Author Topic: 1974 CB750K restomod (Read 5543 times)

meangreen · « **on:** December 06, 2015, 09:07:12 PM »

As a teenager, I had a green '73 CB450 that I really liked. I've had a lot of bikes since then, but the CB450 has always stuck with me. I have always wanted to build up a 750 from the same era.

I' recently found a '74 CB750K on Craigslist. Someone had put a ton of money into an engine and a bunch of other parts for it. Something had happened to it at some point, and it was in non-running order. The current owner had bought it in the condition it was in, and had no information on it other than a huge folder of receipts. I dragged my feet on it for a few weeks, until my wife told me I had better go get it before someone else did.

Originally, I figured it'd need some engine work, and a few other odds and ends. It's turning into more of a build, though, so I figured a project thread was in order. I plan to make some repairs, clean it up, do some refining, and eventually build the original engine to make some power.

meangreen · « **Reply #1 on:** December 06, 2015, 09:18:05 PM »

We managed to get it running where it was, after a few hours of messing around, and I rode it home with my wife following. The engine was obviously very sick. A check of the valve adjustment showed that 3 of the intakes were very loose, and there was no compression on the #3 cylinder.

I decided to mod the frame for a frame kit to facilitate engine work in-frame. I've been working on vehicles for too long to want to mess around with pulling the engine for every little thing.

When I removed the head, I found that three of the intake valves were bent from contacting the pistons. That sucks.

Started polishing the valve cover the other day.

SOHC4 Cafe Racer Fan · « **Reply #2 on:** December 06, 2015, 10:03:14 PM »

That's one cool wife!

Bummer about the pistons.

meangreen · « **Reply #3 on:** December 07, 2015, 03:41:33 AM »

She's pretty incredible!

Best I can tell, the PO threw it together without checking anything.

SF · « **Reply #4 on:** December 07, 2015, 09:08:19 PM »

Cool wife indeed, nice bike to

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meangreen · « **Reply #5 on:** December 14, 2015, 05:44:13 PM »

My original plan of running a spare engine while building an 836 has changed. I found a used head from a 76 750F with very low miles, and have decided to freshen everything up and run the lower end as is for now. The head should be here on Friday, so I may be ready for dry assembly next week sometime.
I'm on the fence with gauges for this thing. It's got one of those crappy 2" mini speedos, and no tach currently. I can't decide if I want to go back to the stock gauges, or do something aftermarket. My first thought was to go with the Koso tach/speedo combo like this:

I also plan to run an old school bikini fairing like one of these :

The original master cylinder is long gone. It's got an Emgo master cylinder with a butt-ugly remote reservoir, which needs to go away. I see there are kits you can buy that have the manual clutch lever on one side, and matching brake master cylinder on the other. I might look into something like that for my build.

Still working on that polished valve cover. They take forever.

SOHC4 Cafe Racer Fan · « **Reply #6 on:** December 14, 2015, 10:23:02 PM »

Beware Emgo. Not the best quality control, especially on their mechanical parts.

meangreen · « **Reply #7 on:** December 15, 2015, 07:14:52 AM »

Thanks. The Emgo MC seems crappy, and I don't understand the idea behind having the huge remote reservoir on it. I'd just as soon go back to the OE parts, but I'm not paying 100 bucks for one on Ebay, only to run the risk of it being junk.

timbo750 · « **Reply #8 on:** December 15, 2015, 06:55:47 PM »

I got a realy good retro sytle unit from DCC. A little more expensive than some but good quality and bled real easy. Haven't ridden with it yet but rolling the bike in the shed it stops with very little pressure.

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meangreen · « **Reply #9 on:** December 15, 2015, 08:13:27 PM »

I'll have to look into that, thanks.

On a different note, the PO (the one who built the bike) emailed me back today. It sounds like he over revved it and floated a few valves. Saves me some guesswork.

meangreen · « **Reply #10 on:** December 16, 2015, 06:17:11 PM »

Here's a few more pics of the bike and progress:

The PO had these nice rims put together.

Here's the huge folder of invoices that came with it:

Finished the valve cover.

meangreen · « **Reply #11 on:** December 19, 2015, 06:39:20 PM »

The bike came with a Mac 4-1 canister system on it that appears a little tough, but looks solid. The muffler sounded pretty corked up, though, and I wondered how much it was actually flowing. I slid the baffles out, and found that the little 1.5" tube on the front side of the baffle was kinked down pretty badly. No wonder it sounded constipated.

