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Honda GB500 thread

Discussion in 'Old's Cool' started by Spurlock, Aug 27, 2016.

  1. neil w

    neil w Been here awhile

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    Northumberland
    There is a really good write up on the web of a GB500 fitted with a tuned 650 Dominator engine
    #41
  2. Spurlock

    Spurlock Long timer

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    Seat cowl repair
    One universal problem spot on the GB500 is the rear seat cowl, which over time develops cracks at the two front mounting screws. My bikes had the cracks, and this is how I fixed them using epoxy and fiberglass cloth.

    First I used a sharp scraper to scrape the inside surface around the mounting holes, and used the sharp corner of the scraper to make scratches in the plastic to give the epoxy something to grab. I mixed up some paste type epoxy, applied it to the cowl, then layed down pieces of fiberglass cloth and added more epoxy.

    [​IMG]

    After curing I trimmed the edges and the inside of the screw hole. The crack is still visible but not obvious.

    [​IMG]

    And to hopefully prevent future problems I modified the mounting screws slightly. They are shoulder screws, with the shoulder tightening against the seat pan so the head only slightly compresses the cowl itself. I added a small collar to each screw to extend the shoulders. That lifts the screw head a bit higher so does not compress the cowl at all. So far so good at 10,000 miles since the repair.

    There is at least one company manufacturing replacement cowls, Mostyn Industries in Australia. They are unpainted.

    -Bill
    #42
  3. Duanob

    Duanob Been here awhile

    Joined:
    Jul 22, 2015
    Oddometer:
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    Spokane, WA
    "There is at least one company manufacturing replacement cowls, Mostyn Industries in Australia. They are unpainted."

    There is a member over at the forums.sohc4.net that makes nice PVC side cover repros for the old CBs. if there was enough demand he might try making a GB500 cowl. His repros are nice quality fit like a glove and paint easily and I think will last longer than the originals as they don't seem as brittle. I bought a pair for my CB550F and I was more than pleased, considering that finding originals in good shape are made of unobtanium. :D

    http://forums.sohc4.net/index.php/topic,74022.msg822417.html#msg822417

    http://www.fb-stuff.com/
    #43
  4. squish

    squish Out of the office.

    Joined:
    Dec 4, 2003
    Oddometer:
    9,444
    Location:
    Where the Ghetto meets the sea.
    Ahhh, the GB500.
    It is the bike that launched my downward spiral into street bikes.
    Early 90's I was walking along and spied one, It stopped me in my tracks

    And I spent about a half an hour just looking at it.

    That in led to me looking into it and researching what it was.

    25 years later I've spent the vast majority of my working life working in the motorcycle industry, I met my wife, and 80% of my close friends through motorcycles.

    I was lucky enough to be in the right place at the right time to get a great deal on a couple of GB's
    I kept them for a couple of years, but the lack of power, the cramped riding position I moved them on.

    Fun Fun little bikes, My only compliant is that there should have been an XR650L/Domminator engine in them,
    That detuned for boredom 500 was too slow, seemed slower even then my FT500 I had years ago.

    One of mine was converted to Pod filter and it would never run right, but to put the airbox back in you have to take the back of the bike off,
    The oil tank was also funky and both bikes had a lot of blow by, I solved it by running a long vent hose out the back and a drain from the crankcase breather. Well that didn't solve the blow by, but it solved the stink that would surround the bike, and getting it out of the airbox cleaned up the combustion chamber

    I bought the last white bros supertrap kit, from who ever had them (Maybe XR's Only) they bought out white bros supply

    It was just ok, to get enough baffles in to make the bike make decent power it was LOUD with a bumpy ass fueling
    Run only a handfull of discs and it was quiet but loaded up.

    We were thinking that we were going to put an dXR650L engine in, run two pipes on each side of the bike, that would give more volume and hopefully make it quieter.
    Seems the electrical system would need a lot of work to make that engine swap.

