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Offline DanJ

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Oil light
« on: August 28, 2015, 06:34:29 AM »
I recently got my 73 CB750 back on the road after 34 years of sitting in someones garage. After about 8 miles or so the oil light comes on. Other than oil level anyone have any suggests on what to look for?
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Offline MCRider

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Re: Oil light
« Reply #1 on: August 28, 2015, 06:52:38 AM »
The pressure light will not react to oil level except when its completely out of oil. Its downstream of the pump and as long as the pump has a quart to pump, pressure is good.

Good thing is that 99% of oil light issues is the sender has gone bad. The sender is located on the back side of the cyls at their base. It threads out and I think there is an automotvie equivalent, like from a civic.
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Offline DanJ

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Re: Oil light
« Reply #2 on: August 28, 2015, 08:41:28 AM »
Thanks I'll take a look at it.
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Offline DanJ

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Re: Oil light
« Reply #3 on: August 28, 2015, 09:28:44 AM »
it looks like the 81 Civic and perlude use the same switch.
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Offline DanJ

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Re: Oil light
« Reply #4 on: September 09, 2015, 09:03:06 AM »
Well the pressure switch isn't the problem. anyone have any other suggestions?
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Offline MCRider

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Re: Oil light
« Reply #5 on: September 09, 2015, 09:10:06 AM »
Sorry.  :(

While on the centerstand, remove the #1 and #4 exhaust tappet covers. Then run it, at idle and blip it. You whould get a spattering of oil coming out, it will make a quick mess so do it outside and watch it. If nothing you've got a pressure problem.  If so, pressure is likely OK.

And/or: buy an adapter and put an oil gauge in the galley plug right above and to the left of the points cover. If it reads less than 20psi at idle and 60 at 4000+ rpm, you've got an oil pump problem.

Either way you go, if you've got pressure, then the light is at fault.
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Offline Henning

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Re: Oil light
« Reply #6 on: September 09, 2015, 01:18:41 PM »
And you are running 20W50 weight oil? A thinner oil like 10W40 won't make sufficient pressure on a hot engine at idle to keep the oil light off, and is only suitable if you live north of the arctic circle.
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Offline rotortiller

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Re: Oil light
« Reply #7 on: September 10, 2015, 02:37:14 PM »
Quote
A thinner oil like 10W40 won't make sufficient pressure on a hot engine at idle to keep the oil light off

I use 10W40 all the time and my light stays off when the engine is running, even under 1000rpm. All the CB 750s I have owned or ridden were the same. It runs 20-30C here all summer. If the light comes on you have a bad pressure switch or low oil pressure in the main oil gallery. So if the pressure is low with 10W40 it boils down to the pump, internal leaks (cracked packings etc), clogged oil filter, or excessive clearances somewhere. They were not stupid when they selected the oil pressure switch rating. What you have to ask yourself is did it happen after I tampered with the machine or did it happen suddenly. I'd check it with a pressure gauge at the main gallery oil passage plug to be safe.
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Offline Johnie

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Re: Oil light
« Reply #8 on: September 10, 2015, 06:40:55 PM »
I have to agree with roto here...I also am running 10w40 in all my 750's and never had the oil light come on even in hot weather at idle.
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Offline Henning

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Re: Oil light
« Reply #9 on: September 11, 2015, 02:49:08 AM »
The 10W40 vs. 20W50 oil issue has been debated a number of times here and the forum seems to be split pretty much down the middle. I am firmly in the latter camp, and there is no doubt that a heavier oil will help his oil pressure. Rotortiller lists a number of things the OP can and should check as well. It would also help to know how hot it is where he is, also makes a difference.

I was a bit sarcastic in my first post. The OP has a serious concern - the oil pressure - and then he cheaps out by buying an oil switch for a car which may or may not be correct. And now he seems to have gone AWOL as well.
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Offline DanJ

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Re: Oil light
« Reply #10 on: September 11, 2015, 04:24:10 AM »
I'll look into things this weekend. I'm using 10W40, new pressure switch, air filter and top end. I'm getting oil to the valves but whether I'm getting enough I don't know.

