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Thread: CB750F Restomod

  1. #1
    CB750 Member Bikemonkey's Avatar
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    CB750F Restomod

    Picked up two CB750F's as I stated in my first post in the Lounge section, and the plan is to get one running and sell it to try and recoup some of the cost of buying them. The other will be a restomod into a muscle style bike with modern running gear like the picture below.






    This is how they looked just after I picked them up.







    The bike in bits is missing the forks and a few odds and ends, but that's the one which will be heavily modded.


    The other night I started work on the complete bike.

    Pulled out the carbs as I'm fairly certain these will need a clean before the bike even runs!





    Engine turns over freely and feels like it has good compression.





    Then I unbolted the exhaust, to find it full of holes. I was planning on keeping it for the other bike, not any more. So that got shoved back on.



    Looking at the wiring, it's a right mess.



    Dumped the oil out the engine and put fresh in, as god knows how long it's been sat in there.



    Drained the tank of the old nasty fuel too. It seems as though someones tried to line the tank before as it's full of big chunks of something.



    Got the battery out my 400 and put it in after "hot-wiring" the bike as it didn't have a key. Good news is that the starter works and will turn the engine.



    Then I sorted out some HT leads and plug caps ready for when the carbs have been cleaned.



    Also whilst I know I'm going to be cleaning carbs I got the set off the parts bike and stripped those too.



    Ordered a battery so I can put a proper one in to get it running.

    Also been drooling over FCR's and titanium exhausts, but that will have to wait!

  2. #2
    CB750 Guru amc49's Avatar
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    Test the compression early so you don't waste much time and effort like so many do when it runs like crap. The engines are valve shim adjusted and virtually nobody ever does that. The valves then close up to lose compression and everybody yanks the carbs to work on them 10-15 times since the issues appear to be those of carbs. Book spec is 170 psi or so, the engine has issues at below 120. 120 will feel 'like a lot of compression' all day long to a finger, sorry, you can't do that.

    The service manual spec of .003" (US, not metric) on the valve clearances leaves much to be desired too, the valves will BURN at .002" because the number is not real, we can discuss that later. If you want it to run long and hard use .005", the engine will love it.

    There is no '84 model in these, all made in '79-'83.

  3. #3
    CB750 Member Bikemonkey's Avatar
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    Thanks for that, I'll get a compression tester once the battery has turned up.

    I'm not new to shim type valve clearances, my VFR's have seen to that!

    The 84 is probably just a late registered 83, however there is a few differences between the two bikes. Mainly the headstocks appear different and one of the carb float bowls on the later bike has an extra vacuum slide bit.

  4. #4
    CB750 Guru amc49's Avatar
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    Yes, there are differences like entire fork (from 35 to 37mm.), wheels a bit wider, twin piston brakes, the carb vac port is for a vacuum operated fuel demand valve. Commonly bypassed when they give trouble.

    Good for you on the test, the valves FU because the cam cap clearances are wider than the valve clearance, thus when you turn the engine the cams get pushed around in the whopping clearance to let the valves back off. Why you loosen them up to .005", it's because roughly .002" of what you see there is not real at all and why you can easily burn (I've done it myself) valves set at the low spec of .002" even though the manual says it's OK. .002" in essence is zero clearance. One added benefit of the loosening is that the interval to setting them again gets bigger too.

    You can easily use a car battery to whirl the motor and all day long with zero issues as long as you DO NOT START the car to have it running.

    Looks like you are already doing it, keep the carb slides with their caps, they are matched. If you have to break the rack then grind the choke shaft screws off as they are staked on the ends and ruin if you pull them without the grind. Ground they come right out and a bit of loctite used going back together you can use them again. Pull one choke plate off either pair, you only have to pull two not all four. Then you can get to the aircuts, which I would be doing seeing the condition of those carbs.

  5. #5
    CB750 Member Bikemonkey's Avatar
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    So I got a compression test done finally, and got 175psi (+ - 5) on each cylinder which was good.

    However I couldn't be bothered to sort the carbs properly, so I gave them a quick scrub, put them back together and whacked them on the bike.







    Managed a couple of seconds of very lumpy running, so I'm happy the engine will run and it's now up for sale as I need to concentrate on the real reason I bought these bikes!

