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  1. #1
    Junior Member 46lukasz's Avatar
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    Hello everyone

    Hello Gentlemen!

    My name is Lukasz, I'm from Poland but i have been living in Ireland for about 14 years now. I am happy to join this community full of knowledge

    Recently I became a very happy owner of a Honda CB750F Super Sport (DOHC variant).

    It has 32'000 miles /51'000 km on the clock. Engine is running, everything is functional for the most part and the bike is stock as far as I am aware, maybe apart from the exhaust.

    I bought it in Northern Ireland which is governed by the UK and I registered it in Ireland for free. The vehicle import tax was zero because its a vintage vehicle.
    It came with a stack of MOT's (which is like a yearly vehicle road-worthiness test in the UK), one for every year since new so the history is there which is nice.
    The seller was visibly sad to see it go, with his eyes actually watering. It seems fairly well looked after and I even got a workshop manual with it from the guy.

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    I still have a few bits to do here and there before I put it on the road. I would like to document all the things here and get input and opinions from you guys. Should I put everything in this thread or start a new one under the 'Project Bikes' category?

    Kind regards,
    Lukasz
    TrialsRider likes this.

  2. #2
    Senior Member seanbarney41's Avatar
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    Congrats! That is a beauty! And you know it's a good one when the seller crys...You can make him smile again by taking good care of it.

  3. #3
    Junior Member 46lukasz's Avatar
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    Hello guys,

    I wanted to continue the thread to ask some questions as well as to give you a brief summary of what i have done on the bike so far, and get your opinions and advice if i should do something else or differently.

    If this is the wrong place for this kind of thing i kindly ask the admins to move the thread to the correct category (eg. project builds).

    I may write quite a lot to keep the write-up detailed, so a lot of people probably won't read the whole thing. So I will put all my questions that i may need help with, in a list at the end.



    Identifying the bike:
    First of all, I would like your help at identifying exactly what year and model the bike and/or engine is.
    The documents say it is from 1983, however to my knowledge the DOHC F model stopped production in 1982. In 1983 they produced the Custom and Nighthawk models.
    I thought that maybe its some kind of European version which was produced in 1982 but sold from 1983 in Europe, but i am not sure about this. Maybe the year in the book is first registration not production.
    To add to the confusion, on a few of the yearly MOT road worthiness tests from the bikes time in the UK, it states that it is from 1982.
    Another confusing thing is that I have single piston calipers at the front, which according to *this* website means that it is a 1981 model, because bikes from 1982 had dual piston calipers.

    I hope that by giving the VIN someone will be able to precisely tell me which year is the bike from and what model it is.
    VIN Number: RC04 2205000
    Engine Number: RC04 E2209816

    According to *this* website, RC04 is definitely pointing to a CB750F but then another letter after the F indicates the year of production. A= '80, B='81, C='82. I would really like to find out what bike I actually have.



    What I have done so far:

    Starter Motor Clutch
    So the first thing I did when I got home from the seller is I let the bike warm up and gave it a more thorough look around to spot things i may have missed.

    Then I realised that there is a clunky knocking sound coming from the left engine cover. It turned out that one of the spring loaded metal cylinders from the starter motor clutch assembly, fell out of place was knocking about.
    It's the thing indicated on the picture below:
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    I have no idea how it managed to get out of its place but its fixed now and the bad sound is gone.
    The clutch itself is a really nice system, simple but clever, only allows free rotation in one direction, while grabbing the shaft when rotated the opposite way, in order to turn the motor over when starting.
    If it gives trouble in the future, there is the mod where you can use the starter motor clutch assembly from an R6 as it fits perfectly as shown in this YouTube video:
    www.youtube.com/watch?v=sYJSUiXDim8



    Oil Change and Filter Casing O-Ring
    Then the next thing I noticed is that there was oil leaking from the oil filter casing. I planned to do an oil change anyway so I ordered a new oil filter which was K&N 401.
    I read somewhere among amazon reviews that the o-ring that they include is too thin and may still leak so I ordered another one separately off eBay, and it indeed was much thicker than the one included with the K&N filter so I used that one.
    Its the one from*this* link and I can recommend it to those that have this problem.

