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  1. #1
    CB750 Member gpounce's Avatar
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    Fixing up a friend's 1992 CB750

    Hi all,

    I'm a Suzi/Yamaha guy but am repairing my friend's '92 CB750. Its the usual sort of thing; he bought the bike in good shape, parks outside uncovered all the time, infrequent maintenance, rides it only the good weather etc. I rescued the bike when it wouldn't start and left him stranded at the bottom of a hill. The bike is in my gazebo & under a cover when I'm not wrenching on it- my R6 lives in the garage

    I have the front caliper off- the usual everything frozen, brakes worn beyond belief, tan fluid etc, rusted slider pin and dead seals, and the pistons are stuck. 100psi compressed air isn't enough to move them, I have a removal kit on the way. I tried reconnecting the brake line and using the lever to pump out the pistons but could not get the brakes to bleed and I'm reasonably proficient at it. I used a hypodermic for vacuum alternating with the caliper drain plumbed to a can of waste brake fluid to keep air from coming back in, got lots of bubbles and moved the last of my fluid thru the system but no luck getting air purged. Are the 750's front caliper difficult to bleed or was I just unlucky?

    Rear calipers and carbs will come off next, probably will be equally annoying. Tank is off and drained- had to cut the hoses they were so stuck on but no rust, battery is out and in the shop on a tender. My friend related idle/low rpm throttle trouble, I'm assuming the typical filty carbs, we'll see what horrors lurk. I read of a typical update putting a few washers on the carb needles which is fine (I did lots to my old Bandit's carbs), is that a generally desirable mod given un-drilled OEM pipes?

    The controls are substantially rusted and sun-faded but nothing too bad, just kind of ugly. No rust under the tank but I'll bet its never been cleaned under there since the bike was new lol. Would much appreciate any suggestions wrt electrical upgrades or other stuff that usually needs overhauling on a poorly maintained stocker.

    Thanks!

  2. #2
    CB750 Addict pidjones's Avatar
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    Most brake bleeders can be replaced with an 8mm grease fitting, the banjo bolt replaced with a 10mm bolt, nut, and o-ring. Then you pump the pistons out with a grease gun. Messy? No more so than the caliper clean-up will be anyway. Use a lot of gease and rags? Yes, and you will probably have to replace the $30-$50 pistons, so the expense is a pittance. If the bleeder won't come out, auto parts stores have rescue kits so you can drill out the bleeder you broke, retap for 1/8"NPT, and install their kit. Use a ASE thread grease fitting in that case. And have nothing to do with people that leave motorcycles outdoors all of the time. Nasty habit!
    "Love 'em all.... Let GOD sort 'em out!"

  3. #3
    CB750 Member gpounce's Avatar
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    Ooo fun, never though of pumping in thru the bleeder port with a plug in the inlet, thanks! I don't mind using a tube of cheapo grease to get the pistons moving.

    I was kicking myself for tossing the old Bandit brake lines I was thinking of rigging something hydraulic but your idea is better. The bleeder is fine- pretty much the only fastener that hasn't been giving me a pain on these calipers. Yeah my friend is an interesting guy, heavy mechanic but he won't touch small stuff and is renting a townhouse basement so has no place to park the bike out of the weather.. .but for petes sake not even a home depot tarp.

    Once the caliper is fixed up I was thinking of pumping new fluid in from the bottom- might set that up if I have trouble bleeding next time.

  4. #4
    CB750 Addict pidjones's Avatar
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    I suspect the master (and maybe lines) will need attention also. Brake fluid pulls in moisture and gets very corrosive. I hate brakes.
    "Love 'em all.... Let GOD sort 'em out!"

  5. #5
    CB750 Member gpounce's Avatar
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    Grease did the trick, doesn't need very much either. I tried a couple different 10mm zerk fittings on the brake line inlet, an odd thread pitch- maybe too fine so that didn't work. So I fit an 8mm zerk in the bleeder, and stacked a bunch of 10mm copper washers onto the brake line bolt to plug the inlet. Just a few pumps moved the pistons just fine, helps to vent the air as the grease goes in or it'll spray a bit when the plugs come out. I think one could easily substitute oil for grease in the gun, would make cleanup easier. Thanks for the hint!

    Pistons are pretty dirty but maybe I'll get lucky on the wirewheel. I have new seals and rings on the way.

  6. #6
    CB750 Member gpounce's Avatar
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    Front calipers now feel great with new rings, gonna work well. A warm day today, so dug the carbs out of the bike. DIdn't break anything and gained a lot of admiration to the engineers & designers, some very elegantly fit stuff in there, much easier to get them out than the carbs on my old Bandit. Anyhow 2 slides stuck solid closed with stuck float valves and some scary looking zombie green sludge in the bowls. Diaphragms in good shape. Carb cleaner washed brown varnish smelling gas... sheesh his bike has surely been running something awful for a while.