    Made myself a roll about work bench as I don't have much space in the garage and will need to put this in the corner when my other bikes need work. Sat the frame and engine on top of it and cracked on with dismantling the engine.





    The wheels will be getting moved on (sold if someone wants them). I'm thinking early fireblade forks, CBR600f3 wheels and a Triumph trophy 900/1200 swingarm. I'll need to find some nice twinshocks too!

    But that's when I've sold the other one to fund those parts.

    Now to get the head off.

    Took the cam cover off, unfortunately the rev counter take off is broken and seems to be part of one of the cam caps. Can I replace this for another or is it matched to the cam?

    Got the rear cam out first, took some work with the rattle gun to get everything loose.





    One of the cam chain cover thing's bolts had already been snapped off by a PO, annoying but not impossible to sort as the head will probably be sent to a specialist to get the exhaust studs out too. Simply because I don't have the tooling to do it properly.



    Got the front camshaft out too, took me a while to figure out I had to take off the gear piece!



    Another broken bolt, this time my fault!



    Got the oil pipe off, and found another bolt broken by a PO! (It's one of the two holding the front of the head down)





    Then I started to lever off the head, being very careful not to damage anything. (Still broke a couple of cooling fins....)





    Bores look good, no scoring or marks. Left everything coated in oil as my garage gets a bit damp.

    Next is to get the barrels off. Are the nuts and the front bolt/nut the only things holding it on? It looks like there's bolts coming up from inside the upper crank case?

  6. #6
    CB750 Enthusiast Chris in Dorset England's Avatar
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    Hi,
    please contact me, re: sale of other bike. P.M. will be OK.Thanks

  7. #7
    CB750 Guru amc49's Avatar
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    The one nut all that's there but based on the pic you will have fits with stud corrosion locking the block in place.

    A 'rattle gun' is death on these, you kill the cam cap bolts and mucho trouble using them. The bolts are pretty much in most cases the lowest grade you can get. You won't find the other broken cam cap bad bolts until going back together.

    The tach cable cam cap is matched to the head and the rest of them but you can use another by careful fitting to get the correct clearance. If you simply bolt it on and go you risk ruining the entire engine.

  8. #8
    CB750 Member Bikemonkey's Avatar
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    Thanks for that, new cam cap bolts all round it is.

    I used the rattle gun as I didn't want to simply force the bolts and shear them.

    Any tips on levering up the barrels without damaging anything?

    The rev cap, you say buy a few and try to find a match? Would that be checking clearance using something like plastigauge?

  9. #9
    CB750 Guru amc49's Avatar
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    You ruin a head every time you remove a cap, buying 'a few' will likely not be an option unless people selling parts are idiots.

    You buy one then hope it is not too loose. then too tight you simply sand away at the split point using a piece of glass until you get proper clearance.

    Rattle guns 'shear' far more parts than by hand EVER could, and by hand you at least know it is coming before it happens, you get no warning using impact.

    Even new cap bolts are dainty as spit and break if overtorqued. Use 6-8 ft.lbs., they commonly strip at 10. Forget the service manual spec.

    The cylinder removal will give fits and depending on how much corrosion, there is no one set way to do it other than soak the studs/holes in WD or whatever passes for it for a day before attacking it.

  10. #10
    CB750 Guru amc49's Avatar
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    http://www.cb750c.com/modules.php?na...iewtopic&t=301

    Go there, register for free and look up 'genesound' who commonly posts and ask about the cam cap bolt set he sells at a whopping good price, indestructible as compared to the OEM ones. He sells the primo Viton valve stem seals as well.

  11. #11
    CB750 Member Bikemonkey's Avatar
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    Personally I've sheared far more by hand than with my impact driver (it's not that strong), but we'll put that down to me still learning really *I'm 24*.

    A new can of plusgas (penentrant) seems to be the way with the barrels then.

    Sanding away at a cam cap sounds fairly rough. Do you mean extremely fine wet and dry or an actual piece of glass? And how do you mean ruin a head? By replacing it or the simple fact of removing and refitting it?

    Thanks for that, just registered on there. Those prices are ridiculously good, think I'll be buying both bolts and seals
    Last edited by Bikemonkey; 08-12-2017 at 04:47 PM.