    The old Oil filter was used up because it came out pretty much black. The seller said that there was an oil change done recently but that may not mean he changed the filter together with the oil.
    The important thing was that I didn't see any metal shavings anywhere during the oil/filter change which is a good sign.
    The oil I used was Castrol Power1 which is semi synthetic. (not the Power1 Racing version which is fully synthetic). Its 10W40 like suggested in the manual and has the JASO MA-2 specification.
    From what I read, oils with this specification are suitable for bikes where the engine, clutch and gearbox are lubricated by the same oil, and oils without this specification can cause clutch slippage, which a lot of people warned about during my research about this.
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    The previous owner also did something strange here. When changing the filter you should put a spring (part #12) followed by a washer (part #11) followed by the new filter into the case in that order like shown on the picture.
    In my case the washer is missing because they often get stuck on the old filter when its taken out and gets thrown to the bin.
    The spring was actually placed on the outside of the filter case crushed between the case and the bolt (part #13) which was retarded.
    I managed to bring it back into shape so that it actually puts pressure on the new filter but both the spring and washer will have to be bought new and replaced as part of next oil change.
    This time around I used the stock oil drain plug bolt but as part next oil change I will install one with a magnet and safety wire it in. I have one bought but it came a in the post a few days after I did the oil change.
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    New Spark Plugs
    I installed a new set of 4x NGK D8EA gapped to roughly 0.6-0.7mm, all according to the book. It really did make a difference when starting the engine, especially when cold.
    Before it had a hard time and kept dying when cold. Even with full choke on, it needed a bit of throttle to keep it from dying until it warmed up.
    With the new spark plugs it fires up no problem on full choke and then it can be steadily turned down until no choke at all is applied. Runs on all 4 cylinders without any problems.



    Removed Rust from Tank
    There was a lot of gunk and rust in the fuel tank. I needed to find a way of removing it. I found out that Hydrochloric acid (HCl) can work wonders based of a couple of YouTube videos and other forum posts online.
    I bought some Ever Build 401 Brick & Patio Cleaner at 6Euros/5L it was a good deal. It has HCl in a concentration of about 10% w/w.
    I first tested on a piece of steel with really good results:
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    So then I removed the tank and took off the fuel petcock so that the acid doesn't attack it. I used a finger of a rubber glove and some tape around the thread instead of a closed petcock and it worked fine.
    I left the cleaner in the tank overnight and then emptied it out. A lot of nasty stuff came out, but it turned out that there was little rust but a lot of something that looked like scabs of resin. It probably was old petrol that has gummed up but I really don't know.
    The rust was attacked straight away but those other things needed more shaking to remove and it still wasn't perfect.
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    After the acid was emptied from the tank, I neutralised it with water and baking soda, but before i rinsed all the water out to give it another rinse with fuel, I fell victim to flash rusting. The tank was then filled to the cap with fuel so that it doesn't rust any more.
    Maybe in the future I will give this another go but rinse with fuel right away instead of water with soda to prevent the flash rusting from having a chance to happen.
    The first picture above is the before state, the second picture is right after dumping out the acid. I didn't take a picture of the flash rusting.



    Added an In-Line Fuel Filter
    Even though I cleaned out the tank and the screen around the petcock tube, i installed an additional fuel filter and replaced some fuel hoses around the engine while I still had some hoses left.
    Its clear so I can also see if fuel is flowing or not, and its an extra safety measure since some of the dirt from the tank may not have been rinsed out. I also installed some new shiny hose clamps while I had some left.
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    Fixed Crunchy Brake Lever
    Another Problem I noticed was that the brake lever was crunchy. Its best described in the first 2 minutes of this video (great YouTube channel by the way):
    www.youtube.com/watch?v=rB5yUOIZIoM
    Its a dangerous one because the lever does not travel freely, but rather incrementally in steps so it's easy to go from light breaking into heavy breaking and you can wash out the front end.
    All it took to fix it was some grease around the lever. I took it off completely and lubricated the hinge and also the part where the lever presses against the master cylinder. And the clicky operation was gone.



    Replaced Shift Shaft Seal
    I could see that there is an oil leak around the shift shaft but when I took the side cover off to get to the seal it was a horror story because of all the dirt mixed in with the leaking oil deposited around the shaft and the front sprocket.
    I cleaned all of it up and took out the old seal. Interestingly enough it didn't have a circular spring around the inner diameter on the inside. It was replaced with a new one, Honda part #91204-425-003 and seems to be doing fine so far.



    Other
    Among other bits and pieces I did, I lubricated the tacho, speedo, throttle and choke cables.
    This helped the tacho somewhat since the needle was jumping, it still does but much less now.

    I also adjusted the chain tension to 20-25mm of slack according to the book.

    I have ordered a new tyre, air filter and fork seals so I will be fitting them soon.
    I wanted to repaint the forks with a fresh coat of black paint since I will have the front end disassembled. Currently it looks like the previous owner painted the forks using a paintbrush.
    I am considering changing the front wheel bearings as well since I will have the front end disassembled, but that depends if there is any play in the current ones.
    If i notice that the axle wobbles in the bearings when I take the wheel off, I will replace them, otherwise the current ones will stay.

