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  1. #1
    CB750 Member canuck1969's Avatar
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    1980 CB750 Custom Restoration- Let the Games Begin

    All,

    Just thought I would introduce myself to the forum. Just picked up a basket base 1980 Custom that will be my next restoration project. Recently finished a restoration of another 80's classic, a 1984 Yamaha XS650. I would post pics of it but not sure the die hard Honda folks want a Yamaha on the forum.

    For the CB it will need a lot of work. Hoping most of it will be cosmetic but starting to see some issues to address in the electrics and engine. Changed the oil and did a quick cleaning of the carbs so they would not leak anymore and managed to get it started but ran rough on idle and very uneven heat from the exhaust headers. From burning hot to warm at best on #4 cylinder At least it started which is more that I had expected.

    Ran a compression before I started it and had some lower numbers than I would like. The good news was they were withing a few pounds across all 4 cylinders and were around the 130 PSI range. After I got it started and managed to get some heat into the engine I rechecked it and 3 and 4 cylinder went up to 150 and 1 and 2 hovered around 130. Blowing some compressed air into the cylinder at tdc revealed air air coming out the exhaust on 1 and 2 so something is not right there. Good news is nothing was coming out the intake or crank case and 3 and 4 seemed sealed up. Need to do a proper leakdown test to confirm.

    Did a quick valve gap check last night (will repeat it again today) and they were all around 4 to 5 thou (that is a good sign) except for #1 cylinder. It was upwards of 9 thou on the exhaust. Not alarming but again, something going on there. Will get them set correctly and see what happens. Suspect I have some build up on # 1 and 2 exhaust valves. Will do a proper leak down test and valve clearance today and report back. Keeping fingers crossed at this point.

    Question on the valve clearance. Every bike I have owned I would check the exhaust and intake on TDC at the same time. The manual and clymer say to advance it 90 degrees between exhaust and intake readings. Can I not just find TDC for each cylinder and take the readings. What am I missing here??

    Here are some before pics. Lets see where it ends up.

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    1980 CB750 Custom (Project In Progress)
    1984 XS650 (Restoration Complete)
    2010 VStrom 650 (Main Rider)
    2012 Moto Guzzi Stelvio NTX (Sold)
    2009 Kawasaki KLR (Sold)

  2. #2
    CB750 Guru amc49's Avatar
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    Fort Worth TX.
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    You CAN check using the TDC method of setting valve but you MUST be at compression not overlap to do so.

    The valves can check wonky and why Honda tried to pick an exact specific spot. The problem is that the cams are as loose or looser than the valve clearances and then the springs push the cams all over the place to alter your readings. Why I say at least .002" of any reading you get there is not real at actually running, the cams then try to center up in the holes and run different clearances than what you find not running. Consider .002" as burning valves as it WILL do so. Why .005" is the number to shoot for, it guarantees not being near zero.

    The valves are not super hard and the heat treat is only on the surface. They were intended for a low lead time and 100% no lead fuel makes the exhaust tend to recede, some body may have found out and tried to set yours, but doing so does not fix burned valves, which then leak, not the rings which are pretty durable if the mileage is not excessive. The valve seals tend to get hard to leak oil and it will likely be them instead of the rings if you get oil smoke.

    Luck with that project, the carbs come off of all of them in an American testosterone knee-jerk when the valves begin to quit sealing and the carbs often have to be gone through more than once to get them back clean enough to run right. Consider defeating the aircuts, they will drive you nuts when they act up. The intake rubber manifolds between head and carbs are almost always bad after a while, they get hard and then they leak when you crack them yanking carbs. Run the OEM airbox, a real hassle working with it but it runs much better than putting pod type air filters on them.

  3. #3
    CB750 Member canuck1969's Avatar
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    Thanks for the info. Setting the valves at TDC on compression makes more sense to me than using that notch. Seems more accurate that trying to line it up consistently. I usually drop a straw down the plug hole and watch for the apex on the rise on the compression stroke. Noticed they are labeled on the pulse generator plate also so all good.

    Carbs will come out again for a more thorough once through. Then will correct the valve gaps. Just waiting for my shim tool and some carb parts to arrive. Would like to get the engine running half decent before a total tear down so I can at least see what will be needed. Should run with the compression numbers I have , although not well, but once the engine comes out I suspect I will be tearing it down for valve and perhaps cylinder work. Might as well do it right the first time.

    No plans on putting pods. Will be as much stock as I can get. Would have liked to get the original exhausts but was not in the cards.

  4. #4
    CB750 Member canuck1969's Avatar
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    So found some time between work today to measure all the valve clearances. I put each cylinder at TDC using the apex of the stroke and the markings on the pulse generator housing. They are as follows. All numbers in thousands or an inch.

    Cylinder 1

    Ex: 7 and 6
    In: 4 and 5

    Cylinder 2

    Ex: 5 and 5
    In: 3 and 3

    Cylinder 3

    Ex: 3 and 4
    In: 4 and 4

    Cylinder 4

    Ex: 4 and 4
    In: 4 and 4

    Nothing alarming other than the couple of 3 thou ones that can be addressed. I redid the measurements using the manual method with the notch on the cam and got readings ranging from 0 to 2 thou difference. Now I know what is meant by variation. I will stick with the TDC method as is seems to error on the low side compared with the manual method.

    All that being said I decided to pull off the exhaust on Cylinder 1 and 2 to look at the exhaust valves and they will need to be pulled. Very carbonized and the stem on a couple of them were white. As you predicted AMC, can see some oil on the stems as i move them up and down so will need new oil seals also. No need to try and get it running before the tear down. I know it starts but will pull the bike apart and pull the heads for a closer look and repair. Lets see what adventures that brings.
    1980 CB750 Custom (Project In Progress)
    1984 XS650 (Restoration Complete)
    2010 VStrom 650 (Main Rider)
    2012 Moto Guzzi Stelvio NTX (Sold)
    2009 Kawasaki KLR (Sold)

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