  12. #12
    CB750 Guru amc49's Avatar
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    The cam caps are match machined to the head, all is one piece, why you can't buy just the caps from Honda. When you yank a cap you wreck the entire head. The cam bores are lined bored at once and they can be slightly high or low and every head is slightly different. The split in the middle real world tends to move up or down a little bit. One could actually use shim stock with a hole cut for bolt to shim up a tight one I suppose, doable but I haven't done it.

    #400 sandpaper and yes on a piece of glass. I use like a small cheap picture frame so cheap and no edge cuts. Sand in a figure 8 motion and check the fit every so often. You of course do NOT want to do an entire set.................

  13. #13
    CB750 Member Bikemonkey's Avatar
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    Thanks for that.

    To be honest I've been toying with the idea of a digital speedo/tacho set-up, which wouldn't need the camshaft take off.

    But I guess I'll cross that bridge when I come to it. I don't think the engine will be going back together any time soon!

  14. #14
    CB750 Guru amc49's Avatar
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    Going back over the thread, weren't VFR clearances (post #3) adjusted by ROCKER ARM?

    The difference, they require no shims. Why nobody adjusts these, you have to bother to get the parts and it breaks the operation into two steps, first finding which needed then getting and installing them later. No, I ain't buying shims, I want to ride it NOW, the American half as-ed Way. Even the Honda dealerships used to cop out on that, I had to reset several on mine at new engine break-in when they looked it over and said it was 'OK'. It was NOT. The fee for that work was big and the work often partially done to rake in the cash.

    FYI, if mine that tach drive cam cap might even be able to be repaired enough to work and fine other than looks.

  15. #15
    CB750 Member Bikemonkey's Avatar
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    VFR400 NC30 is shim under bucket, you don't need to remove the cams though.

    I have heard plenty of stories of dealers charging for shim changes not done, it is a big job. Piece of cake when you've done it a few times before however.

    Looks is going to be a big focus of this build, so I'll probably go with a digital speedo/tach, just need to find the right one

  16. #16
    CB750 Guru amc49's Avatar
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    We only got the VF500 here stateside. I went looking around at parts lists and those were rocker so further looking revealed that the 500 was essentially a modded VF400 bottom end merged with 2 VF250 top ends and apparently the difference there. Interesting.

    I GOTTA ask............how on earth do you get shim out if you don't yank cams???? Usually they are hard pocketed in the retainer tops and/or tappet bottoms, you have no way to get to that as the tappet has to remove to even think about it. I've done shim under and they SUCK. Shim on top is so much easier to work with.

    Not arguing at all but my curiosity always gets the better of me when it comes to things mechanical.

  17. #17
    CB750 Member Bikemonkey's Avatar
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    The rocker arm is held in place by a spring, push it to one side and you uncover the shim.

    Simply fish out with a good magnet on a stick and have fun getting back in without dropping it!

  18. #18
    CB750 Member Bikemonkey's Avatar
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    So I've been having trouble getting the barrels off...

    Managed to get out 6 of the studs, the rest won't budge and neither will the barrels. So I turned my attention to stripping the rest of the engine.

    I'm storing all parts bagged and sealed in a few of these boxes as the engine won't be going back together any time soon!



    Got the clutch out with help from the rattle gun.



    Removed the gear selector and alternator bolt. Just need a great big bolt to arrive to pull off the rotor.



    Got the oil pump off too, which put up a small fight. Broken bolt was annoying, how on earth do they get that stuck when constantly surrounded by oil!



    Also got the starter clutch and idler gear off with the help of a gear puller.



    Once the bolt turns up I'll be splitting the cases and hoping to apply some force from the underneath of the barrels to get them free.

  19. #19
    CB750 Enthusiast Chris in Dorset England's Avatar
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    Is that a hole in the crankcase? The rear wheel spindle can be used to get the rota off, the threads are the same taper, it might be very difficult to get off, mine was
    That engine is in a fearful state, good luck!

  20. #20
    CB750 Member Bikemonkey's Avatar
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    Hole in the crankcase!

    I think/hope the light is playing tricks as I haven't noticed any hole.

    I'm sure the rotor will put up a fight though.

    The engine is pretty minging externally, hopefully the internals are within limits. The engine will probably be getting powdercoated, at minimum vapour blasted.

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