Topic: 1976 CB750F: Mystery component of oil system leaking

[rieshunter]

Hello again!

After adjusting the massively-out-of-whack idle screw (thanks ekpent!), the bike idles at a comfortable 1.2k RPM. The bike runs and rides (woo!), but thoroughly marks its territory down the road (boo!). The oil level was fine after a few miles, but I'd rather the bike not leak on the road. Of note, the bike does not leak when idling in the garage, and old oil residue on the rear swing arms suggests this is not a new problem—perhaps it is part of the reason the previous owner left the bike to die in a barn.

After noticing the trail yesterday, I narrowed the leak down to this mystery component and its connections. The part is located directly under the battery (bolted on a bracket to the battery holder) and is positioned to the left of the oil reservoir. Tube #1 brings oil from the oil reservoir to the part. Tube #2 brings oil to the top of the heads. Tube #3 brings oil to the transmission area.

I noticed two odd things: 1) Tube #3 had a bolt clamped in it , and 2) another tube was on the transmission area nipple where I am almost certain Tube #3 connects to. I removed the bolt, connected the bike to the transmission area, then took the bike for a quick ride. Much less oil dripped from the bike, but it still had residual drips at the end of the ride and continued slowly dripping for about 5-10 minutes.

I can't seem to find anything online or in my Clymer manual regarding this part, so...here I am.
Does anyone know what the part is? I greatly appreciate any and all help!

[Jerry Rxman Griffin aka MuthaF'er]

Sure, it's a flimsy attempt at an oil/air separator and/or an oily air reburn apparatus. Otherwise known as a breather assembly.

Likely what you may be calling tube 3 with the bolt is the oil drain that accumulates oil from the crankcase air and is drained out at the bolt. It originally was a pinch fed hose. 

[rieshunter]

Sure, it's a flimsy attempt at an oil/air separator and/or an oily air reburn apparatus. Otherwise known as a breather assembly.

Likely what you may be calling tube 3 with the bolt is the oil drain that accumulates oil from the crankcase air and is drained out at the bolt. It originally was a pinch fed hose.

Thanks for the help! This is a stock piece, right? Are they known to leak?

[david 750f]

No leaks with stock parts, post some pictures

[david 750f]

Jerry and I both have 750F’s

[ekpent]

Make sure that the breather hose coming off the back of the valve cover does not have any blockages, I had one blocked on a 1974 K model once and it spewed oil all over the back of thee bike. Here is some of the stuff that came out of the rubber hose that was blocking it from an insect of some sort like a paper wasp.

[rieshunter]

No leaks with stock parts, post some pictures

[rieshunter]

Make sure that the breather hose coming off the back of the valve cover does not have any blockages, I had one blocked on a 1974 K model once and it spewed oil all over the back of thee bike. Here is some of the stuff that came out of the rubber hose that was blocking it from an insect of some sort like a paper wasp.

I’ll check that today and clean the area thoroughly. Thanks for the response!

[rieshunter]

UPDATE

The official name of the mystery component is...drumroll, please..."Breather Assembly" (Jerry Rxman Griffin nailed it). This breather assembly is my 1976 CB750F's method of separating air and oil; whereas, older models had a mesh screen in the oil reservoir. The reason for the change, according to HondaMan, is "to capture the 'noxious crankcase fumes' and send them back to the engine to be burned." Instead of routing these fumes to the airbox for the birds and small children to ingest, they are routed to the top of the engine. Sounds on par for the '70s.

Here are some of my pics from today (https://imgur.com/a/1muDzed). My logic is that oil overflow from the oil reservoir will flow to the middle inlet. From there, oil vapors (the "noxious crankcase fumes") will rise through the top and exit to the top of the engine. Un-foam-ified oil will sink through the bottom and exit to the transmission. Please let me know if you believe I swapped a hose or if anything seems awry.

I've cleaned, snugged, and clamped the tubes to the breather assembly. Hopefully, the leak is fixed. The rubber gasket bit at the top of the breather assembly seemed to be clean and seated fine, so I don't believe that will be a problem. Regardless, I cleaned and reseated it to be sure. Hoses were cleaned and air-chucked. They seemed perfectly clear of blockages.

Thank you all for your helpful input. Amazing, as always!

[madmtnmotors]

I'm not sure the connections are described properly in your picture. I have noted your picture to reflect the connections as they are on the later F2/F3/K7/K8 (77/78) vapor/moisture separator. The separator on the later model is a little different shape (shorter and fatter) but serves the same purpose. The "oil return" at the back of the engine/transmission on the later models (both F and K) connects to the oil tank itself. I don't have an F0/F1 (75/76) F model to refer to so I can't say for certain that they are plumbed exactly the same but I would expect them to be similar.

