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  1. #1
    CB750 Member P_e7's Avatar
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    Requesting some guidance

    There's a local posting on craigslist for a 1979 Honda CB750k. The seller said he had it running for a year and then it started taking a long time to start up. Looks like he tore the carbs off and never put them back on.

    The bike has a little over 50k miles so it's fairly high for a bike I'd tear apart and build back into a brat style commuter bike.

    I'll include a link to the ad, but from what everyone can tell is this too far gone to salvage? It does have a title.

    Here's the bike: https://portland.craigslist.org/clc/...617547995.html

    I was also thinking of this one as a parts bike since it doesn't have a title: https://portland.craigslist.org/mlt/...614617066.html

    Any help on this would be awesome.

  2. #2
    CB750 Addict Chris in Dorset England's Avatar
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    Never mind the mechanics, go and see it, talk to the seller. Face to face. Don't get scammed.

  3. #3
    CB750 Member P_e7's Avatar
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    Quote Originally Posted by Chris in Dorset England View Post
    Never mind the mechanics, go and see it, talk to the seller. Face to face. Don't get scammed.
    I plan to see it Monday evening. I have a compression tester and I'm going to see what the results are as well as check out some other stuff. I've never purchased a bike with that many miles though so I'm worried I will get way in over my head with the build.

    Is there a handy dandy chart with parts compatibility? Like for instance, could I take that engine off the 1980 cb and place it into the 79?

    Thanks for the response.

  4. #4
    CB750 Addict Chris in Dorset England's Avatar
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    Quote Originally Posted by P_e7 View Post
    I plan to see it Monday evening. I have a compression tester and I'm going to see what the results are as well as check out some other stuff. I've never purchased a bike with that many miles though so I'm worried I will get way in over my head with the build.

    Is there a handy dandy chart with parts compatibility? Like for instance, could I take that engine off the 1980 cb and place it into the 79?

    Thanks for the response.
    The engines are identical so there should be no problem there. (assuming both are DOHC) You can download a parts manual from the internet or find the CMS site where you can access their parts manual.
    Compression pressure when new is 170 psi, down to 150 is OK. lower than that needs investigation. Don't go pulling heads off if the pressure is low, first check the valve clearances. Ignore the manual, it's wrong, Honda have admitted that in the past. Set them to between 5 & 6 thou. It's a fiddling job, the clearances are set with shims. You will probably find the clearances are too small. The valves don't shut fully and then burn.

  5. #5
    CB750 Member P_e7's Avatar
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    So I went and checked the bike out. Didn't run any tests other than hooking up the battery and checking to see how the electrical system was. All the lights turn on and and everything functions.

    Tried the electronic start and it works and turns the engine over. Gas tank is not installed, nor are the carbs. The boots on the airbox are trashed and will need to be replaced if I decide to stick with the CV carbs. Checking inside the gas tank doesn't show any rust, but it will still need a deep clean since there's been gas sitting in it for a year.

    Also noticed it already has a 4 - 2 exhaust system on the bike. The exhaust shows no sign of wear and tear so it's fairly new.

    Won't have much time to check it out and start the work until next weekend.

    Didn't pay the 600 asking price. Bike with title and some extra parts, including a Clymer manual - $200

  6. #6
    CB750 Addict Dave's Avatar
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    Gasoline lasts much longer now then it did in the past. It can actually last several years in an air tight container.

    Smell and color are good indicators. Does it smell bad or just smell like gas regularly does? Is it fairly clear or has it turned brown?

    If it smells and looks ok, then it's ok. If you want a real test, drain the tank, remove the petcock, toss a handful of 6mm nuts in there. Shake it around for several minutes then pour out the nuts onto a clean paper or rag. Look for anything other then the nuts, pieces of dirt, gunk, rust, etc etc.

    After you do that compression test. Good luck.
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  7. #7
    CB750 Member P_e7's Avatar
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    Welp, the gas in the tank was inspected. Smelled like regular gas and when dumped out, it was a few shades darker than what I'd consider "normal" colored gas. Looked inside a bit and didn't see any rust. May throw some nuts in there at one point, but for now I'm okay with the tank.

    Did the compression test and the results were not ideal.

    Spark plug hole 1 - 120 PSI
    Spark plug hole 2 - 135 PSI
    Spark plug hole 3 - 100 PSI
    Spark plug hole 4 - 145 PSI

    The carbs are already stripped off when I bought the bike, so I've hit that point where I need to decide whether I want to invest the time in fixing up the carbs or doing a swap and saving myself the headache. I realize that running pods on the stock carbs is out of the question, but has anyone got any solid lead whether or not an aftermarket air box would be possible? The stock air box is giant and clunky. I've seen a few out there that are much smaller and better looking. Has anyone run one for an extended period of time?

  8. #8
    CB750 Addict Dave's Avatar
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    Sorry but, anything you do not related to fixing those compression numbers is a waste of time and energy.

    Even 145 is a bit low but having such a big difference between cylinders is tragic.

    On the bright side, it's usually the valves and not the rings. Check the valve clearances first. Adjust them if needed. Retest.

    If your compression still sucks pop the head off and look for burnt valves.

    My favorite way to test them is put the head upside down and level and fill the chambers with WD-40. It will get past any leaks and run out the ports.
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  9. #9
    CB750 Member P_e7's Avatar
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    Okay so I finally got back to the bike after taking care of my dog after she was spayed.

    I dropped the engine which was a huge pain in the butt as I need to do an engine rebuild anyway. I pulled the valve cover off to check the clearances and my numbers were pretty bad.

    My gauge doesn't have a shim that's lower than .0015" so that was the lowest I could go.

    Exhaust:

    1) .0015", 2) .0015", 3) .0015", 4) .0015", 5) .0015", 6) .0015", 7) ~.0015", 8) .0015"

    Intake:

    1) .0015", 2) .0015", 3) .0015", 4) .0015", 5) .0015", 6) .0015", 7) .006", 8) .005"

  10. #10
    CB750 Addict Dave's Avatar
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    Those should all be between .08 to .09 mm (.003 to .0035) (Shims come in mm sizes so I suggest you get some metric feeler gauges as well as spending $12 on a shim wrench.

    Now comes the pain in the butt part. Each valve cap has a shims under it. Every single valve can and usually does have/need different thicknesses of shim.

    First you need a gap you can measure. The ones that are too tight to measure accurately you'll have to put in something thinner to even start.

    Take say #8 Intake. It's at .127 mm and you want to shoot for .08mm to .09mm (.085 is a good target and shims increase by .005mm) so you want to add .04mm to the shim thickness. Now you remove the shim and measure it with a mic and get say (random number here) 2.5mm. You add .04mm to that for a shim thickness of 2.9mm.

    Get an empty egg carton and mark it so you can put the shims in it in the same positions they are in the engine. Pull all the shims out and measure them all noting it all down. Hopefully you'll have one that's thin enough to use for measuring the clearance of every valve. Then do your math from those measurements. If your lucky, sometimes you can move a shim to a different location to get the proper gap. Your sure to end up needing to replace some and they ain't cheap.

    Myself, I'd want to somehow check if the valves were sealing before spending the cash on shims because if you have do any valve work, those shims won't fit any more. My favorite test is with the head off. Squirt WD-40 into the port and see if any gets through the closed valve.

    I've tested valves on the engine by using air too. You need an air compressor or air tank and a custom adapter that screws into the plug hole and connects to the air hose with an on off valve in it. I made one by welding an air hose end to a spark plug I knocked all the porcelain out of once. You pull the exhaust and the intake off, (and your oil cap, don't need to pressurize the engine case) put the cylinder on TDC, grab your stethoscope, if you don't have one you can use a piece 3/8' rubber hose and hold it to your ear, turn your air on with the pressure set about 140 psi and listen inside the ports for air leaking past the valves. Don't worry about the air leaking past the rings, that's normal as well as why you need the stethoscope, to be sure if air is leaking past the valves.

    PS Make sure you have the minimum or more valve clearance (you can just remove the shims to do it) before leak testing the valves


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    Last edited by Dave; 07-02-2018 at 12:41 AM. Reason: add chart
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  11. #11
    CB750 Member P_e7's Avatar
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    Okay...I'm finally getting back to this. I have the cylinder head yanked and the valves are pretty dang dirty, I'm setting that aside temporarily to focus on the cylinder block. The pistons were quite dirty, but not as bad as I expected.

