Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: adespin on August 03, 2018, 12:35:57 PM
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The last time i had a cb750 was back in 1976 with a new K6, with a new bike i didn't worry about things like, is the oil circulating correctly, are the oil jets clear, but on reading some of the posts on this forum, it's now got me thinking about my bike, a 1971 K1 with 14000 miles on the clock, judging by the unrestored condition of the bike it could be genuine.
So this morning i took the front tappet covers off and started the bike from cold, it took a couple of minutes for oil to appear, i was expecting oil immediately, is this normal ?
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2 mins sounds like a long time... does the oil light go out straight away?
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Yes Rob, instantly.
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Yes Rob, instantly.
Good news oil light goes out instantly but if you think where the oil pressure switch is located on the engine, the oil still has a fair way to go, including up through the infamous jets, until it gets to the camshaft area.
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Exactly.... so we know we have oil pressure. Is the 2 minutes actually timed? Is there plenty of oil on the tappets and spring tops or do they look dry? Have a good look when she has been stood overnight, then start her up and have another look after 20 seconds of running... if there is no extra oil after running i would definitely be looking at the jets... it may be the safest option anyway.
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No, i havn't timed it, it just seems like a couple of minutes, thinking about it, 2 mins is a log time, so tomorrow i will time it accurately from cold.
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You have to get a normal confirmed timing of oil arrival visible at the tappet covers to compare this one too.
On the contrary, the "oil jets" should be doing exactly this. They prioritise the crankshaft as it's turning with as close to instant pressure build up. You really don't want the crank running while waiting to build oil supply to the cam area which has, in effect, a holding bath for the critical cam lobes to dip into while it's waiting for oil to arrive from the gallery.
I know it's probably pedantic :), but jets ordinarily speed up output from the flow coming in to the flow going out as in jet engines, carb jets etc. These are "pressure limiting restrictors" and do exactly that, to protect the oil pressure supply to the crank and big ends as their first objective.
I'd clarify if it's normal before considering taking it apart.
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So am i to understand that there is no pressurised oil feed to the top end except for the jets, therefore basic splash lubrication Briggs & Stratton style, if so quite primitive.
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There is some pressurised oil getting to the top end but because they are restrictor jets giving priority to the bottom end, the pressure is diminished alot by the time the oil gets to the top end. It really is a gentle spray by the time the oil get to the top end. Basic but works fine as long as the jets are 100% clean and clear and you have good oil pressure in the engine. The minutest blockage will reduce the spray effect as the holes in the jets are very, very small.
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Yes, correct. You've in effect got two oil systems, high and low pressure, supplied by the same pump and divided by those restrictors to hold the main shells at maximum pressure.
They just push it through the cam journals so that they have constant supply, which leaks out and variously fills what I'd guess you'd call a series of wiers to retain oil at levels around the components and always give them a supply.
In all fairness, many engines do the same as this as they have no need to do anything more complex. The cam lobes on any engine are effectively open/ non pressurised bearings really so you can't do much more than chuck oil at them or dip them into the oil as here. They mostly are very rudimentary but work well. And only rotating at half crank speed demands aren't that high realistically.
They are more likely to be compromised by too tight a tappet clearance as it prevents the oil film establishing round the cam lobe if it's continously wiped even round the base circle diameter of the cam.
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Apologies Julie, I crossed over on your reply as I was typing it, and argueing with my predictive text :)
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So should we all be pulling our engines out to check the jets, or am i getting a little paranoid.
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I was worried about this when I first got my 78 k started. Removed tappet covers and it threw oil over my front mudguard after running a few mins.
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If you've got oil to both sides of rocker cover (it only comes through those two vertical supply) then it confirms both are flowing. And unlikely to need attention. At first impression it looks normal and wouldn't unduly be a concern.
The cam journals start squeaking if there is no supply ;D
The oil there is supplied via the filter as is the crank, so it's not usually a problem. They often have trouble purely because someone building the engine has introduced gasket sealer or similar to the oil system that comes after the filter housing, with nowhere to go it can get caught in the restrictions if it travels that route.
Normal use with oil and filter changes doesn't ordinarily result in these being compromised as they generally see nothing but clean oil. For blockage, it low risk in normal services and they don't generally require maintenence outside of if the engine had a problem otherwise.
It's the same with all of these Honda engines, clean oil is paramount.
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Well you have settled my paranoia (for now)
Many thanks,
Adrian.
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Agree with Nigel. Usually too much gasket sealer used during rebuild or blast media from a muppet blasting the cases with sand or glass bead, 1 spec of either can block the jets.