I cut down the baffle, so that all that's left is the end cap and about 6" of pipe going into the muffler body. The exhaust note is quite a bit louder, but I know that noise does not always equal flow. The muffler body has flaking chrome, huge scrapes underneath, and rust setting in, so I doubt I'm going to build a baffle for it, and will probably just toss it at some point. I'm thinking about going with a Supertrapp like this:

I'll just refinish the rest of the exhaust with VHT and replace the wrap.

meangreen · « **Reply #12 on:** December 22, 2015, 10:37:35 AM »

I had some extra time yesterday, and decided to tackle the carbs. The bike had been sitting with fuel in the carbs for at least two years before I bought it, so all the jets were plugged up, all the slides were stuck, and all of the fuel bowls were full of green gunk. Although I managed to free everything up enough to get it to run, they still needed to be thoroughly gone through.

I used the Pine Sol and crock pot method, and the results were amazing.

The new head showed up yesterday. It's in great shape, and will just need minor work before I can use it.

754 · « **Reply #13 on:** December 22, 2015, 06:54:19 PM »

If its an F head make sure you got the right cylinders under it..

meangreen · « **Reply #14 on:** December 23, 2015, 04:21:36 AM »

What should I be looking for?

754 · « **Reply #15 on:** December 23, 2015, 08:46:33 AM »

The oil drains are changed on F, you can mix one way i think but not the other..

That orange Duc you posted looks a lot like my XL 350..

meangreen · « **Reply #16 on:** December 23, 2015, 12:06:52 PM »

I double checked the oil drains on both heads. Both look to drain the same way. Also, my new head is a 392.

meangreen · « **Reply #17 on:** January 04, 2016, 02:21:47 AM »

I spent some time here and there working on the new head over the last week. I swapped over the HD springs and shims to the new head, replaced the valve stem seals, lapped in the valves, and gave everything a thorough cleaning. The gasket surface looked good, but I hand planed it, along with the deck. There was still a good cross hatch pattern present in the cylinders, and the rings only had a few thousand miles on them, so I didn't do a thing there other than clean everything.
I had planned to leave the cylinders and head silver, but the finish was beat up and weathered. I sprayed them with RustOleum high heat black, which turned out pretty nice.
I sprayed the base and head gaskets with Permatex copper gasket dressing before assembly. Everything went together well, with the only issue being an upside down valve cover gasket.
The engine runs and is leak free, but idles like crap. It's got pods, and the factory air cleaner is gone. I static timed it and have it close, but need to set the timing dynamically. I'm guessing that the slow jets are way too small, since the idle air screws have to be almost seated for it to idle at all. I don't have a tach right now, but it sounds like I have to set the idle so high that it's running on the main jets. I'm revisiting the carb adjustments and jet sizes this week.
The bolts pulled out of the crappy Emgo master cylinder, so now it really has to go.
Overall, it's about 90% there. I lucked out and ended up with a solid, good running engine for less than $200 worth of parts.

<-click to play

meangreen · « **Reply #18 on:** January 05, 2016, 05:16:17 PM »

Pulled the carbs apart this afternoon. I found one of the main jets laying in a float bowl, and another one loose. Guess it was idling on the mains after all ha ha.
I set the floats at 26mm and verified it has #40 pilots, #120 mains, and the fuel needle clips were at the #4 position. Put it all back together (about a 15 minute job with pods and a frame kit), and it runs quite a bit better. It's still got a rough idle, but based on what I've been reading about these bikes, the Web #41 and pods pretty much ensure a crappy idle. I'll recheck the sync and see if it helps. I may experiment with bigger pilots too. I want one of those plain clear points covers, but can't find one anywhere. I think it would be cool to illuminate the inside of the cover with LEDs.
Went for a quick spin in the 30 degree weather, and it ran quite a bit better than it did when I first put it together. I can't really do much more with jetting until it warms up. I'm excited about being able to swap jets in just a few minutes with the carbs in place. I tell people that the SOHC 750s were made before they really put a lot of time into making them tough to work on.
I have lowered it 1 1/2 inches, by pulling the spacers out of the front fork tubes, and chopping a coil of of each rear spring. I may decide to go with some 12" rear shocks at a later date, because I worry about losing a spring retainer in the rear if I top the suspension out on the stock shocks. For now, it looks cool, and it is just high enough to where the stock stand still works.

meangreen · « **Reply #19 on:** January 05, 2016, 05:48:24 PM »

I would like more of an upswept exhaust than what's on there now. I'm considering cutting the muffler off the angled extension pipe, rotating it for more upsweep, and going with a slip on muffler. I like the look and sound of the 12" automotive glass pack and will probably try one. If it doesn't work out, I'll probably go with a Supertrapp and competition end cap. I plan to relocate the passenger pegs higher on the frame to accommodate the new exhaust.

I still haven't made any decisions on gauges. All of the original hardware is gone. I like the look of the stockers, but I would like to go digital without spending a lot. I like the Koso TT, as well as the Trail Tech Vapor. I may also Ebay one of those crappy 2" tachs to match my crappy 2" speedo and get me down the road.