    I replaced them with a Ducati 750sport and a Kawasaki W650,
    That W was a great bike, I enjoyed the heck out of it. I hated the Ducati

    For years there was an active GB500 mailing list/group on Yahoo. I hung out there for years after I sold mine
    It was great group of riders, hardly any drama or noise.
    #44
  5. Spurlock

    Spurlock Long timer

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    A look inside the GB500 engine

    Last year I overhauled my "spare" 1990 GB500, and thought I'd share some details and pictures here. It was an ebay bike with around 11K miles on the odometer. But many things about the bike showed it had to have been more like 30-40,000. The engine was mechanically quite noisy compared to my '89 model which currently has close to 25K miles and is all original. Wanting to see the inside of this engine anyway I tore it down and discovered two sources of knock: the piston/cylinder clearance was at the limit of .004", and worse the wrist pin/con rod small end clearance was also way loose at .004". So I started looking at different connecting rod kits. Turns out this engine uses the same kit as the XR600. I chose the Pro-X brand, kit #1654.

    But there appeared to be one issue with the new rod kit: the oiling holes for the main bearings are not positioned the same as the originals. Below is the original right side view. Note the tiny hole in the inner edge of the crank pin plug that directs oil through a hole in the balancer drive gear and into the side of the left main bearing:
    [​IMG]


    But here is the Pro-X rod kit oil hole which is centered in the plug instead of at the edge:

    [​IMG]

    I was disappointed that the main bearing oil jets were located differently from the original, so their oil stream would entirely miss the main bearings. After talking with crankshaft rebuilders and other rod kit suppliers, the consensus was that I was "overthinking it", that there is a blizzard of oil flying around in the lower end and that there is no way the ball main bearings would not get enough oil. I can accept that maybe Honda's main bearing oil jets were a solution looking for a problem and not really necessary, after all older two strokes get by fine on a 2% oil/fuel mix flowing through the crankcase. But I found a local motorcycle machinist with a great reputation who went ahead and pulled the original crank pin end plugs and installed them in the new crank pin for me.

    The main bearings looked fine but the left side bearing has to be removed by pulling the outer race to get it off the shaft. Putting high side force on the races and balls ruins bearings, at least potentially, so I replaced both mains while I was at it. I had one problem though. The bearings are listed in the parts fiche as 6307 and 6308 bearings. Since they have no special grooves for retaining rings or holes for locating pins I assumed generic bearings of that size would work. Wrong! When I began to snug up the crankcase screws the crankshaft bound up. It turns out that the actual bearings supplied by Honda have a more heavily radiused inner race than the generics, which kept the bearings from sliding up close enough to the crank wheels, essentially making the crankshaft too wide for the cases. So off they came and I installed the genuine Honda bearings.

    Here is the difference, Honda part on left, generic on right:
    [​IMG]

    The only piston I could find for this engine was an OEM Honda first over, and that was only available from CMSNL. With .004" piston to cylinder clearance I wasn't sure the first over bore would clean it up, so I sent it down to LA Sleeve expecting them to re-sleeve and bore to my piston, but they were able to just rebore so now I'm at a nice snug .001" clearance still with the original sleeve. I could not find an OEM wrist pin anywhere, and the only substitute I could find was a Wiseco #S-651. It is about 2mm shorter and a few grams heavier than stock which is no problem.

    The valves were pitted and the seats worn very cockeyed so I took it to a local shop for seat grinding, then installed new Kibblewhite stainless valves and OEM seals. The guides were fine. By the way, to measure valve stem to guide clearance I make 2mm wide feeler gauges from thin brass shim stock. These guides measured at .001" which is within Honda spec for new valve/guide clearance. The home made shim stock feeler gauges also work great for measuring wrist pin fit in the small end of the rod.

    As with many bikes with vertically split cases, the left main bearing is a press fit both on the crankshaft and in the case. To reassemble, I heated the left case half so the bearing would easily drop in. Then I made a quick and dirty puller to pull the crankshaft through the inner bearing race. In this shot a PVC pipe coupling serves as a collar to bear against the inner bearing race, then a steel plate and the alternator rotor bolt screws into the crank to pull on the shaft. This way there is no force put on the bearing balls themselves, only the inner race:
    [​IMG]

    Here I used a wood dowel to keep the crankshaft from rotating while turning the puller bolt.
    [​IMG]

    One interesting feature of these engines is the gear driven balancer shaft. It has a small counterweight just like a crankshaft. To reduce noise from the straight cut gears, the balancer gear is actually two pieces, spring loaded so they are slightly out of phase. The movable spring-loaded outer gear takes up any lash between the crank gear and balancer. There are also triangular rubber blocks between the balancer gears, I suppose to act as dampeners of some sort. Here are the two parts of the balancer gear assembly:
    [​IMG]