  7. #7
    CB750 Addict pidjones's Avatar
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    I've found lightly greasing the insulators with silicone vacuum grease then heating them with a heat gun helps the carbs slip back in place without so much damage to the rubber. The first two times I assisted this with a rachet strap to help pull them in, but the last time I must have the knack and they went in without the strap. As with many multi-carb bikes, it is best to connect cables before final mounting. On the '79 CB750F, get them rested vertically on the right side of the transmission while connecting, then turn the rack horizonral and slide in place.
    "Love 'em all.... Let GOD sort 'em out!"

  8. #8
    CB750 Member gpounce's Avatar
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    Cleaned the carbs today- f'ing mess. 2 enrichener jets clogged, 2 main jets clogged, 2 slides stuck not all on same cylinders. Used .010" copper wire to clear the jets, blew everything out with carb cleaner and compressed air- all passages clear now, slides move properly and diaphragms move air. I guess the green stuff was some corrosion residue, became powdery and brushed/wiped clean after drying. A testimony to the Honda engineers that the thing would start and run... going to change his oil heaven knows whats in the crankcase lol If the weather stays good maybe I can get it started next week, will use my 0% ethanol. Looking forward to test riding it, seems like a fun bike

    thanks for the hint for cable re-attach on the right- my Bandit's went in the same way.

  9. #9
    CB750 Addict pidjones's Avatar
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    In my experience, the green is due to moisture brought in by ethanol. Real gasoline will evaporate to varnish. My '79 carbs had zero green stuff - but heavy layers of varnish. Last parked ~1999, before the mandates. We can still buy 100% gas here in Tennessee. I usually get 3 five gallon jugs at a time, treated with Stabil. For all carbureted engines. I do run regular 10% in my fuel injected cages and GL1800.
    "Love 'em all.... Let GOD sort 'em out!"

  10. #10
    CB750 Member gpounce's Avatar
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    Got the carbs back in today- throttle retract cable had parted via rust..sheesh, so got him a new set of cables too. New ones fit fine. I used a 37" carpenter's bar clamp to seat the carbs- worked well; the plastic pads on the clamp jaws are quite soft, one jaw on the carb, the other (w/ screw handle) on the header side of the block. Doesn't take much force, they eased in nice and easy no levering necessary. Also needed the clamp to seat the airbox manifold onto the carb intakes. The carbs were LOTS easier to get connected to cables and seated than my Bandits were. Next halfway warm day I'll see if it starts up.

    I assume the hole w/ screw seen in each cylinder intake manifold are balancing ports? I have a homebrew manometer, would be happy to sync them- does the engine care much as long as balance isn't too far out?

  11. #11
    CB750 Addict pidjones's Avatar
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    If you were careful to match the butterfly openings it should start Ok, but manometer balancing is always indicated after compression, timing, etc. are correct. It WILL run better in sync. The ports should be the same that other bikes use.
    "Love 'em all.... Let GOD sort 'em out!"

  12. #12
    CB750 Member gpounce's Avatar
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    The af mix screws are all against the stops, butterfly closed positions are close- doesn't seem like anything is really crazy. No idea how the thing will idle- I guarantee my guy has done no tuning whatsoever on this bike and it clearly hasn't run properly for many years. Going to replace his plugs too- looking at the headers pipe oxidation is almost nonexistent on the rightmost cylinder quite heavy on the left two- maybe theyve been running lean and hot for a while. Heaven knows whats up with the valves, but I'm not a good enough friend to find out and fix them lol. There's a good salvage yard near me, will be replacing his forks (seals are literally flopping around loose because their seats are rusted out).

    btw- is there any particular magic wrt servicing the rear brake? I've not been into a drum for a long time, but will probably hold off on pulling the rear and digging into that till its a bit warmer out.

  13. #13
    CB750 Addict pidjones's Avatar
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    Wait, a drum on a '92? Is it a Nighthawk? https://upload.wikimedia.org/wikiped...thawk750-2.jpg
    BTW, if you haven't been in there thoroughly yet - critters love nesting in the airbox of derelict motorcycles. Mine had nesting materials, nut hulls, etc. ( but the filter was intact). The exhaust must have housed other families - it spit a cloud of hickory hulls when started the first few times. Luckily, they had not dined on wiring or other plastic/rubber (chipmonks love rubber hoses).
    Last edited by pidjones; 01-10-2019 at 05:59 AM.
    "Love 'em all.... Let GOD sort 'em out!"

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