Topic: Oil light

[Walt]

I have a 400F and once it's warmed up, the oil light comes on when the revs get to 2K or less. Is it time to take the oil pump off and check the clearances? Maybe there's something I'm missing here. I checked the electrical connections and they seem to be ok.

Thanks,
Walt

[bwaller]

I quess there are a number of things to find out before recommendations can be made. One thing first is, how does it run and how many miles are on the engine?

These switches do fail, but usually the light will stay on if that's the case. Maybe the best approach is to actually install a guage to check the oil pressure. I'm not sure if replacement parts or an NOS pump are still available, but you can probably find a spare from someone on this site.

It isn't a good thing if the low pressure light comes on at all when it's running, let alone below 2K. 
 

[Bodi]

Well, there are 3 reasons the light could come on - low oil pressure is two reasons: weak pump or worn engine. The switch is the otherreason : it could be bad and/or switching at too high a pressure.
The oil pressure switch (attached to the pump) is physically the same size as almost any automotive one, but most car ones switch at about 20ps while the 400F one switches at somewhere between 5 and 10psi. So a car one will put the light on at low rpm with a pressure Honda thinks is OK.
It's a fine idea to check the pump, you just unbolt it and disassemble the working bit. Seals and parts inside the pump are not available and never were, so don't ruin an O-ring inside or you'll have to figure out the size and get one from a rubber supplier. The O-rings connecting the pump to the engine cases are available from Honda. If the larger rotor set looks really nasty (gouges on the edges and lobes) that could be the problem. The smaller rotor set is just for the transmission and does not have any effect on the pressure switch for the warning light.
You might try a pressure gauge. You can remove the gallery plug directly below the points cover. There's room in the hex to drill and tap a 1/8 NPT thread (and still have the hex to wrench it in with) and directly install a small 100psi pressure gauge to see what you really have. It's impossible to see when riding and rather vulnerable so get a 1/8 NPT threaded plug and seal the hole after testing - then you can install the gauge again when desired. With a hot motor you should have 10 psi or more at idle, and 60psi or more when cold. If you really have below 5psi at idle you really do have a bad pump or a worn engine. Some bikes can wear through the case into the oil gallery and lose pressure with slack cam chains but as far as I know the 400 doesn't do that.

[Johnie]

Excellent and informative post Bodi.  This should go into the archives.  I would start with the switch and move on from there if needed.

[Walt]

The bike has 27K miles on it and is running great for the little bit I've been able to ride it. I got it as a basket case and have only put about 200 miles on it once I got it running. I put a new head gasket on about 50 miles ago, the first ride was about 25 miles and the light didn't come on. The second ride was about 12 miles and the light came on at the end. The third ride was the same as the second.

Maybe I screwed something up while doing the head gasket?

Thanks everyone for your help. I think I'll be pulling the pump off tonight to have a look at it.

Walt

[Bodi]

The two "oil control orifice valve" things between the head and cylinder section could have been damaged when you replaced the head gasket. These have full pressure oil from the oil gallery underneath and a tiny hole that lets a bit of oil through to dribble onto the cam lobes: if there's  a problem you either get not enough oil to the cams (blocked hole) or too much oil up top and a loss of oil pressure in the main gallery (missing or broken orifice valve).
Do you recall any problems with these orifice valve things? Can you open a tappet cover at each end and see if there's a LOT of oil getting to the top end? It should be oily in there and a few dops might fly out of the cap when running, overall not a lot of oil should be in the cam gallery.

[Walt]

I took the oil pump off last night and the clearances are well within spec and there is no scoring on the lobes.

I took the plug off the hole that is below the points cover and will try to get some fittings and a pressure gauge to read the pressure.

I'll also try Bodi's suggestion about opening a tappet cover, but I think for peace of mind, I may end up taking the head back off and inspecting the orifice valve and o-rings. A head gasket can be re-used right?

Thanks everyone.
Walt

[Walt]

I still haven't hooked up an oil gauge yet, hopefully I'll get one from Napa today and take Bodi's suggestion of drilling and tapping the plug.

I took the head off and inspected the orifice valves and o-rings and they seemed fine. I blew out the orifice valves and passage in the head and it didn't seem like there were any clogs. I checked online to see if the oil pressure switch on a 75 750 was the same as the 400F and it was. I swapped them out and the bike ran fine and the oil light didn't come on. However, at about 10 miles into a ride, the bike died and wouldn't re-start. The wife had to throw a 2x10 in the back of the truck and come and get me.

I'm going to change the oil pressure gauge back to the original and get the gauge to check the oil pressure. It just seems odd that it died on me. It did re-start the next day though and seemed to run fine although I didn't take it out. Anyone have any insight as to why it may have died? Maybe something completely different? The battery is somewhat weak and won't run the electric starter most of the time. But I can kick it over and it will run just fine.

Thanks,
Walt

[Klark Kent]

swap out the sender.

i wish i saw this before you took off the head and disassembled the oil pump- most of the time it is the sender's daiphram that is giving false oil lightings. 

good luck

[Walt]

what and where is the sender?

[Klark Kent]

it is the cheeseburger shaped thing that threads into the top of your oil pump under the sprocket cover.

[Walt]

That's the pressure switch with the wire going to it right? I did swap that out with the one from my 750.

[Klark Kent]

that's right. my apologies i missed that. check out this thread

[Walt]

Thanks Klark, that's a good thread. I'm getting a gauge tomorrow, so hopefully I'll be able to report back with the pressures.

Walt

[JAG]

If you look back at the thread Klark Kent sent you to, I believe I have gotten everything in order.

Do you know for sure your tach is spot on? I don't believe mine is. I have my tach set at '1400rpms' and the pressure shows 5-6psi. Oh my other, smoothly running 750, after it has warmed up (it shows 1200rpms and 5psi) I can back off the throttle screw and drop it past 1000rpms, and the light will flicker, the come on, and when I get down to 800rpms, it pretty much dies..

If you got the tappets in order try and get it sync'ed and see if that helps.

Have any more questions let me know, i've went through that whole dang bike and I think I just about have it whipped!

Good luck with it!

JAG

[sandcastcb750]

My CB750 did the same thing;hot below 2000rpm and the light comes on. It was a bad sender on top of the motor, actually leaking, obviously low pressure.

[JAG]

where can I get a sender at? How much?  What was your oil pressure gauge showing?

[sandcastcb750]

You can probably get a sender off ebay cheap. Not the thing that everybody wants. Mine read 5 lbs when hot and the red oil light was on too.

[JAG]

so putting in a new sender, fixed the light where it didn't come on?