This thing has the crappiest brakes of any bike I've ever ridden. It needs some of those EBC sintered pads and shoes, minimum, and maybe an EBC rotor if it still needs help.

Lots to do.

timbo750 · « **Reply #20 on:** January 05, 2016, 06:14:05 PM »

Cross drill the rotor and fit braided brake lines, that should make a big difference, from what I know there is no aftermarket bolt on discs available. You can also convert to twin discs on the front which I have done but my bike is not on the road road yet.

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meangreen · « **Reply #21 on:** January 05, 2016, 10:18:00 PM »

Thanks Timbo. I have braided lines but never thought about drilling the stock rotor. I'll have to look into that.

Stev-o · « **Reply #22 on:** January 05, 2016, 10:30:25 PM »

These bikes seem to run best with the orig air box, but with proper jetting and tuning, it can be made to idle and run good.

Here s the MC Timbo mentioned...

https://www.davidsilverspares.com/CB750F2-(Single-overhead-cam)/part_126550/

Cone Eng makes some very nice mufflers.....

http://www.coneeng.com/motorcycle_components.html

meangreen · « **Reply #23 on:** January 09, 2016, 04:17:04 PM »

Thanks, Stev-o. Those Cone Engs look pretty nice. I'll have awhile to think about it, since I'll be slowing down on this project for awhile. Winter break is over for me on Monday, and I'll be back to a super hectic schedule until April. While I'm not exactly mothbaling it, I won't be making much progress during that time.

I have set the points a few times now, and I had issues with the screws loosening up. When I went to replace the screws, I found what appeared to be guts from the condensers leaking out onto the condenser bodies. I looked up symptoms of bad condensers, and sure enough, I have several. Rather than continue to screw the pooch with points, I'm just getting a Dyna S and being done with it. I'm building this one to ride.

I have decided on a Trail Tech Vapor for gauges. I like the option of displaying engine temp plus ambient temp, as well as all the other things that can be done with the Vapor. I'm building a little mount to tuck it up on top of the triple near where the factory gauges mounted, and it should all be neatly hidden by the cafe fairing. Should be pretty sweet.

My goal is to have this project mostly wrapped up by May, because I'm riding it to my commencement and it would sure suck if it broke down ha ha.

In the meantime, I removed the exhaust wrap from the header. The PO only wrapped about half of the length of the headpipes, and they looked really tacky that way. I knew the header would be rusty as hell under them, and I was right. I sanded down the rust, hit everything with naval jelly, then sprayed it with Rust Oleum BBQ paint. The plan is to wrap everything from cylinder head to muffler, once I have the exhaust sorted out.

meangreen · « **Reply #24 on:** February 01, 2016, 04:27:03 AM »

It's been slow going on this project, but I have managed to get a few things done.

My points were a mess. Not the points, specifically, but the condensers were friggin' shot. With the engine idling, my points plate area looked like a fireworks show, and when I pulled them, I found they were leaking their guts all over the place. I've only set up a few sets of points in my life, and I've had to mess with them enough already on this build to know that I really don't enjoy it. So, Dyna-S it is.

For the exhaust, I wanted something basic, stripped-down, and aggressive looking. The Mac muffler was beat up, didn't flow very well, was too low, and too big. I messed around with a few different designs. What I ended up doing was cutting off the muffler pipe, and adding a 2" ID extension pipe which gave me the ability to use H-D drag pipe baffle parts. Rotating everything around just right gave me a nice upsweep that just clears the passenger footpeg, and I put enough length on it to get the outlet behind my ears in an effort to reduce drone. I added a wrapped 10" drag pipe baffle with an internal diameter of around 1 5/8", to cut back the bark and raise back pressure a bit. I wanted it to look like a straight pipe system, so I tucked the baffle way up into the pipe to hide it. It sounds really nice, albeit loud, and looks pretty cool too. And for those of you who care, it cost less that $30 to make.

5
I plan to wrap the whole thing once the rest of the bike is finished, run it for awhile, and see what I think. If it doesn't work out, I'm going with the Delkevic system, which would be pretty pricey since I don't have any of the original hardware or spouts.

Due to some unseasonably warm weather, I took the bike out for a little 10 mile rip yesterday. It runs great, but idles like crap, and has some sag in the upper mid to full throttle range. Also, the rear shocks suck, and almost skittered me right into the ditch after I hit a sharp dip while cornering. Despite those things, it was fun to get it out and into the wind for a little bit. It's torquey, smooth, and gets lots of looks.

Next on the agenda will be 130 main jets, new needle and seat in all four carbs, some new 12" shocks, a cafe fairing, and getting that ugly dent pulled out of the tank. Oh yeah, and front brake upgrades.

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Author Topic: 1974 CB750K restomod (Read 5543 times)