Topic: CB750 k5 oil light at idle

[heymak]

Hi Guys ,I am new to the site not posted here before, need some help and advice , have read a lot of posts ,
I have got my self a 1974 k5 USA imported to Uk , very nice bike only 10,900 miles all working and sounds good only done 15 miles, except for the oil light at idle stays ON , but goes OFF around 3000 rpm,
I changed the oil 10/40 semi synthetic and filter ( will 15/40 semi or mineral be better ), dropped the pan and cleaned the oil strainer but didn't need it, was really clean inside like the day it came out the factory. changed the oil pressure switch, cleaned up the connector terminals . checked the idiot lights and no holes in bulb mounting rubber , is it a poor earth. when I take off the wire to the oil pressure sw. the light goes OFF, when I ground the wire on the engine the light comes back on ?? , running out of options . maybe I need to pull the oil pump out ? doing my head in lol

[bjbuchanan]

Enough oil? I had an oil leak that I would use the oil light as a reminder bc it got bad enough. One qt low will trip it a little over half did it sometimes too

[Nic]

I'd be going 20/50 oil for starters, then you can take out the pump and replace the 3 Orings and also the one behind the countershaft bearing retainer. Should be good then.

[PeWe]

Here a thread about the o-ring replacement, oil pump 3 and 1 on the counter shaft.
http://forums.sohc4.net/index.php?topic=78643.0

Make sure you use bike oil for wet clutch JASO MA/MA2 on the label. This has nothing to do with low oil pressure.

[robvangulik]

If it were mine I'd first check with a pressure gauge what the actual oil pressure is, who knows what has happened to the bike in its previous life. For all we know one of the crankshaft bearings has given up or something like that.

[bryanj]

Did you fit a Honda switch or a car switch as the car ones trip at a higher pressure

[acollin]

Double down on that pressure check with a gauge. This is, without a doubt , the first step.

In cars they are called" idiot lights" for any number of reason. You do not want to be be chasing  something with repair after repair that does not really exist.

Be certain what the problem is before you start repairing and replacing.

Good luck

[MCRider]

I don't think the oil light can be used as an indicator of level on a 750. The bulk of the oil is held in the tank, and the light has no way of knowing how much is in the tank. Maybe it can, I don't see how.

The light is triggered by pressure, or lack thereof.  A warning I give is that we can no longer look at a low mileage bikes the same way we used to. Sheer age is now a factor. Those rubber parts like oil pump ORings, transmission bearing ORings etc. get old, hard and shrink up, not holding the oil pressure at low RPMs.

[Black 750K8]

To sum up all the good answers you have from the others. Do this in this order.
1. First put a gauge on it. You need to know if the light is telling you the truth.
2. definitely change the four O-rings Take your reading from step 1 and compare it to the reading after new o-ring you will be surprised .
3. Change the oil weight for the temperature you plan on riding in.

[Henning]

20W50 oil will be a better choice for your location, it's what I use here in Denmark, which is probably a little colder than you. It's thicker, so it will hold pressure better. But follow up on the other suggestions, starting with a pressure guage to check actual pressure.

[robvangulik]

20W50 oil will be a better choice for your location, it's what I use here in Denmark, which is probably a little colder than you. It's thicker, so it will hold pressure better. But follow up on the other suggestions, starting with a pressure guage to check actual pressure.

Using more viscous, or thicker, oil in a colder climate may not be the wisest of choises.
Without doubt there will be more pressure, but the oil will be slower to reach those places dependend on it like the camshaft and crankshaft bearings.
Too much pressure will activate the pressure relief valve at the oil pump, and the oil that escapes there doesn't reach those places at all.

[Henning]

20W50 oil will be a better choice for your location, it's what I use here in Denmark, which is probably a little colder than you. It's thicker, so it will hold pressure better. But follow up on the other suggestions, starting with a pressure guage to check actual pressure.

Using more viscous, or thicker, oil in a colder climate may not be the wisest of choises.
Without doubt there will be more pressure, but the oil will be slower to reach those places dependend on it like the camshaft and crankshaft bearings.
Too much pressure will activate the pressure relief valve at the oil pump, and the oil that escapes there doesn't reach those places at all.

