Topic: CB650 - Low oil pressure when hot @ idle

[k_650]

Hi, it's my first post here but I've had this bike as a project for a while. I've already looked through the forums and googled as much as possible before posting, but I can't seem to figure out what's causing my issue.

I just finished overhauling the engine (new rings, honed cylinders, valve lapping, stem seals, and all new gaskets/o-rings). The bike sat for many years but only has 15k actual on it. When the bike gets up to temperature, and when at idle (1200 rpm), the oil light has been coming on. It goes away as soon as you give it even the slightest amount of throttle. I hooked up my oil pressure gauge to where the pressure switch normally is on the pump after riding, and it showed only 5psi at idle. It was around 50psi at 3k rpm, which seems to be consistent with Honda's measurements.

Fresh oil and filter (15W40), no visible leaks anywhere. There's a bit of crankcase pressure and blow-by out of the PCV, but it seems to be getting better. I'm thinking its just a result of the new rings bedding in. A lot of people seem to run 20W50 in their 650s, but if Honda says 10W40 is fine year-round, would something else be wrong with the bike? When I was riding today it was 60*F, I can't imagine it'd be any better in 100+ summer heat.

Some possibilities I've seen are worn crankshaft bearings, missing primary nozzle (I put a new o-ring on when I had the engine apart, it's definitely there), clogged pickup screen (just pulled off oil pan and checked), or a worn oil pump. When I had the engine apart, I checked the bearings for wear and it seemed consistent with the mileage the bike has. I wouldn't think any of these things would be worn with this low mileage.

Any advice or suggestions would be appreciated, thanks! 

[fizzlebottom]

I think the oil pump is the place to look. I suspect worn components, possibly due to previously running with low oil pressure.

[scottly]

The bike sat for many years but only has 15k actual on it. When the bike gets up to temperature, and when at idle (1200 rpm), the oil light has been coming on. It goes away as soon as you give it even the slightest amount of throttle. I hooked up my oil pressure gauge to where the pressure switch normally is on the pump after riding, and it showed only 5psi at idle.

What is the oil pressure when you adjust the idle speed to an indicated 1300 RPM?

[k_650]

Not that much more, reading 7 psi @ a touch over 1300rpm. I guess I should also say that I replaced all the seals on the oil pump, including the one that notoriously leaks. Is the best way to diagnose the oil pump still the feeler-gauge method in the FSM?

What's an acceptable oil pressure for these bikes at idle? I couldn't find any hard number but it seems to be around 15psi ideally...

[scottly]

The pressure switch normally turns the light on when the pressure drops below 7 PSI, more or less. As long as the pressure is good at higher RPMs and there are no other symptoms, I wouldn't be too concerned about the light.

[RAFster122s]

Keep your revs to 1300-1500 at idle if that will keep your oil light off and ride it…
Trying to have a motor barely tick over at idle isn’t always a good idea. You mix the oil pump orings in your replacement? An undersized oring could contribute to a leak inside the pump area

[fizzlebottom]

If you replaced seals in the oil pump as well, then I'm leaning towards either a flawed new part or a mistake during assembly. Both are perfectly conceivable and unfortunate.

I'm not a fan of the idea of just letting the bike idle above 1300 to keep oil pressure up. You rebuilt the motor to get it running like new and now it runs with risky oil pressure. You'll either get away with it for some time or cause even more damage when the issue gets worse.

You may end up treating things down a little just to find out which part isn't behaving.

[Little_Phil]

If it's the same as the 550 the o ring under the oil pump cover (91305-426-003) must be 46 x 2mm  cross section I think. Bigger will cause lower pumping capacity.

[Nicklopic]

You should open up the oil pump just for the peace of mind. My 650 makes about 15psi at idle and about 60 at whatever RPM the manual says, I forgot to read what RPM it was in my post.
You don’t need a ton of oil pressure at idle but it does seem a little low and I’d be second guessing and worrying myself

[HondaMan]

First note: I've posted many times about this - Honda America incorrectly 'retranslated' the Owner's Manual beginning in 1973 for the SOHC4 bikes because the poorly-written Jinglish in them was sometimes confusing (the Shop Manuals are even worse). Prior to that, they all stated clearly (to those of us who understood the Jinglish versions) "Use 20w50 oil above 40 degrees F", and that's what you should be running. Use 15w40 or less only in cold-weather riding. What Honda originally said about this oil was: "Use NOT LESS than 10w40 oil in warm temperatures. Normally use 20w50 weight." American Honda left out the 'not less than' phrase in most of the Owner's Manuals. The term 'warm temperatures' meant 40-50 degrees F or higher, depending on which manual was read.

Your crank bearings are likely worn some (like most of the 650s I see today), since the bike likely suffered at the hands of the American re-translation of the manuals, as many of these SOHC4 bikes have. The "full" fix is to install new, usually the next oversized, crank and rod bearings, but you can run for quite a while by just using the correct oil. DON'T use oil with detergents in it: if the label states anything like "Cleans your engine while you drive" it must not be used, as it WILL foam at the RPM these engines run, and this results in low oil pressure throughout the engine. This is/was the 2nd most common reason these bikes started showing their oil lights at 10K miles or more: the owner used typical car oil (like Valvoline 10w40) in many of them.

