SOHC.co.uk Forums > CB750
Oil pressure issue / motor rebuild
K2-K6:
You've a very unusual set of circumstances that look to be a possible cause.
If you think about "traditional" low oil pressure, that would be the engine bearings worn enough to allow the oil feed to leak out faster than the pump can put it in to fill the gaps.
What you appear to have is "within reason" a relatively unworn bearing set (unless further running totals it)but a restricted supply so the leak out is normal but refilling the bearings is not happening fast enough. The con rod upper shell is effectively prevented from getting much oil into the area during the power stroke and relies on the non compressive part of the stroke to replenish the oil that separates the shell from the crank. If you restrict the supply, that replenishment won't complete in the time available so the next power stroke gives you a clonking as the shell hits the crank pin.
The mains have nothing like the percussive load inflicted on them so as previously posted they'll generally not give the same sort of noise.
There is no real restriction going to the pump in a dry sump system but a collapsing feed hose would definitely make one that the pump could not overcome.
That's very informative from Bryan (hat off to him for remembering that) as it could be key to your problem. There's not many forum with that in depth level of knowledge available.
Bryanj:
I remember it because my one time boss, Neal Frazer, who owns Frazers of Gloucester bought a K? from Skellerns of Worcester with all the goodies (Tank. bars, sidepanels,etc) removed and had the frame and swingarm nickel plated then built a "New" Dunstal. Two weeks later it started knocking and he had to take just the engine to Skellerns who took over a month to fit new crank, rods and shells also supplying the new oil hose. It may even be in the bulletin list--not sure
MrDavo:
I have the hoses off, although I cannot be certain we've found the problem until I fit new hoses and refill with 10/40, the supply hose has a kink (not terrible but still a kink) where it bends 90 degrees, a simple but messy experiment pouring oil into the tank and watching it come out of the motor end saw oil coming out faster when the hose was straightened.
New OEM hoses are NLA (thanks again Honda), but an eBay search found a man in Germany who does a service exchange service fitting new stainless braided hoses to used fittings, not cheap and priced in Dollars, but the best I'll get.
Even if it doesn't solve my problem and I have to pull the motor I reckon that's what caused it, it will look better and I will have eliminated the poor feed. Interestingly my Haynes manual (I have little faith in these but at least I can take it in the garage unlike my virtual Honda manual) mentions bad oil hoses sometimes causing low pressure, so they must have read the bulletin too. A Google search tells me Honda bulletin 750#35 dated 11/72 was called 'Revised Oil Tank Feed Hose', but no trace of it online anymore, Ash will know, but it seems that the US site bottled it and pulled their online service bulletins - why the hell would Honda sue their loyal customers over copyright on 40 year old bulletins?
While I'm at it, Ive pulled the oil pump again, all looks well and I checked the pressure release valve too. A couple (literally) of metallic flecks on the strainer, also tiny bits of my new clutch friction plates but nothing scary.
Green1:
That kink looks pretty severe to me.
If that was a rad hose you would bin it.
MrDavo:
Well the good news is that the oil hoses arrived from Germany, and very nice they look too:
They were a struggle to fit, as they are not a millimeter longer than they have to be, and very stiff - it took a few attempts with each hose until I found the correct sequence of assembly so they would fit. I thought of spraying the stainless braiding black, but decided against it as they'd just look scruffy when the paint starred to flake off.
Anyway, that's the least of my worries, after I fitted a new filter and some new 10/40, I went for a test ride. From this picture, taken later in the day, you can guess how that went:
Fine until it got hot, then the knock was back. I limped home slowly, and dropped the oil. This time, because it was new and clean, I could see the dreaded 'glitter' in it, no question. Although I've fixed the cause, the crank is already goosed, and isn't going to fix itself. B*gger!
I've wondered about a second hand crank, DK have one, and I know a guy in Stockport with a load of used CB750 stuff. However what you don't know is why the bikes they came from were stripped / scrapped in the first place. Unless I go with a micrometer, measure every journal and compare it to the spec sheets, I could easily buy a whole load of more trouble. What do you think?
The motor is nearly ready to come out, I'm stupid busy this week though, so it may not get pulled and stripped straight away. :( I am keen to see how bad it is though.
The contact in Kegworth seems promising, if only because its a lot nearer than Bedford, however there's always the post. How feasible is it to post a CB750 crank though? What would I have to pack it in? Probably not a Jiffy bag.
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