Fair enough. I buy oil at the local Honda shop - the owner is old enough to remember these bikes when they were new. He asks how long you ride, meaning do you ride in sub-zero conditions. When I say no, he says 20W50 oil. I've seen that recommendation a lot of times on this forum.

[heymak]

hi guys i really appreciate any input from you all , I did use a genuine Honda pressure switch ,  I will go see a mate who will hook it up to his gauge 1st, after getting some readings , then I will pull the oil pump and change the 3 O rings on the pump and the 1 on the counter shaft , change the oil to 20/50,
 but i do really think the oil is the problem and might just change the oil for 20/50 see how that goes 1st ?

[robvangulik]

If the pressure is good, you'll be allright, that's most important.

[commando1954]

I would only use the oil that Honda recommends and I think that is 15/40.It looks like an oil pump problem to me.

[70CB750]

Honda does not recommend 15/40.

[commando1954]

OK - it must be 10/40 then?

[70CB750]

Oil discussions are long and if you use the Google search, you will find material to read for many hours.

Anyway, most of the people who know thing or two about old Hondas use oil 20W50, such as Gastrol 4T

http://www.autozone.com/autozone/accessories/Castrol-1-qt-20W-50-4T-4-Stroke-motorcycle-oil/_/N-255s?itemIdentifier=493839

or Diesel oils for the zddp.  Do a search and read up.

[robvangulik]

Anyway, most of the people who know thing or two about old Hondas use oil 20W50

Except for the Honda factory, who, from my first CB200 in 1975 to my current CBR1000, advised to use 10W40 via the owners manual.
And as all those bikes kept up quite well for hundreds of thousands of km's, I see no reason to use anything else.
But I grant everyone their own choice, those bikes can take it, as long as the quality is good enough!

[commando1954]

Yes if Honda thought that 20/50 was the best oil they would recommend it!

[70CB750]

Do your homework, search on oil and read. For me 20w50 is the best oil, based on what I researched. 

[Dream750]

Yes if Honda thought that 20/50 was the best oil they would recommend it!

The CB750 owners manual does call for 10W-40 oil, but it’s not the final word. The Honda CB750 shop manual does spell out the use 10W-40 or 20W-50 oil. This spec is further supported by the OE oil tank side cover decal, either early or late style.
 

[heymak]

Thanks Fellas I am doing an oil change tomorrow with 20/50 hopefully the oil light will go off at idle ,  iff not am pulling the pump , will let you know how I go on

[HondaMan]

The Honda manuals recommend the use of 10w40 BELOW 30 degrees F () degrees C) and 20w50 above that temperature, or for long rides (more than 15 miles at once). This was true in the Owner's manuals until a serious mistake was made in 1974 that was not corrected until the 1977 manuals came out: it was a mis-translation problem from the original Japanese to English (in the US manuals). The original Japanese read:
"Use NOT LESS than 10w40 oil in cold air. Use 20w50 oil for normal temperatures."

This somehow became, in English:
"Use AT LEAST 10w40 oil."

Those of us who were fortunate to have a Honda Service Manual or their Service Bulletins saw the strong recommendations for the 20w50 oil "unless the weather is cold and the rides are short". In essence, this means that 20w50 is the NORMAL oil for these engines, and always has been.

On the 500/550 and later SOHC4 engines, the MINIMUM oil weight was printed and/or cast onto the dipstick. Those who did not read their Owner's Manuals then thought that the "10w40 Oil" imprint meant to use that oil: instead, it was a warning to NOT USE LESS than that, ever.

So went the confusion: of course, today it is just history, with many worn engines to show for it. The first thing to wear out for thin oil is the valve guides, and almost 80% of the engines I see today have seriously worn-out guides for that reason.

[commando1954]

Using 10/40 oil will not wear out the valve guides any quicker than 20/50.In fact just the opposite as when cold 20/50 will take longer to get to the top of the engine. The reason that you find wear on the valve guides is probably due to not changing the oil and filter at regular intervals and oilways get blocked.