[fizzlebottom]

The "full" fix is to install new, usually the next oversized, crank and rod bearings ...

Oh, what a dream it'd be to have new bearings available for these bikes.

[HondaMan]

The "full" fix is to install new, usually the next oversized, crank and rod bearings ...

Oh, what a dream it'd be to have new bearings available for these bikes.

You can order the sizes you want here:
https://www.southsoundhonda.com/--xpartsstream#/Honda_Powersports/CB650A_(80)_MOTORCYCLE%2c_JPN%2c_VIN%23_RC05-2100002_TO_RC05-2108005/CRANKSHAFT_%2b_PISTON/511bb203-758c-422f-aa64-40fd161c6be4/9fb18781-e614-4560-b3e5-bdb2c4e2d485/y

This is where I get them for these bikes.

[fizzlebottom]

You can order the sizes you want here:
https://www.southsoundhonda.com/--xpartsstream#/Honda_Powersports/CB650A_(80)_MOTORCYCLE%2c_JPN%2c_VIN%23_RC05-2100002_TO_RC05-2108005/CRANKSHAFT_%2b_PISTON/511bb203-758c-422f-aa64-40fd161c6be4/9fb18781-e614-4560-b3e5-bdb2c4e2d485/y

This is where I get them for these bikes.

Luckily I'm not in need of new bearings right now. They only seem to have the brown crank bearings though, and that's the most stock I've seen anywhere. Rod bearings are all available which is great.

I really should just make a parts order and head down there someday. Not a far trip from Seattle.

[scottly]

Perhaps the original poster should try switching to proper 20-50 wt oil before committing to a full bottom end rebuild? It wouldn't hurt to double-check the oil pump, as others have stated.

[rotortiller]

Been running 10W40 oil for over 58,000 miles on my 750 K7. No matter how low I make it idle the light will not come on. One thing about oil pressure gauges is they are most often accurate at mid scale, sometimes having notable errors at the bottom of the scale, say with a 150 psi full scale deflection. 15K miles is nothing for these bikes regardless if you used 10W40 or 15W50 weight oil etc. You may have messed something up on the reassembly, we all have at one time or another.

[k_650]

Thanks for all the replies, sorry I was away this weekend and didn't have a chance to respond. I'll make the switch to 20W-50 and see what changes, but I'm starting to agree that there may also be something wrong with the seals I put into the oil pump.

If it's the same as the 550 the o ring under the oil pump cover (91305-426-003) must be 46 x 2mm  cross section I think. Bigger will cause lower pumping capacity.

I'm assuming that the o-ring that goes under the oil pump metal plate/cover thing is what would play the most important role with oil pressure (affecting tolerances and letting oil slip past the side of that pump mechanism). When I was putting the engine back together I found this post: http://hondacb650.com/viewtopic.php?t=5101, which found a McMaster o-ring that works. That's what I ordered and put in. It's imperial but translates to ~2.4mm cross section. It seems like the 650s are supposed to have a 3mm o ring there, so perhaps this is fine? Would this be any better if I switch to a regular circular o-ring?

I looked at the bike again with fresh eyes today, and noticed a very slight leak from the bottom left o-ring on the back of the oil pump. Would this really have that much of an effect on oil pressure? I did replace all of these seals with what's specified in the parts diagram so I don't know...

If the primary chain nozzle was leaking out of the o-ring (that I replaced), would it also cause this much drop in oil pressure? High crank-case pressure doesn't affect oil pressure, does it?

Crank bearings would suck, but definitely plausible. They looked really clean when I pulled them apart. I'll see if I can dig up a picture of when I had the bottom end apart, I wish I checked them with plasti-gauge but it was a free bike so I didn't really care, guess I'm paying now haha.

Thanks so much.

[Flyin900]

Buy the correct O rings from Honda and not something that is close, but not to the Honda spec. You can get away with close to spec O rings in some areas. I would only use a Honda part in critical areas such as the oil pump seal assembly.

Then switch to the correct oil spec and see where your at with the oil pressures.

[scottly]

Crank bearings would suck, but definitely plausible. They looked really clean when I pulled them apart. I'll see if I can dig up a picture of when I had the bottom end apart, I wish I checked them with plasti-gauge but it was a free bike so I didn't really care, guess I'm paying now haha.

Thanks so much.

If the bearings were worn enough to lower the oil pressure, you would have noticed it.

[Little_Phil]

Buy the correct O rings from Honda and not something that is close, but not to the Honda spec. You can get away with close to spec O rings in some areas. I would only use a Honda part in critical areas such as the oil pump seal assembly.

Often Honda don't list the cover O ring as a separate part. But it is important if it is leaking there and has to be replaced and info has come from experienced mechanics back in the day.

[Little_Phil]

Not on any parts lists I have seen. 550 Oil pump is listed as a single part.

https://www.cmsnl.com/honda-cb550f1-super-sport-550-four-1976-england_model17113/partslist/E09.html