    I'd like to clean the cylinder block for the most part so I can check the clearances and then go from there. I've found that pinesol kicked butt when test cleaning one of the pistons, but how about for the cylinder block itself? Would a pinesol soak be okay for a day or two so I can then scrub it and get it as clean as possible? Or should I try something less acidic for a longer soak?

    Also when measuring, is there a way around buying a more gauge? I need to buy calipers to measure the pistons themselves..

  12. #12
    CB750 Addict Dave's Avatar
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    Buy a "t-bore hole gauge" along with your calipers, they don't cost much. It's basically a spring loaded upside down T with a twist lock handle. They come in many sizes so be sure you get the right one.

    You stick in the cylinder (inline with the bore) unlock the handle, the T legs pop out, you lock it, remove it, and measure it with those calipers you bought. You usually do so at at least 3 different depths and twice at each depth the second time 90 degrees to the first to check for out of roundness.

    I've never used pinesol, I've always used Coca-Cola Classic

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  13. #13
    CB750 Member P_e7's Avatar
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    Quote Originally Posted by Dave View Post
    Buy a "t-bore hole gauge" along with your calipers, they don't cost much. It's basically a spring loaded upside down T with a twist lock handle. They come in many sizes so be sure you get the right one.

    You stick in the cylinder (inline with the bore) unlock the handle, the T legs pop out, you lock it, remove it, and measure it with those calipers you bought. You usually do so at at least 3 different depths and twice at each depth the second time 90 degrees to the first to check for out of roundness.

    I've never used pinesol, I've always used Coca-Cola Classic

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    Good to know about the t bore hole gauge. I'll need it eventually and they weren't as expensive as I thought.

    I'll post pictures, but my cylinder head and cylinder block are pretty grimy. Will coca cola work for that? If so I'd save a ton of money. I may try it on the pistons if it works well or I could use pine sol for the smaller parts and coke for the bigger pieces.

    As always, thanks Dave.

  14. #14
    CB750 Addict Dave's Avatar
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    Coca-Cola works good for breaking up corrosion caused by moisture in cylinders. EI It's good for breaking loose seized engines because of the citric acid and the carbonation. Scrubbing bubbling! (ok, the bubbles help lift loose stuff)

    GUNK brand foaming engine degreaser is the product I prefer for removing built up grease n grime from the outside.

    For small stuff a 1 gallon can of carburetor cleaner, this should come with a parts basket inside, works great, just don't leave stuff in there too long and use the rubber gloves like it says.

    You should make some phone calls in your area. If you can find an automotive machine shop nearby, they probably wouldn't charge much to "hot tank" your cylinder and head.

    A hot tank is basically the automotive version of a dish washer. It's just bigger and uses stronger cleaning chemicals.
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  15. #15
    CB750 Member P_e7's Avatar
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    Another update. I leak tested the valves before removing them all to give them a slight soak, all of the them passed the leak test so I'm getting a good seal there. Yanked all the valves out and put them into pinesol (that stuff has been magic for me as I've just been reusing it again and again).

    I pulled the cylinder block to measure the cylinders. Going off the clymer manual it looks like I'm right there at that point where I will need to have them rebored to the next largest dimension.

    I took a total of 6 measurements for each of the cylinders. Starting at the top and working my way down, making sure to take both horizontal and vertical measurements for top, middle, and bottom. I will post a chart down below. Unless someone advises different, it looks like I will need to have them bored out and will need new piston rings assuming my stock pistons are still okay to use (have yet to measure them).

    Cylinder 1:
    Top - Horizontal: 2.438" Vertical: 2.408"
    Middle - Horizontal: 2.448" Vertical: 2.436"
    Bottom - Horizontal: 2.440" Vertical: 2.440"

    Cylinder 2:
    Top - Horizontal: 2.439" Vertical: 2.442"
    Middle - Horizontal: 2.441" Vertical: 2.433"
    Bottom - Horizontal: 2.441" Vertical: 2.440"

    Cylinder 3:
    Top - Horizontal: 2.433" Vertical: 2.442"
    Middle - Horizontal: 2.447" Vertical: 2.444"
    Bottom - Horizontal: 2.440" Vertical: 2.443"

    Cylinder 4:
    Top - Horizontal: 2.439" Vertical: 2.441"
    Middle - Horizontal: 2.441" Vertical: 2.439"
    Bottom - Horizontal: 2.440" Vertical: 2.440"

    Given that I've never done this in my life I'm assuming there will be a small user error on my part or possibly not using a micrometer, I'm going to assume the numbers aren't 100% accurate. That being said I'm still safely going to say they're slightly out of spec and will need to be worked on.