    Another problem I have is a stripped bolt in the left side engine cover. I am aware that these bolts are a Japanese standard driver bit, but I will try with a suitable Phillips head on a manual impact driver. Then I will replace all 3 bolts on that cover.
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    To Do List:
    • Identify the exact model of machine
    • Fix seized steering lock or get new ignition and swap over key lock barrel
    • Replace front tyre and balance the wheel
    • Sync carbs
    • Check compression (out of curiosity)
    • Inspect cylinders (I bought a small inspection cam that works with a phone, I'm curious how the cylinders look inside after all those years and milage)
    • Replace air filter
    • Replace fork seals (leaking is quite bad, leaks down the fork tube and onto brake disk on one side)
    • Replace indicator relay (indicators flash but there is a suspicious high frequency clicky noise coming from the relay before they start working)
    • Extract stripped bolt on on spark advancer cover and replace all 3
    • Replace front brake pads
    • Replace engine bolts with Allen head ones
    • Fix hole in exhaust collector


    Done List:
    • Lubricate tacho, speedo, throttle and choke cables
    • Replace shift shaft seal
    • Replace oil filter and oil
    • Remove rust from inside of fuel tank with HCl
    • Lubricate brake lever
    • Adjust chain tension
    • Replace spark plugs




    Questions:
    1. When should I measure the oil level? Warmed up or cooled down engine? I am wondering because when you let the bike sit for a while all the oil drains down and it shows a higher oil level on the dipstick.
    When you let the bike warm up the oil is distributed around the engine and it shows a lower level on the dipstick.
    When you do an oil change you also warm it up first so that it drains better so should you wait for it to cool down before filling?
    I am asking because when I poured in the new oil it was cold, it took 3.5L like it said in the book and it was up to the mark on the stick.
    Next day I fired up the engine and checked again when warm, and it was too low so I ended up pouring 4L of oil in total into it, and now when cold its slightly above the max mark on the stick. Should I worry about overfilling by 0.5L more than recommended?


    2. Some of the bolts on the engine are in bad shape and I am afraid that one day i will break the head on one of them and it will be impossible to take out. I would like to replace all engine bolts with new ones, specifically Allen head bolts.
    I saw that there are engine kits available with all bolts needed. What is your opinion on those? is it easier to strip an Allen head or a standard hex head bolt? Because that is what i want to avoid.


    3. When the engine was running I noticed that there is an opening in the exhaust, where the 4 separate pipes join at the collector and turn into one. There is a hole there because you can feel exhaust gasses when you put your hand near there.
    Is that normal? If yes, what is its purpose? if not, should I weld it up or is it not a big deal?
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    4. I also want to replace the air filter. I took out the old one only to see some kind of liquid around the rubber gasket on the filter. Is is greasy but sticky at the same time, and smells similar to petrol or brake fluid. Its brown in colour too.
    Initially I thought its some kind of way of creating a seal between the air filter and the gap for it inside the airbox, but I managed to put my hand into the airbox.
    I find out that it is also coated with a similar sticky substance, even on the insides of the velocity stacks around the carb mouths inside.
    What could this be? Is it normal or should I be worried that something bad is happening on the intake side?
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    5. Is it worth it to take the carbs apart and clean/rebuild them? The engine starts and runs no problem. Idles fine and climbs to high RPM without hesitation. Runs on all 4 cylinders without any problems.
    I was thinking to leave it like that if it is good but maybe sync them or at least check how in sync they are. This doesn't involve taking them apart completely and is an easy adjustment if you have the gauges.

    6. Is a jumping tacho needle a normal occurrence in these bikes or should it smoothly move up and down? Its still quite jumpy in my case.





    I think this is everything I have to say so far. I hope that you all will enjoy my write up and I am open to all suggestions and comments.
    Please also tell me what you think i should do to get this bike on the road in a safe condition, and what generally is worth doing or replacing on these bikes.

    Kind regards,
    Lukasz
    Last edited by 46lukasz; 09-25-2018 at 03:16 PM.

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  5. #4
    Senior Member seanbarney41's Avatar
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    I don't know too many specifics for the dohc bikes but here are some answers...
    1.I doubt .5 extra litres will harm anything but I would guess if you put in 3.5l cold and it came up right on the dipstick, that is gonna be right.

    2.Allen or hex bolts are fine, just be aware that a 6mm bolt is very easy to strip the THREADS!...no big wrenches allowed!

    3.The hole in your exhaust collector is not normal and serves no purpose. Weld it up or replace it or live with extra noise and sooty gases escaping. I doubt it will affect the running condition or performance of the bike very much.

    4.I have seen similar goop on air filters before, not really sure what it is or where it comes from...my guess is gas fumes degrading the glue used to manufacture the air filter. Probably time for a new one.

