Author Topic: '80 CB750F SuperSport build (DOHC, don't hate me)  (Read 10196 times)

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Zpack

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'80 CB750F SuperSport build (DOHC, don't hate me)
« on: April 23, 2010, 01:05:15 PM »
    I know I'm committing a grave offense, so I will apologize up front. This is a 1980 CB750F DOHC *GASP*. OK, I'm glad I got that off my chest.

    Now, on to business. My first build was a bit of a practice run, I didn't do quite everything I wanted due to time/money/fear of destroying my bike. This time I'm not holding back: frame up, no-holds-barred. There will be several new challenges with this build which I'll get to later, but first, on to the bike!

    What I was really looking for was a blank slate, just an engine and frame, something I wouldn't feel guilty about tearing up. This is what I found:




    Not running? Check. Missing parts? Check. Paint-by-numbers flame job? Check. Absolutely perfect!




    It was sitting for quite a while; outside, but under a carport




    And the main reason I bought this bike: the big, beautiful engine




    Admittedly this was a risky buy. I brought my compression tester to the seller's house to check the engine, but the starter wasn't working, it has no kick starter, and to top it all off: no chain! So there was no way to even see if the engine was a solid, melted-together chunk of steel and aluminum inside :-\. The guy who was selling it, Steve, told me it had been running not long ago when it was owned by a friend of his. This guy had torn it apart to paint it, but before he could put it back together his house got foreclosed on so he had to sell it for some extra cash. Steve bought it because his wife has a bike and he thought it wouldn't take too much to get this one running; he thought he could use it to learn on and then go on rides with her. Well, Steve had some medical problems so he put it off for a while, then he found out that this was a sport-bike and probably not the best thing to learn on. To Craig's list it went, and after talking him down $200, it came home with me. As an interesting addition, he picked up a front end from a swap-meet that appears to be from a '85 CB700 nighthawk. It's pretty cool, and I think I'm going to use it... more on that in future posts.

    The first thing I did when I got home was hot-wire the starter to see if the engine turned.... success! It turned over strong and it sounded like it should; so it wasn't seized, check it off the list. Next I drained the oil, luckily it was full, and it didn't look too bad. No gear teeth came out, and the oil was black and had no signs of moisture inclusion. Check two. Now the wife and I are really getting into it, tonight we check the compression and continue the tear-down.

     My inspiration for this build is the Wrenchmonkees CB750 KZ



     According to BikeExif.com this type of build starts at around $13,000- I'll be doing this myself, thank-you.

     There are a couple of challenges in this build that I am looking forward to:
  • Welding up a new rearframe, I'm a good welder, but it's challenging non the less.
  • Making and upholstering a custom seat
  • Making a clean, hidden, functional and safe custom wiring harness
  • Adapting the '85 front end and brakes to fit
  • Making my ambitious idea for the paint look professional
  • And the really hard one: do it all for less than $2,500. Including the initial cost of the bike

    The cost is, of course, going to be the hardest one, but I think I can do it. I will be keeping you posted on the build and at the end I'll let everyone know how much it ended up costing. SO! off the computer and down to the garage!

Offline RustyOlive

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #1 on: April 23, 2010, 01:18:05 PM »
good luck senior, keep us posted on your progress! =)
Untitled by cameron.bateman, on FlickrRustyOlive, on Flickr

Offline weekend_junkie

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #2 on: April 23, 2010, 01:35:33 PM »
I see nothing wrong with that machine, except for the flame paint.  The place with the DOHC answers you'll eventually need is www.cb1100f.net .  I'm a convert from there since I got my cb350.

Biggest tip I can offer is find an airbox or get aftermarket carbs.  These are vacuum lift carbs, not mechanical like SOHC carbs, and pods will just leave you frustrated.    Keihin CR-29 will be your weapon on choice, probably can be had for $400.
Dan
2012 Triumph Tiger Explorer / 1981 CB900F / 2002 VFR800 / 1973 CB350F / 1973 CB350F mistake / 1976 CB360T Cafe /1976 CB200 Cafe / 1989 GL1500 w/ sidecar / 1949 IMZ w/ sidecar

Zpack

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #3 on: April 23, 2010, 05:44:30 PM »
   $400 FOR CARBS? did you even see my budget? LoL, thanks for the tips Weekend_junkie, I guess I'll be updating two forums now!

Zpack

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #4 on: April 23, 2010, 09:09:49 PM »
Ok so the wife and I did a compression test tonight and I don't really understand the results so maybe you guys can help. Here are the results of the first test:

cylinder 1: 130PSI
cylinder 2: 110PSI
cylinder 3: 130PSI
cylinder 4: 155PSI

Remember this is a cold engine that hasn't run in a long time, so while the results aren't great, they aren't terrible either. So I have heard that you should pour a tablespoon of oil into the cylinder and recheck the compression: if it stays the same then you have worn valves, if it rises to normal pressure it is the rings. So I tried it; one at a time I poured about a tablespoon of oil into a cylinder, turned it over about 4 times to clear the excess, then checked the compression. Every cylinder was OVER 300PSI :o, not at all what I was expecting. It almost seems like the exhaust valves aren't even opening- did I do something wrong? Any input would be greatly appreciated!

Offline BeSeeingYou

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #5 on: April 23, 2010, 09:56:30 PM »
To much oil?  You have decreased the size of the combustion chamber and raised the CR.  Fluid does not compress.

Zpack

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #6 on: April 23, 2010, 10:11:03 PM »
  I suppose that could be it, but like I said I cranked the engine 4 times to get the excess out. I'll try it again with less- thanks!

Offline weekend_junkie

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #7 on: April 25, 2010, 03:28:42 PM »
You had a budget of $2500, minus $200 for the bike... I could go a long way with that budget on one of these bikes.

