

Mostly the Koso uses its signal wire wrapped around the spark plug wire. If that wire is dirty or if there's insufficient wraps, you are prone to a fluctuating tach signal. An acetone wipe of the spark plug and 6-8 tight wraps with some electrical tape over them helps.

[JoeCooley]

I didn't use the plug wire method. I tapped into the coil positive lead  (blue wire). The instructions had both methods.

[calj737]

Isn't the Blue wire a negative? The BLK/WHT should be positive.

[JoeCooley]

Isn't the Blue wire a negative? The BLK/WHT should be positive.

Yes you are correct. I didn't realize that but just looked at the diagram again. The tach can be set to either coil positive or negative. I currently have it set for positive but have switched the tach to negative coil input and the tach reading was way off. I will try tapping into the black/white wire and see if it make a difference. Thanks for the input.

[JoeCooley]

Ok so time for an update. I picked up more parts for my bike. "My name is Joe and I have an addiction..."

So I was searching CL and found a great deal for a running K2 836, extra set of cast 836 pistons, and a flat brat seat.
[IMG]https://imageshack.com/i/pnz01RS7j[/img
[IMG]https://imageshack.com/i/po2Ia7Ddj[/img
The "836" is kinda unknown. It is supposed to have a hi-po cam in it. I'm going to tear it down and see if it really is an 836 w/ upgraded cam. If not, I have a piston kit. It does need some love tho. The po used red rtv on everything, including the headgasket. 
So yeah, it's leaking oil from a couple places. I'm going to get a full gasket kit from cycleX and replace most gaskets. Not sure if I will split the cases. They don't appear to be leaking.

The seat will be used later but will require some fab work. The engine came with some chrome and finned covers. The stator and clutch covers are near perfect and already made it onto my bike. The points cover is damaged and needs a little work but I think I can fix it.
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[IMG]https://imageshack.com/i/plv1sMTMj[/img