    All of that being said, if someone much smarter than me agrees they need to be bored out, who are the online gurus I can send this to and not worry about it being bored incorrectly? Unless there's someone local in Portland Oregon that people have gone to and have been proven capable I'd rather send it to someone who will do it right the first time.

    Thoughts?

  16. #16
    CB750 Addict Chris in Dorset England's Avatar
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    Quote Originally Posted by P_e7 View Post
    Another update. I leak tested the valves before removing them all to give them a slight soak, all of the them passed the leak test so I'm getting a good seal there. Yanked all the valves out and put them into pinesol (that stuff has been magic for me as I've just been reusing it again and again).

    I pulled the cylinder block to measure the cylinders. Going off the clymer manual it looks like I'm right there at that point where I will need to have them rebored to the next largest dimension.

    I took a total of 6 measurements for each of the cylinders. Starting at the top and working my way down, making sure to take both horizontal and vertical measurements for top, middle, and bottom. I will post a chart down below. Unless someone advises different, it looks like I will need to have them bored out and will need new piston rings assuming my stock pistons are still okay to use (have yet to measure them).

    Cylinder 1:
    Top - Horizontal: 2.438" Vertical: 2.408"
    Middle - Horizontal: 2.448" Vertical: 2.436"
    Bottom - Horizontal: 2.440" Vertical: 2.440"

    Cylinder 2:
    Top - Horizontal: 2.439" Vertical: 2.442"
    Middle - Horizontal: 2.441" Vertical: 2.433"
    Bottom - Horizontal: 2.441" Vertical: 2.440"

    Cylinder 3:
    Top - Horizontal: 2.433" Vertical: 2.442"
    Middle - Horizontal: 2.447" Vertical: 2.444"
    Bottom - Horizontal: 2.440" Vertical: 2.443"

    Cylinder 4:
    Top - Horizontal: 2.439" Vertical: 2.441"
    Middle - Horizontal: 2.441" Vertical: 2.439"
    Bottom - Horizontal: 2.440" Vertical: 2.440"

    Given that I've never done this in my life I'm assuming there will be a small user error on my part or possibly not using a micrometer, I'm going to assume the numbers aren't 100% accurate. That being said I'm still safely going to say they're slightly out of spec and will need to be worked on.

    All of that being said, if someone much smarter than me agrees they need to be bored out, who are the online gurus I can send this to and not worry about it being bored incorrectly? Unless there's someone local in Portland Oregon that people have gone to and have been proven capable I'd rather send it to someone who will do it right the first time.

    Thoughts?
    Hi,
    Your best course of action is to take the cylinder block to a motorcycle engine specialist who has a good reputation and who can take accurate measurements looking for bore tapering, and ovality. If a rebore is required than that will involve new oversize pistons and rings. The old pistons etc would be too small. I am several thousand mile away from you so I cannot recommend anyone to you. Get in contact with amc49 on this site he will be able to give you further advice.

  17. #17
    CB750 Member P_e7's Avatar
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    Out of curiosity how many miles do you have on your bike? My bike has gone through two different gauge clusters so I can't say for certain how many miles the bike has.

    I will get in contact with AMC and go from there!

    Thank you

  18. #18
    CB750 Addict Chris in Dorset England's Avatar
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    Quote Originally Posted by P_e7 View Post
    Out of curiosity how many miles do you have on your bike? My bike has gone through two different gauge clusters so I can't say for certain how many miles the bike has.

    I will get in contact with AMC and go from there!

    Thank you
    Well the speedo says 18400, but that doesn't mean much, with a bike this old. It's condition that's important.

  19. #19
    CB750 Member P_e7's Avatar
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    That would somewhat make sense with mine. Mine is estimated to be around the 26k mile range. The engine doesn't look neglected, but definitely needs a rebuild before I do any serious riding on it.

  20. #20
    Moderator dirtdigger's Avatar
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    AMC is gone, he didnt like being told he needs to talk nicer to people.

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