    And here is the balancer installed in the case. You can see how the two parts of the balancer gear are slightly out of phase:
    [​IMG]

    The cylinder head is where things get really interesting. This is Honda's RFVC engine, for Radial Four Valve Combustion chamber. This design was widely used in a variety of their engines since 1983.[​IMG]

    [​IMG]

    So how do the rocker arms push on valves that are splayed sideways? Honda uses "sub rockers" between the rocker arms and the valves to convert the main rocker motion to radial motion:
    [​IMG]

    Closeup of the exhaust rockers:
    [​IMG]

    Closeup of the intake rockers:
    [​IMG]

    To make the engine easier to crank over there is an automatic compression release on the camshaft. At cranking speed it delays closing of the right exhaust valve, effectively lowering compression. Once the engine starts it disengages. There is also an automatic reverse compression release to prevent the engine from kicking back. This one uses a sprag clutch similar to electric starter clutches. If the engine backfires and tries to turn backwards it immediately opens the right exhaust valve. This shot shows the right exhaust rocker arm has a foot extending to one side for the compression release cams to engage:
    [​IMG]

    And here is the camshaft with comp release mechanism:
    [​IMG]

    The cam and rockers were fine except for two of the sub-rockers showing slight wear, which I replaced. Also replaced were all gaskets, oil seals, O-rings, cam chain, and clutch discs.

    Cam chain tensioning is automatic. There is a curved rigid rubber/metal blade that is pressed against the back (slack) side of the chain by a spring loaded tensioner. The tensioner has a one way roller clutch, again like the electric starter clutches, that will only let the tensioner move toward the chain, not away from it. The front side of the chain has a fixed guide blade. Here is a shot of the tensioner and guide poking out of the cylinder:
    [​IMG]

    And here the tensioner, chain and cam are installed in the head:
    [​IMG]

    Another thing I noticed, new to me since I last worked as a Honda mechanic about 35 years ago, is that several of the gaskets have sealant on them. The clutch cover has precise little lines of black sealant only on one side and only in the areas between screws:

    [​IMG]

    And the head and head cover gaskets are steel, coated with what appears to be something like paint. After running the engine the head gasket joint had a tiny bit of squeeze-out where I suppose the sealant had bonded to the engine surfaces.

    I put 500 miles on the rebuild before putting in storage as my backup GB. It runs great, and surprisingly my '89 with almost 25,000 miles still sounds as good as this tight new rebuild.

    -Bill
    #45
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  6. Spurlock

    Spurlock Long timer

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    How to adapt an XL/XR600R CDI box to work in the GB500
    This information came from the now defunct "Mighty Honda GB500 forum" in some posts by a user named Jack. I printed it out at the time because that fellow had clearly done his research and provided us with a means to use an aftermarket CDI box for the XL/XR in place of the no-longer-available GB500 part. Jack, if you're out there, please check in and many thanks to you for your research and posting! I'm not sure if this info currently appears anywhere else, so thought I would plant it here.

    First, symptoms of a bad CDI box can be not starting, intermittent dying, or rough running. If the bike will not start, one symptom of a bad CDI module is that the tach needle will not jump while cranking. Failure of Honda's CDI boxes from the GB era are not uncommon and affect several models. And about the only way to test a CDI box is to replace it with a known good unit.

    First, buy an '86 or later aftermarket CDI for the XL/XR600R such as this one:
    https://ricksmotorsportelectrics.com/Hot-Shot-Honda-CDI-Box-15_601

    NOTE: CDI boxes from the XBR500, NX500 and NX650 will NOT work because they are powered by 12v battery power whereas the GB500 CDI uses AC from the ignition side of the stator just like the XL/XR dirt bikes.

    IMPORTANT: To adapt the aftermarket CDI to the GB a couple of simple changes to the wiring are needed. First, in the 4 pin CDI harness plug, remove the green/white wire by slipping a straight pin down the side of the harness pin to release the catch. Pull the pin out and insulate it with a rubber tube or shrink tubing and push it aside. It will not be used. The function of this wire is to kill the engine if you put the bike in gear with the side stand down. However the side stand-down light will still work. If this G/W wire is left plugged in, the 10 amp starter fuse will blow when the starter button is pushed.

