Cams not getting oil

Joseph New York

Active Member
The 81 CB750 DOHC is at a shop, I told em I needed them to check the valves as I was hearing slight sounds. They had the bike for other reasons I wasn't equipped to fix myself...

Under the valve cover was a melted gasket, a lightly scorerd lobe, and NO OIL.

They ran it with the top off and no oil was coming up. To rule out the pump they went in with a scope and it was operational.

Obviously the thing to do is remove the top end and find out what's going on. Maybe it's the head gasket. Unfortunately they have the bike and I can't afford to pay for a rebuild that costs more than the bike. So we're trying to brainstorm possible reasons the head is dry and hopefully y'all can throw your hat in the ring.

Clogged oil passage is the only thing I can think of. Going to have them go through the filter and pan and see if anything is there, but I can't think of another way to check for a clog without the head removed.

Anyone here have any ideas?

Worst case senerio is I fill up the head manually and ride it to my garage, but they are going to insist I tow it...

Thanks
 
Fill up the head manually? You know how this works right? Those cams are riding on pressurized oil film straight on the cylinder head alloy. Run it without pressure = its toast.
 
does your oil pressure light work

is it on when you turn the key on and off when you start the bike?

maybe you just dont have any oil pressure
 
I'm trying to picture how they stuck a scope in there to work out that the oil pump is OK.... ::)
 
"Filling up the head" hah I'm talking about lubing the cams so I can ride up the street. I'm not really serious about that. No shop would let that happen anyway.

"Scoped the pump" I didn't do this myself, but we were checking if the screen came off. No, ya can't tell it's OK with the scope, but we were checking if it's intact. The bike was layed down in traffic, so we thought it wouldn't hurt to look at its physical condition. Oil pressure is the real test...

"Oil pressure" I wish I knew man. She doesn't have any dummy lights or gauges. The pressure is the real test, I'm sure they are looking into it, I didn't want to insult them in asking, although in tomorrow's meeting I will be bring it up if they don't.

Sorry I don't have all the info for y'all to help right now, just looking for ideas I can include in tomorrow's discussion. I'll post back with what they say tomorrow.

Thanks guys
 
this is not rocket science you use a test light and check

or unscrew the sending unit and put a mechanical pressure gauge in and crank it over

and if you are going to a meeting and ask them if they have done a pressure check and they say no

we will look into that

you have your bike at the wrong place ! they should have done that first
 
cxman said:
this is not rocket science you use a test light and check

or unscrew the sending unit and put a mechanical pressure gauge in and crank it over

and if you are going to a meeting and ask them if they have done a pressure check and they say no

we will look into that

you have your bike at the wrong place ! they should have done that first

Hah they won't say that. They checked the pump screen first because it takes two seconds to rule out. I also told em it's always ran fine for years and without oil pressure I'm sure things would have been much worse.

Reguardless, I'm trying to get out of the shop. Hoping I could hear some ideas here of possible causes yall could share. I'll report back tomorrow after I hear the update.

Thanks all
 
you need to do some checking its going to be a investigation

lack of oil

no oil pressure

a blockage in the oil galleries

if the motor has not been tampered with or anything has changed

it hard to say
 
Yeah man, it's why I'm here grabbing for straws. I don't want to pay for an investigation, I didn't bring it in the shop for engine work, especially at their busiest time.

Let's say it's a blocked passage, is there any way to test for that without pulling the engine? I guess that's my real question I have for you guys. I've been looking at diagrams and the passages look like the catecombs. Pushing air through, somewhere?

In case the head gasket is leaking, I've read that it's possible to re-tighten head bolts. Dunno, it looks like I'll have to have it towed from the shop, find out more in a few hours...
 
i take the side cover off pull the electric sending unit

screw in a mechanical test gauge and start it up and see what the oil pressure is

that should have been step one

i would not think past that till you even see if it has pressure

it may not be pumping oil period
 
The're tell'n me 4-5lbs at idle. Of corse the head wouldn't get oil.

I'm just going to tow her home and rebuild it myself. It has a head gasket leak so it will have to be done anyways.

I'll post back once I find out what's going on in a few weeks, thanks for y'all help.
 
Have you cleaned your pick up screen? It could be blocked and the passage from the screen body is narrow. There is a mod from a member on the cb1100f.net site that sells modified screens to allow more oil to flow through. Your oil pipe could be blocked as well. It delivers the oil from the case to the head. It’s the s shape pipe on the outside of the case. I’d try these first. You’ll have to drop your pan to inspect the pickup screen but it may be worth it.
 
What is stock tickover pressure specification? That sounds really low.

If it's as low as it sounds, teh reasons could be blocked inlet or a spun bearing that's allowing oil to just piss out back into the sump.
 
esmoojee said:
Have you cleaned your pick up screen? It could be blocked and the passage from the screen body is narrow. There is a mod from a member on the cb1100f.net site that sells modified screens to allow more oil to flow through. Your oil pipe could be blocked as well. It delivers the oil from the case to the head. It’s the s shape pipe on the outside of the case. I’d try these first. You’ll have to drop your pan to inspect the pickup screen but it may be worth it.
I haven't. During my rebuild I'm cleaning everything. I'll look into that aftermarket screen, thanks. Without the bike in front of me, I totally forgot about that external pipe, interesting.

teazer said:
What is stock tickover pressure specification? That sounds really low.

If it's as low as it sounds, teh reasons could be blocked inlet or a spun bearing that's allowing oil to just piss out back into the sump.
Yes, I think that's super-low. All I was told from the shop is it was 5-6 at idle and that's all I needed to know before deciding to take it back from em. Once I get her in my garage, I'm checking oil pressure, reviewing oil pan/filter, running compression and leak down test, then disassembly, remove all carbon, replacing all gaskets, adjust valves, etc. I've never inspected the cause of low oil pressure, any tests or reviews I should preform to find the culprit, before disassembly, and after?

Guys, on a side note, I'm suspecting the shop didn't check the oil pressure (I could be wrong) but during my call with them, we talked about causes of the dry cam. Then I asked about the oil pressure and he said it was very low. If the oil pressure was low, why would we even be discussing why the head wasn't receiving oil?
 
like i said you had the wrong shop

60 psi is about the relief pressure

i would expect around 20 psi hot idle 30-40 cold idle
 
From what I read on the cb1100f.net forum you want about 10psi at idle and 80 when riding. Check your oil pressure relief valve as well.
 
cxman said:
like i said you had the wrong shop

60 psi is about the relief pressure

i would expect around 20 psi hot idle 30-40 cold idle
Once I get it out of there I'll be happy. Wow that high, hu?


esmoojee said:
From what I read on the cb1100f.net forum you want about 10psi at idle and 80 when riding. Check your oil pressure relief valve as well.

The shop told me they'd want around 12 idle, I'll find out soon enough what the pressure is and go from there. The fact that the bike has been running for as long as it has with low pressure (so they say), dry cams, and a leaking head and valve cover gasket, is nothing short of a miracle. Y'all wouldn't believe what this bike has been through.

Just ordered gaskets, not sure what else I will be needed, will be my first DOHC motor build. Shim over bucket :eek:
 
10 lbs per 1000 rpm hot is a universal minimum for 4 stroke engines till relief pressure limit is reached
 
if its been running with dry cams you are going to need at least another head and cams

the cam runs right on the aluminum head no oil film you have scrap
 
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