[rieshunter]

I'm not sure the connections are described properly in your picture. I have noted your picture to reflect the connections as they are on the later F2/F3/K7/K8 (77/78) vapor/moisture separator. The separator on the later model is a little different shape (shorter and fatter) but serves the same purpose. The "oil return" at the back of the engine/transmission on the later models (both F and K) connects to the oil tank itself. I don't have an F0/F1 (75/76) F model to refer to so I can't say for certain that they are plumbed exactly the same but I would expect them to be similar.

Thanks for the reply!

"Air to engine"

• When I bought the bike, this hose was routed from the breather assembly to the valve cover. You are right; it makes much more sense for these fumes to go to the airbox rather than the engine. And the parts diagram in the HondaMan post also supports this.

"Oil overflow"

• In HondaMan's reply (linked above), he says "In the attached pic, the 'mystery hose' that is shown from the separator with no part ID is the one that goes to the oil tank."
• Should I connect the oil tank or the valve cover hose to this inlet?(EDIT: oil tank overflow connects to transmission; valve cover hose connects to middle outlet on breather assembly.)

"Transmission" 

• HondaMan's reply says, "The #24 hose goes to the back of the engine."
• It's an oil drain into the transmission. I think this one is routed correctly. (EDIT: not routed correctly. This is a drain to nowhere.)

[madmtnmotors]

It depends on the oil tank. I believe early K tanks (before 77) had a vent/overflow near the top. Later K and F (77/78) had a vent/overflow connection at the bottom with a stand pipe in the oil tank. This vent/overflow on the bottom of 77/78 F and K oil tank connects to the back of the crankcase behind the transmission. I do not know which oil tank was used on the 75/76 F models. It is my understanding that the "breather" mounted near the battery provides a means to separate water vapor from oil vapor. The water would settle to the bottom. The drain connection on the bottom of the breather on the 77/78 F and K models is routed around the front of the swingarm and then to the rear, terminating at the stock muffler bracket. This drain tube on the later models has a plug in it and should be opened to drain at every oil change per the service manual, at least on the later models. If this collected water vapor simply drains back to the engine on the earlier (75/76) models then the point seems moot. The same thing could be accomplished with a simple tee fitting. Just connect the valve cover breather hose to the overflow connection at the back of the crankcase with a tee fitting connected to the airbox to vent crankcase pressure. That scenario does not seem right to me and I find it hard to believe that the oil vapor/moisture separator would drain back to the crankcase.

Just my two cents, since I no longer have a 75/76 F model for reference.

[rieshunter]

It depends on the oil tank. I believe early K tanks (before 77) had a vent/overflow near the top. Later K and F (77/78) had a vent/overflow connection at the bottom with a stand pipe in the oil tank. This vent/overflow on the bottom of 77/78 F and K oil tank connects to the back of the crankcase behind the transmission. I do not know which oil tank was used on the 75/76 F models. It is my understanding that the "breather" mounted near the battery provides a means to separate water vapor from oil vapor. The water would settle to the bottom. The drain connection on the bottom of the breather on the 77/78 F and K models is routed around the front of the swingarm and then to the rear, terminating at the stock muffler bracket. This drain tube on the later models has a plug in it and should be opened to drain at every oil change per the service manual, at least on the later models. If this collected water vapor simply drains back to the engine on the earlier (75/76) models then the point seems moot. The same thing could be accomplished with a simple tee fitting. Just connect the valve cover breather hose to the overflow connection at the back of the crankcase with a tee fitting connected to the airbox to vent crankcase pressure. That scenario does not seem right to me and I find it hard to believe that the oil vapor/moisture separator would drain back to the crankcase.

Just my two cents, since I no longer have a 75/76 F model for reference.

Good points. Breather assembly delete it is!

I will connect the valve cover breather hose to the rear crankcase as you described.
I'll let the oil overflow just overflow onto the chain.

[madmtnmotors]

What do you mean "oil overflow"? Are you planning to use a tee and route that hose toward the chain? I still think you're going to have an oily mess, I suggest you try to connect it the way it was designed.

Can anybody clarify/verify the plumbing for this on the 75/76 F model?

[rieshunter]

What do you mean "oil overflow"? Are you planning to use a tee and route that hose toward the chain? I still think you're going to have an oily mess, I suggest you try to connect it the way it was designed.