    Next, in the 2 pin harness plug, remove the BL/W and BL/R wires and swap them. That's it! Now you will have a brand new functioning CDI box for your GB500. The XR/XL CDI reportedly has a full advance of 31 degrees rather than the stock 29, but the bikes still run fine on regular gas. Also beware that the rev limiter will be 8,500 rpm which is higher than stock.

    I bought one of these aftermarket CDI modules and tested it and the bike ran perfectly. I put the stocker back in but carry the new one as a spare to avoid being stranded in case of failure.

    Hope this helps someone out there!

    -Bill
    #46
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  7. JPCollinsworth

    JPCollinsworth Compulsive Spender

    Joined:
    Jun 13, 2009
    Oddometer:
    177
    Location:
    The Republic of Vermont
    Great wrire-ups!
    #47
  8. Mobil1

    Mobil1 Long timer

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    Check out ignitech.cz . Programmable CDIs. Plug and play. Tell them what you have and what you want. They'll tell you what you need. Even their preloaded standard maps are impressive. :nod
    #48
  9. Spurlock

    Spurlock Long timer

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    Interesting, I might just email and ask whether they already have the specs for the GB500.

    -Bill
    #49
  10. Spurlock

    Spurlock Long timer

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    Repairing a bad CDI module

    Another alternative to replacing a bad CDI is to repair it. A year or so ago my 1990 GB had an elusive poor running problem which turned out to be a bad CDI module. With the OEM part unavailable, I was able to repair it thanks to information gleaned from other forums.

    First the symptoms: Intermittent vague misfire/hesitation at low to mid throttle when running at even speed or slightly accelerating, as though it were about to run out of gas. Wide open throttle power was down by about half, and when backing off from hard acceleration it would often backfire like a shotgun blast. The bike would always start and idle fine. The symptoms really felt like fuel starvation to me so I started off barking up the carburetion/fuel supply tree, checking tank vent, fuel filter, float valve, carb CV piston, etc. Finally I thought to check ignition even though I'd always found ignition problems to cause missfiring mainly at high speeds & hard acceleration. But that was on older points systems and this was my first bike with capacitive discharge ignition. Lesson learned, CDI modules control timing, advance, red line and other functions so symptoms can be unexpected.

    To add to the diagnostic fun the only way to test a CDI module (short of very specialized equipment) is by swapping it for a known good unit. That's the shop manual statement, and luckily I have two GB's so was able to swap modules and viola, the problem disappeared. Next I read where certain years of XL/XR 600 dirt bike CDI's will work in this bike with a couple of wiring mods as I described in post #46 above. So with the problem solved using the aftermarket CDI I then ran across references to CDI repair. I had assumed that they failed due to internal electronic components going bad, but it turns out that a very common problem is bad solder joints on the circuit board. I found many discussions in other forums all saying the same thing: cut the module open and re-solder the connections. Most talk about using a Dremel tool with circular saw blade to get inside.

    Here are a couple of good threads on the subject:
    http://www.thumpertalk.com/forum/showthread.php?t=980822
    http://www.advrider.com/forums/showthread.php?t=684756

    Not wanting to do it freehand I made a quick and dirty "table saw" for my rotary tool to make sure I could control depth of cut and easily cut a straight line. Depth can be adjusted by shimming under the rotary tool, and location of cut by moving the blade in or out of the collet:

    [​IMG]


    [​IMG]


    The circuit board is up against the top of the case, so the saw is set up to cut 2mm deep and 2mm from the top of the case. The bottom edges of the case measured just slightly over 2mm, so I figured better to err on the safe side than go too deep:

    [​IMG]


    Here's the module after cutting around the perimeter:

    [​IMG]

    There is a thin layer of rubbery potting compound between the circuit board and the top of the case, so next step is to carefully pry the top loose without damaging anything inside. I used a paint scraper, putting the sharp edge into the saw cut and prying to the side. Holding light but steady pressure caused the compound to gradually tear loose:

    [​IMG]

    Most of the potting compound stuck to the case, just a bit needed to be peeled off the circuit board using fingernails:

