Author Topic: it lives again... now how much noise is normal?  (Read 1396 times)

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Offline superchode

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it lives again... now how much noise is normal?
« on: July 14, 2006, 06:36:02 AM »
so after spending a lot of time rebuilding the bike after a top end seize... it ran again last night for the first time.  once i figured out that you need the choke cable hooked up to start it - it fired up first kick. good stuff.

as you might expect... after the first cb750 i'd ever ridden siezed the top end in the first week of our relationship - i'm a little paranoid about what noise is normal and what indicates problems out of the top end.  what should i be hearing?  it obviously sounds better than it did pre-failure... but that's not saying much, and it's certainly still making noise in the top end - but i'm sure some of that is normal and that there's going to be camchain noise no matter what one does.  anyway - looking for some helpful advice from people who know these engines better than i do. what should i be hearing and what indicates problems? i don't hear any sort of ticking/knocking from the tappets or valves... but i hear what sound like friction and i don't know if it's just the cam chain rolling around or what... like i said - i'm paranoid about the camshaft operating and being lubed properly.

it won't idle without the choke holding the revs up.. so i'm assuming the slow jets are nearly completely clogged - as it chugs to a fairly quick halt after you let off the gas without the choke.  it revs all the way up on all cylinders, though - which is very nice to see.

i'm off on my other bike for a couple weeks starting today - but i'll be right back at this project soon enough... should be only a few more weeks of work until it's rolling down the highway.

edit: it's a '78 CB750F
« Last Edit: July 14, 2006, 06:44:04 AM by superchode »

Offline Mark M

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Re: it lives again... now how much noise is normal?
« Reply #1 on: July 14, 2006, 07:42:55 AM »
Did you find out what the cause of the first sieze was. With out addressing this you could be heading for groundhog day.

Is your oil warning light working? Is it lit up beofre you start the engine and off almost imediatly after startup.
If it was me I would start it up, get it to idle as SLOW AS POSSABLE by adjusting the throttle stop screw and pull the
valve adjuster caps off one by one to check for oil flow.

Dont run the engine for any longer than you need to to perform this check, DO NOT LET IT GET HOT!
You can speed up the checking by making sure the caps are not siezed in place before you start.
In the UK anything over 40 years old only needs insurance and Fuel.

Offline superchode

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Re: it lives again... now how much noise is normal?
« Reply #2 on: July 14, 2006, 07:57:28 AM »
the cause of the last sieze was mating of the camshaft to the blocks above the 2nd and 3rd cylinders.  this was likely due to a combination of partial blockage of the oil passageways to the head and the exhaust valve guides being out of spec - burning oil off and significantly reducing the effective life of the oil that remained in the bike. the journal bearing over 2 and 3 eventually closed off their oil holes and i imagine the sieze wasn't too far behind that occurance.  both causes have been corrected and the camshaft and valve towers have been replaced, although they aren't a specific match to each other - both are in good shape (sourced them from two different spots).

oil light is working and indicates as you described... on then off shortly after the bike fires up.  all the caps are not locked as i had them off when the engine was out of the frame.

what am i looking for when i pull the caps? oil splashing around? don't believe i'll be able to see a whole lot looking in those holes.