Can anybody clarify/verify the plumbing for this on the 75/76 F model?

The oil pan has an oil overflow valve. This valve was connected to the breather assembly.
See HondaMan reply linked above.

[bryanj]

DO NOT connect the top breather to the cranckcase UNLESS you do it at a tee piece and have the third leg open to atmosphere preferable at the end of a long hose. If you just connect them together the engine can not breath and could blow every seal and gasket

[rieshunter]

Thank you all for the help! I was really turned around with all of this routing, but I think I’m starting get this straightened out thanks to you all.

I plan on connecting the…

• top breather assembly outlet to the airbox
• the middle outlet to the valve cover
• the bottom outlet will be the drain (could be plugged and function as a catch can)

The oil overflow hose will connect to the crankcase.

If anyone with a 1975/1976 CB750F could send pictures of their breather assembly routing, that would be amazing. If anyone else can verify this, I’d greatly appreciate it!

[rieshunter]

I believe part of my confusion, and the main contributor to the leak, was that the oil tank overflow was connected to the breather assembly. The bottom “drain” was plugged and used as a catch can, while the middle outlet was connected to the valve cover (not good because top outlet was not airbox).

My working hypothesis: At higher RPMs, oil overflowed and filled the breather assembly. The gases from the valve cover were shuttled to the breather assembly with nowhere to go except up into the oil reservoir (it should have been shuttled to the airbox). Perhaps this created a high pressure environment in the breather assembly and top of oil reservoir that pushed oil out of any/every orifice in the breather assembly.

[madmtnmotors]

Thank you all for the help! I was really turned around with all of this routing, but I think I’m starting get this straightened out thanks to you all.

I plan on connecting the…

• top breather assembly outlet to the airbox
• the middle outlet to the valve cover
• the bottom outlet will be the drain (could be plugged and function as a catch can)

The oil overflow hose will connect to the crankcase.

If anyone with a 1975/1976 CB750F could send pictures of their breather assembly routing, that would be amazing. If anyone else can verify this, I’d greatly appreciate it!

I believe you are now correct based on my understanding and experience of how this breather canister functions on the later models. 

[Jerry Rxman Griffin aka MuthaF'er]

Send me your cell phone number for a picture.

Top is to the air cleaner
Middle to the valve cover
Bottom is the oil collector drain

[Jerry Rxman Griffin aka MuthaF'er]

The drain tube is a no longer available part number. I picked up a couple for the CB500 but it may now also be NLA FROM Honda. EBay etc perhaps? Close replacement as it fits the bottom fitting on the breather canister and the size decreases from there as does the stock one except the pinch end of it is different. It also was longer so I just cut it off. Closest replacement I had found. 17335-323-000 Tube assembly, breather. Google search the part number for images and more links.

The 12mm hose is/was available. 95005-1200120M. Order only ONE. It is 120cm not one Meter. It will cut into enough for all the pieces. Just make your cuts appropriately because there is NO leftover. 

From another discussion: http://forums.sohc4.net/index.php?topic=173442.0

[rieshunter]

The drain tube is a no longer available part number. I picked up a couple for the CB500 but it may now also be NLA FROM Honda. EBay etc perhaps? Close replacement as it fits the bottom fitting on the breather canister and the size decreases from there as does the stock one except the pinch end of it is different. It also was longer so I just cut it off. Closest replacement I had found. 17335-323-000 Tube assembly, breather. Google search the part number for images and more links.

The 12mm hose is/was available. 95005-1200120M. Order only ONE. It is 120cm not one Meter. It will cut into enough for all the pieces. Just make your cuts appropriately because there is NO leftover. 

From another discussion: http://forums.sohc4.net/index.php?topic=173442.0

Thank you!

[rieshunter]

Update!

I just took the bike for a nice little 4-mile ride to the gas station and back. I am happy to report that I didn't leave a massive oil trail and the bike has not dripped after the ride.

Thank you all for your help, seriously. Odd how properly routing the oil hose fixes oil leaks...Who'd a thunk?



Edit1: That's the old plate btw. The new one is in the mail and I have a copy of my MV1 application under the seat.
Edit2: I'm not committed to the pods, and I am wholly aware that my pod dreams will likely be crushed.

[madmtnmotors]

So where did you route the "Air Cleaner" connection with pods?

[rieshunter]

So where did you route the "Air Cleaner" connection with pods?

Straight to the air. No connection at the moment.