    [​IMG]


    I had to use a small microscope to see the problem, but sure enough the damaged solder joints looked exactly like the close up pictures I saw in the ADVrider forum link above. As others have found, the most common problems are on the connections running to the plug terminals, but I found a few others as well. I didn't have a good macro lens for this shot, but you should be able to see crescent-shaped cracks in the solder in this image. Some connections were clearly cracked, others were shiny at the base turning to dull grey higher up, indicating solder breakdown most likely from heating/cooling cycles:

    [​IMG]


    And a couple more close ups taken with a digital microscope-

    [​IMG]


    [​IMG]


    [​IMG]

    To make sure of a good fix I decided to re-flow all of the connections even if they looked OK. Ideally it is probably best to remove the old solder using de-soldering wick and re-solder with new, to ensure there is no contamination. But I didn't have the wick or solder sucker on hand so just went ahead remelting the old solder and adding a bit of new where needed. I stuck the case top back on with tape temporarily, and after a few more test rides, went ahead and seal with a bead of silicone around the cut to seal against moisture. The bike ran fine.

    The question is, why do these connections go bad? I really doubt that it's vibration, since the board and all electronics are embedded in potting compound and the module hangs from a soft rubber mount. So maybe there is some fault with the original soldering that leaves the connections vulnerable to breakdown from repeated heat cycling. Whatever the reason it's nice to know it's possible to repair these obsolete parts!

    In addition to the GB500, this CDI problem aflicts similar era XL/XR, Transalp, and NX Hondas and possibly other models and makes.

    -Bill
    #50
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  11. Mobil1

    Mobil1 Long timer

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    They're not real quick to respond , they may need to translate. I sent two E-mails about a week apart before I got a response. You have to sound serious about purchasing or they seem to think you're just asking stupid questions.
    #51
  12. bran.c

    bran.c Adventurer

    Joined:
    Sep 10, 2016
    Oddometer:
    29
    [​IMG]
    Just got this on Thursday
    #52
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  13. Spurlock

    Spurlock Long timer

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    Nice! Looks like the Japanese version decals and front fender?

    -Bill
    #53
  14. bran.c

    bran.c Adventurer

    Joined:
    Sep 10, 2016
    Oddometer:
    29
    Thanks yeah its a japanese import, was wondering how to check oil level on it and what grade of oil to use , ill be giving it an oil change soon.
    #54
  15. Spurlock

    Spurlock Long timer

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    When the bike is not running the oil level in the tank drops as it slowly drains back into the crankcase. So the book method of checking is to run the engine a bit, then let it idle for a couple of minutes and then check with the dipstick within a minute of two of shutting off. Be sure the bike is level when checking, not leaned over. I and others have found that the level in the tank varies at different times though. You might change oil and filter, then put two quarts back in and ride 5 miles, idle, turn off, then find the level reads too high. But if you go ahead and ride it and check another day, it will likely read lower. After many oil changes I have settled on just adding 2 quarts or a bit more (assuming filter change) and calling it good. Exact level is not critical.

    I'm in the habit of leaning the bike over to one side and then the other a few times while draining. You'll find more of the old oil will drain out that way.

    -Bill
    #55
  16. bran.c

    bran.c Adventurer

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    Thanks, thats how I thought it was done, mine has a bit too much oil in it then, its almost to the top of the dipstick
    #56
  17. Spurlock

    Spurlock Long timer

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    So is it a 400 or 500?
    #57
  18. Spurlock

    Spurlock Long timer

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    Yeah, way too high if that's the reading after running and idling. Oil will get carried into the separator tank and air box if too full. Both those have drain hoses down on the left side. You need to uncap the drain hoses once in a while to drain excess oil out.

    Have fun with your bike!

    -Bill
    #58
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  19. bran.c

    bran.c Adventurer

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    Thanks Bill, I'll have to drain out a bit before I go back out on it, what oil should I use when changing it? It's a 500
    #59
  20. Spurlock

    Spurlock Long timer

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    Well, I hesitate to start a "what kind of oil" discussion, since they are notorious for going on forever! :lol3 Everyone has their favorite oil and theory. But here's the Honda service manual recommendation:
    [​IMG]
    -Bill
    #60
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