Compression around 160 would be a lot better, but at least you've got compression across the board.  Make sure you have the right tools and check your valve clearances, trying for .005".  Also, NO COMPLAINING that you have 16 valves instead of 8!  If you can't find shims, toyota and triumph also have 25mm shims. 
Dan
2012 Triumph Tiger Explorer / 1981 CB900F / 2002 VFR800 / 1973 CB350F / 1973 CB350F mistake / 1976 CB360T Cafe /1976 CB200 Cafe / 1989 GL1500 w/ sidecar / 1949 IMZ w/ sidecar

Zpack

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #8 on: April 25, 2010, 04:35:22 PM »
Oh no, I didn't get away for $200 with this baby- I talked him down $200; from $950 to $750. Think it was too much?

Offline weekend_junkie

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #9 on: April 25, 2010, 05:37:58 PM »
$950 for a non-runner 750 would have been too much, but $1 per cc is better.

Look me up when you come over to the cb1100f.net site.
Dan
2012 Triumph Tiger Explorer / 1981 CB900F / 2002 VFR800 / 1973 CB350F / 1973 CB350F mistake / 1976 CB360T Cafe /1976 CB200 Cafe / 1989 GL1500 w/ sidecar / 1949 IMZ w/ sidecar

Koshin

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #10 on: April 25, 2010, 06:44:18 PM »
that KZ is awesome.  I hope yours ends up looking like that thing

Zpack

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #11 on: April 26, 2010, 08:14:00 PM »
Progress pics ;D

The wife and I are deep into it now-



after a bath-





got the carbs soaking-



all that's left is the engine-



I mentioned that I got the front end of an '85 CB700 with the bike which I wanted to put on, but after much deliberation I decided that it will be too much trouble to make it work. For starters, the triple clamps that came with it have the wrong steering stem, and both the old and the new are welded into the lower tree so no-swappy. Also the '85 shocks are a good 2" longer than the '80's, so it would look funny with that much shock sticking out above the top clamp. Oh well, they'll probably find their way onto ebay :P.

One nice discovery I made was a pair of $300 Dyna coils already installed- something I was probably going to buy anyway. The stock ignition modules are both fried, probably because the new coils are 3ohm and not 1.5 like the stock ones, but no matter; my Dyna 2000 ignition is on it's way!

We started rebuilding the breaks. The master cylinders are trashed, but the front one that came on the spare front end is in good shape, so it wasn't all lost. Up next: pull the engine out and lift off the valve cover, here's hoping I don't need to replace 16 shims!
« Last Edit: April 26, 2010, 09:04:45 PM by Zpack »

Offline Pinhead

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #12 on: April 26, 2010, 10:29:24 PM »
I've always liked the look of the '80s SuperSports. This should be a nice project!

Quick! Someone with a '79 CB650 head over to Zpack's place and see if the DOHC's tank will fit!!!
Doug

Click --> Cheap Regulator/Rectifier for any of Honda's 3-phase charging systems (all SOHC4's).

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By the way, I'm going for the tinfoil pants...so they can't read my private thoughts.
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Offline weekend_junkie

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #13 on: April 26, 2010, 11:54:27 PM »
I don't even need to look, but you have the green DYNA coils.  I have the grey 2.2 coils on another bike.  Unless you're going to get the DYNA ignition (not a fan, lots of quality control/overheating issues), opt for CBR coils and stock ignitors.  $20 on ebay and the performance/reliability is remarkable.

Looks like decent progress!
Dan
2012 Triumph Tiger Explorer / 1981 CB900F / 2002 VFR800 / 1973 CB350F / 1973 CB350F mistake / 1976 CB360T Cafe /1976 CB200 Cafe / 1989 GL1500 w/ sidecar / 1949 IMZ w/ sidecar

Zpack

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Re: '80 CB750F SuperSport build (DOHC, don't hate me)
« Reply #14 on: May 18, 2010, 12:29:52 PM »
New parts! The UPS guy has been making more trips down my long gravel road lately to bring me motorcycle goodies:



Some maintanance items- master cylinder kits, gaskets, exc... some upgrades- Dyna 2000 ignition, Dyna 2.2 ohm coils, plugs, wires, filters... and a couple of appearance upgrades- a stainless steel bolt kit, and Norman Hyde "M" bars. I'm really excited about the bars, I think they look great, and they have a very similar bend to the ones on my CB550 which I find very comfortable.



The wife and I have been doing various chores on the bike. The alternator cover gasket was bad, so some water got in there at some point and made some nice surface rust on the rotor and stator. While she took the sandpaper to those I got to work on the rear master cylinder.



It was not pretty, the retaining ring was so rusted I broke both of the circlip eyes off and busted the tip off of my circlip tool in the process. I finally worked it out with a screwdriver and a lot of WD-40. It took a claw hammer to pull the piston out of the cylinder, and inside was some nice thick and chunky mud. The cylinder wall had a good bit of corrosion and pitting so I used my tried and true small cylinder honing technique to polish it up. I take a drill bit that is a good 1/8" smaller than the I.D. of the cylinder and I wrap it with 2000 grit wet-dry sandpaper. Just take a strip about 2"x8" and fold it in half long-ways so it is 1"x8", then wrap it really tight around the bit in the direction of rotation. I add or remove some sandpaper so it fits snugly in the bore



Then I work it back and forth at about 300 RPM with a drill and flush it with a lot of soapy water to keep the sandpaper from loading up and to keep any chunks from scoring the wall



It works like a charm every time, you may need to do it in a couple of steps depending on how bad it is. I would have taken an after photo but the master cylinder is in a nice bath of MEK to prepare it for paint.