    5.My grandpappy told me, "If it ain't broke, don't fix it" and darned if he wasn't exactly right when dealing with these carbs...vacuum sync can be a good thing

    6.Should move smoothly but the jumpiness can be a sign of imminent gauge death...or sometimes they smooth out with more use. These weren't really designed for easy service but many have done it successfully, and yeah many have royally fucked em in the attempt(me included)

    Really, nice careful work so far but beware that fuel filter sometimes causes fuel flow problems. There should be a screen/sock on the fuel pickup inside the tank. If yours is intact, that filter is unnecessary.

  6. #5
    Junior Member 46lukasz's Avatar
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    Thanks for the quick reply to my questions.

    I have updated the to do list on the first post to include the Allen engine bolts kit and fixing the exhaust hole, as well as syncing the carbs.

    There was a filter as part of the fuel petcock, but I put the inline fuel filter in there as an additional safety measure especially after cleaning the rust out of the tank. If it gives me trouble with fuel flow i will remove it.

    Kind regards,
    Lukasz

  7. #6
    Junior Member 46lukasz's Avatar
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    Hi again guys,

    Just another update, i forgot that i recorded some short videos of the bike running.
    The first one was recorded when the bike was bought and the second one was recorded after i have done everything i wrote about so far. You can probably hear the difference in idling because the idle RPM was reduced to what it should be according to the book.
    Enjoy!

    https://www.youtube.com/watch?v=4xLjtX3GXjM
    https://www.youtube.com/watch?v=Qs9fTcL07_g

    Kind regards,
    Lukasz

  8. #7
    Junior Member 46lukasz's Avatar
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    Hi guys,

    So i have found out that the gunk on the air filter and inside the air box is coming from the crankcase ventilation system. I had a look on my bike and I found something funny, there is no catch box for this gunk underneath my battery and there should be one.
    It is meant to be a regular maintenance step to empty this box once it fills up.
    There is plenty of rust underneath the battery holder so maybe there was one there before but it's gone now. On the other hand, I don't see any hoses that would indicate that anything was there to start with.
    So correct me if i am wrong but i think there are only two hoses involved in this crankcase ventilation system. One going upwards on the gearbox side and one going upwards on the clutch side.
    Both of these on my bike are connected to a big T which in turn connects to the air box. That is it as far as I could see. I would appreciate if someone could post some pictures or diagrams of how its really meant to look.
    Or maybe its region/model specific?



    Another thing going back to trying to identify the actual model of the bike. I know for sure that in Ireland and UK, what's in the book is the year of first registration and not year of production, so I definitely shouldn't go by that, 1983 is wrong anyway.
    According to this website
    1979 VIN started with RC04-2000023
    1980 VIN started with RC04-2100003
    From 1981 onwards the VIN naming convention was different, eg: JH2RC040*BM200004

    My VIN is RC04-2205000 which confirms that the bike is from 1980. However there is something else.
    The same website says that the 79 model speedometer had a 150 mph (240 kph) limit while from 80 onwards it had a 85 mph (135 kph) limit.
    My bike speedometer goes up to 240kph as if it was taken from a 79 bike.

    I also know that the bike was repainted once (change in colour in the MOT documents and visible filler on the tank) so this would indicate that it was damaged.
    My idea is that it originally had the speedo that went up to 135kph, but it was crashed at some point and the clocks were damaged and replaced with ones from 79.

    If that is to be true, and it all adds up, it then is exactly a CB 750F-A


    In the mean time, I'm preparing to take off the front wheel and forks to replace the tyre and fork seals. I have build a stand that props up against the engine cover crash bars and holds the front wheel off the ground.



    Kind regards,
    Lukasz

  9. #8
    Senior Member TrialsRider's Avatar
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    Just a suggestion; if you have a strong overhead structure or even engine hoist? is generally a lot safer and easier to hang a bike then prop it up on stuff imo.
    ... I almost suggested Deer or Moose hoist but I bet they don't do a lot of big game hunting in Ireland.

  10. #9
    Senior Member TrialsRider's Avatar
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    lol let me rephrase that
    Last edited by TrialsRider; 08-09-2018 at 12:15 PM.

  11. #10
    Senior Member TrialsRider's Avatar
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    oh and yes,
    your crankcase will spew whatever stuff gets by the piston rings and valve guides along with a little oil mist which is pretty much typical of any engine,
    ventilation is very significant on a single cylinder engine, singles pretty much move their displacement of in/out air plus blow by gases through the crankcase on every stroke.

    ... is very nasty environment inside an engine and the older the engine gets the more disgusting it is. My airbox typically collects relatively clean engine oil in it which is a good sign, if it was black with soot, contained water or smells rich with gasoline, those are probably not such good signs.

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