Page 1 of 2 12 LastLast
Results 1 to 20 of 25
  1. #1
    CB750 Member gpounce's Avatar
    Join Date
    Nov 2018
    Location
    Baltimore, MD
    Posts
    15

    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
    Join Date
    Sep 2018
    Location
    East TN
    Posts
    272
    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
    Join Date
    Nov 2018
    Location
    Baltimore, MD
    Posts
    15
    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
    Join Date
    Sep 2018
    Location
    East TN
    Posts
    272
    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
    Join Date
    Nov 2018
    Location
    Baltimore, MD
    Posts
    15
    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
    Join Date
    Nov 2018
    Location
    Baltimore, MD
    Posts
    15
    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
    Join Date
    Sep 2018
    Location
    East TN
    Posts
    272
    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
    Join Date
    Nov 2018
    Location
    Baltimore, MD
    Posts
    15
    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
    Join Date
    Sep 2018
    Location
    East TN
    Posts
    272
    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
    Join Date
    Nov 2018
    Location
    Baltimore, MD
    Posts
    15
    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
    Join Date
    Sep 2018
    Location
    East TN
    Posts
    272
    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
    Join Date
    Nov 2018
    Location
    Baltimore, MD
    Posts
    15
    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
    Join Date
    Sep 2018
    Location
    East TN
    Posts
    272
    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 04:59 AM.
    "Love 'em all.... Let GOD sort 'em out!"

  14. #14
    CB750 Member gpounce's Avatar
    Join Date
    Nov 2018
    Location
    Baltimore, MD
    Posts
    15
    It is in fact a Nighthawk. With the weather mostly bad progress has been slow, but I did get a warm enough day to get the carbs in and hooked up, replaced the rusty plugs and put in some 0% ethanol gas and started up. It ran fairly well, all 4 cylinders firing and the pipes heated symmetrically. There are a couple holes in the exhaust and theres an oil leak somewhere dripping down onto the pipes, but good behavior from the carbs & throttle was gratifying. His forks need work, so they're coming off next. Naturally everything is screaming tight, so if there are any magic pointers for fork disassembly I'm all ears. Going to get out the pneumatic wrench to open them up. I found a pair of salvage forks on ebay if his need parts or straight out replacement.

  15. #15
    CB750 Addict pidjones's Avatar
    Join Date
    Sep 2018
    Location
    East TN
    Posts
    272
    Good job! All 4 firing equally is a major plus!

    I've not had any problems getting GL1000 forks apart (my CB had Kawasaki leading axle forks that were tossed) - although you need to use an air impact to get that bottom allen loose. Get the bottom allen loose before taking the top off, and the spring will hold the inside from turning. Otherwise, you'll need to jamb a broomstick in to hold it. Propane torch helps in seal removal, but that's no different from most forks. Nighthawk forks may have the anti-dive feature. Just get a manual for that.

    Glad you have it running. Remember, carbs demand fresh, clean, 100% gasoline. Stabil or another treatment is Ok, just don't overdo and avoid alcohol. When you give it back to him, demand that he ride often and store it properly.
    "Love 'em all.... Let GOD sort 'em out!"

  16. #16
    CB750 Member gpounce's Avatar
    Join Date
    Nov 2018
    Location
    Baltimore, MD
    Posts
    15
    Ugh my salvage ebay forks were nasty nasty, could not budge them with air impact and oxy/acetl torch so drilled the heads and punched. At least they were cheap. That worked fine but when I flipped the 1st one back upright and the fork stroked & sprayed the grey watery oil all over my bench and R6 console down onto the pipes etc... gross. Was more careful with #2 lol. Got all the bushings and seals off in case they're in better condition than my friend's, anyhow live and learn we'll see how his come apart. Maybe going to use his tubes with the ebay lowers (assuming they're in better shape than his which I suspect have been full of water for years sheesh ).

    The condition for all this work is that he USE the bike cover I'm giving him

  17. #17
    CB750 Addict pidjones's Avatar
    Join Date
    Sep 2018
    Location
    East TN
    Posts
    272
    I once changed the oil in a Penton's forks. Made the mistake of removing the drain plug without making sure first that it was vented. Shot fork oil all the way across the room, and it smelled like they had used oil from sardine cans. Glad that house is long ago sold!
    "Love 'em all.... Let GOD sort 'em out!"

  18. #18
    CB750 Member gpounce's Avatar
    Join Date
    Nov 2018
    Location
    Baltimore, MD
    Posts
    15
    Finally got a break with this bike- got the forks off, not only is there oil in them (dirty but recognizable), but the bottom screws came out without drama.. unbelievable. Might be all they'll need is new seals and fork oil. The last technical issue I'm working on is an oil leak somewhere around the front of the engine, oil weeps when the engine is running, I'm going to investigate the oil cooler first; hoses, pipes etc, I would much appreciate any hints related to the usual suspects.

  19. #19
    CB750 Addict pidjones's Avatar
    Join Date
    Sep 2018
    Location
    East TN
    Posts
    272
    If they still have the cam chain adjuster on the front of the head in the center, it is supposed to have an o-ring. Don't over-tighten it!!! Very easy to strip or break the bolt. Also, on the early dohcs the tach drive can leak some.
    "Love 'em all.... Let GOD sort 'em out!"

  20. #20
    CB750 Member gpounce's Avatar
    Join Date
    Nov 2018
    Location
    Baltimore, MD
    Posts
    15
    There oughta be a law... man the oil seals gave me a fun time on the forks, had to chew up one of them with a screwdriver before it would move and it took some hammering for sure. The solidified gunk in there smelled like dirty diapers when I wirewheeled it out but almost no damage at all to the seats so the new seals should go in clean. Sliders are actually pretty good, so not high mileage forks but substantially degraded by long outside storage. Please tell me I can pour the new oil into the inverted tubes and reassemble upside down. The tube caps aren't moving either and I'm tired of being just short of destroying things on this bike just to get them apart lol.

    I'm <really> hoping to not go inside the engine at all, but thanks for the cam chain pointer- likewise the tach sender, will look at that closely. The oil leak is next once these forks are back on, naturally the front bearings are dry, speedo gear also and fillthy, so will clean all that up too...

Page 1 of 2 12 LastLast

Similar Threads

  1. 1992 CB750 Scrambler Build (first time builder)
    By Mcbuckets13 in forum The DOHC Garage (1984+)
    Replies: 18
    Last Post: 11-06-2017, 10:22 PM
  2. Removing Plastics 1992 CB750
    By mdsjay in forum The DOHC Garage (1984+)
    Replies: 9
    Last Post: 08-31-2017, 07:52 PM
  3. Cb750 1992 nighthawk brain wiring
    By finisterra in forum The DOHC Garage (1984+)
    Replies: 0
    Last Post: 07-18-2015, 10:59 AM
  4. For Sale - 1992 Honda CB750 Nighthawk
    By dbroten in forum Classifieds
    Replies: 0
    Last Post: 08-29-2014, 10:26 AM
  5. Removing Plastics 1992 CB750
    By mdsjay in forum The CB750 Garage
    Replies: 4
    Last Post: 08-25-2014, 11:54 AM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •