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  1. #1
    CB750 Member docholiday's Avatar
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    Unhappy First Ride, first trouble need help badly

    I have an 80 super sport that I have been converting to cafe racer for almost 2 years now... I have not really taken her for a long drive until yesterday... While cruising at around 3000 rpm there was a "pffft" sound, like a misfire, then the engine died.. I tried to start her again but sounded like there was no compression. I got the bike 2 years ago and PO said the engine had some work done on it and I have talked to the mechanic who did the "work" and said he just replaced the journal bearings and nothing else... I had the carbs rebuilt and syncronized a couple of months back as well as replaced the intake manifolds. I was planning to have the timing chain tension adjusted as well as have the valve clearance checked then this happened.... It is hard to find mechanics over here who are knowledgeable on cb750.... I am going to bring the bike to a local mechanic to have it checked tom. Any advise on what to look for and what to do?? is a full rebuild advisable? parts are hard to find here let alone cost...so I hope we can fix it with as little cost as possible.. Thank you for any help..

  2. #2
    CB750 Junkie Dave's Avatar
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    A "pfft" sound, died, no compression - head gasket, stripped spark plug hole? That's only a guess because there's not a lot of information to go on.

    If you don't have the means to do a compression check yourself, your pretty much at the mercy of those mechanics you mention.

    He said "he just replaced the journal bearings and nothing else"? That's pretty odd.

    You'd have to tear the whole engine apart to change the crank bearings (I assume that's what was meant) why would you not do "anything else"?
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  3. #3
    CB750 Member docholiday's Avatar
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    I had my engine checked... the timing chain broke.... Does this means that I need an engine overhaul? what needs to be replaced other than the timing chain?

  4. #4
    CB750 Junkie Dave's Avatar
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    It depends on which chain, there are two. One goes from the crankshaft to the rear cam, then another goes from the rear cam to the front cam.

    Best practice is to replace both chains and the 3 sprockets all at once but, I realize you are working with limited resources here. The problem is as a chain stretches, the sprocket teeth wear to fit the extra length between the pins on the chain. We are talking fractions of a mm here but it all ads up. Often a new chain will not fit properly on a used sprocket. The fact the chain broke usually means it was stretched beyond it limits. Cam chains can break for other reasons and it is possible a new chain might work on the original sprockets. The only way to know for sure is a visual inspection of the sprockets, one of which is the middle of the crankshaft

    The top chain could be changed with the engine in place though it would be much easier to do with the engine out of the bike. Changing the main chain requires splitting the case and pulling out the crankshaft to get the chain around it as they don't have master links in them.

    Either way, if the engine kept turning after the chain broke, just for one full revolution, there is a very good chance you bent some valves too.

    Here's a good place to look at all the parts of your bike. https://www.cmsnl.com/honda-cb750f-1...856/partslist/ (There are several models with slight variations, I chose the general export for reference, it might not be the one you have)

    I got this picture from there.



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    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  5. #5
    CB750 Member docholiday's Avatar
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    Thank you for the reply Dave. I guess it will be a very costly repair....
    is the cmsnl.com the cheapest place to get the parts?

  6. #6
    CB750 Junkie Dave's Avatar
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    I've never purchased parts from CMSNL as they are in the UK and I'm in the US. I'd pay more in both sipping and currency as the dollar is worth less then the euro. I just like the way the documentation is presented on their website.

    I would think shipping costs are going to be your biggest expense and have no idea who can provide you with the lowest cost overall.

    I have a friend who lives in Pili, Camarines Sur and sending anything to him from the US is insanely expensive.

    These are Japanese bikes and Japan is a lot closer to you then the US or UK, so is Australia. Maybe you can find parts from a closer source.

    If you can find a shop specializing in Japanese imports, maybe they already have supply lines setup?

    Don't rule out anything. You might find a complete engine for less then the cost of repair.
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  7. #7
    CB750 Member docholiday's Avatar
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    Quote Originally Posted by Dave View Post
    I've never purchased parts from CMSNL as they are in the UK and I'm in the US. I'd pay more in both sipping and currency as the dollar is worth less then the euro. I just like the way the documentation is presented on their website.

    I would think shipping costs are going to be your biggest expense and have no idea who can provide you with the lowest cost overall.

    I have a friend who lives in Pili, Camarines Sur and sending anything to him from the US is insanely expensive.

    These are Japanese bikes and Japan is a lot closer to you then the US or UK, so is Australia. Maybe you can find parts from a closer source.

    If you can find a shop specializing in Japanese imports, maybe they already have supply lines setup?

    Don't rule out anything. You might find a complete engine for less then the cost of repair.
    Thanks Dave. I have been looking at ebay and saw some parts that are inexpensive like the overhauling kits and piston rings. are they good enough?

  8. #8
    CB750 Junkie Dave's Avatar
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    Personally, Ebay is the last place I would look for motorcycle or auto parts.

    I prefer to buy from someone who knows exactly what it is they are selling and exactly what it is I need.
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  9. #9
    Moderator dirtdigger's Avatar
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    I buy probably 75% of my parts through ebay, just rebuilt a cbr600 and it was 99% ebay parts, cost me 1/3 what oem and new store bought parts would have cost me. Thing is to know what you want and if possible have a part number to check with the seller. Ebay backs up purchases and will refund money if seller wont deal with you.

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