I'm not awol and I've tried a Honda pressure switch from a friend so lets say i didn't "Cheap Out" . The Honda Perlude uses the same switch as the bike just like they use the same tappet adjusting screw so whether its from the Honda car or Honda bike it's still Honda, same part number.
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Offline MCRider

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Re: Oil light
« Reply #11 on: September 11, 2015, 05:32:07 AM »
Quote from: DanJ on September 11, 2015, 04:24:10 AM
I'll look into things this weekend. I'm using 10W40, new pressure switch, air filter and top end. I'm getting oil to the valves but whether I'm getting enough I don't know.

I'm not awol and I've tried a Honda pressure switch from a friend so lets say i didn't "Cheap Out" . The Honda Perlude uses the same switch as the bike just like they use the same tappet adjusting screw so whether its from the Honda car or Honda bike it's still Honda, same part number.

Yes, I've read here on more than one thread that members have used a Honda car sender from local auto parts stores. I think it was less money than the MC part, which as you say, carries the same part number. 

The oil pump may be at fault, though I've never seen one fail. But we've learned that age is causing failures of certain parts, that we ordinarily attributed to mileage. The ORings can fail which causes the pump to leak internally and lowers pressure.

Good luck.
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Offline martin_uk

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Re: Oil light
« Reply #12 on: September 11, 2015, 05:49:42 AM »
I seem to recall that the oil pick up strainer was included in service/maintenance checks, requiring a clean periodically.

If the exhausts do not obstruct, may be worth dropping sump to take a peek at this and debris in sump, especially in view of length of time standing?

Unfortunately, there are also quite a few rubber parts in lubrication system inc pump body, which will have gone very hard over the years and may not work as originally intended.

It would be a shame to run engine with low/no oil pressure and cause major damage. :'(

« Last Edit: September 13, 2015, 03:00:02 AM by martin_uk »
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Offline DanJ

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Re: Oil light
« Reply #13 on: September 11, 2015, 08:40:20 AM »
Thanks for the help guys I'll look into this weekend hopefully I'll have time. Have my son's stag party Saturday evening and we have Rice-O-Rama Sunday.
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Offline przjohn

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Re: Oil light
« Reply #14 on: September 11, 2015, 07:24:20 PM »
Have a great time at the stag Dan. We may be out for Rice O Rama, life happens and I heard it may rain and get my outfit wet.  :D Get that bike running will ya, let's go for a ride!
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Offline rotortiller

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Re: Oil light
« Reply #15 on: September 13, 2015, 04:13:58 AM »
Quote
I seem to recall that the oil pick up strainer was included in service/maintenance checks, requiring a clean periodically.

I think the screen in the sump is for the scavenge oil pump pickup. The pressure pump takes it's inlet supply from the oil tank. Now if all the oil went into the sump and could not scavenge back to the tank (unlikely) then you could have cavitation at the pressure pump inlet (oil tank outlet). Certainly worth looking at through the oil tank opening to see if the oil goes down too much during operation.

rt
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Offline Desert-SOHC

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Re: Oil light
« Reply #16 on: September 13, 2015, 08:40:41 AM »
Have you checked to see if the wire that runs through the starter chest isn't shorting out? My K8 had a light on all the time problem. It ended up being a frayed wire in the starter case. replaced the wire and all is good.
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Offline Restoration Fan

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Re: Oil light
« Reply #17 on: September 13, 2015, 08:58:14 AM »
Quote from: Desert-SOHC on September 13, 2015, 08:40:41 AM
Have you checked to see if the wire that runs through the starter chest isn't shorting out? My K8 had a light on all the time problem. It ended up being a frayed wire in the starter case. replaced the wire and all is good.
+1
If your pressure switch isn't the problem, I would look at this as the next issue.  Something in the connection from that switch back through the wiring harness to the indicator light is possibly shorted.
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Offline martin_uk

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Re: Oil light
« Reply #18 on: September 14, 2015, 08:20:05 AM »
If you remove the lead to the oil switch and light stays on, then you know it is a short, as the oil presure warning light is live on one side with ignition on and the switch earths the lead when pressure below preset value.
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Offline DanJ

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Re: Oil light
« Reply #19 on: September 14, 2015, 09:26:07 AM »
Thanks for all the suggestions. I haven't had time to look at it yet but will definitely will be this week.
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