Topic: Need Help with CB650 1981 Petcock and Hose Configuration

[Backisen]

Hi everyone,

I recently purchased a 1981 CB650. The previous owner removed the original vacuum petcock and replaced it with a standard one. I'm trying to ensure everything is correctly connected and could use some advice.

According to a diagram I found (link below), hoses numbered 10 and 11 are not connected to anything and are just hanging loose. Should these hoses be plugged, or do they need to be connected somewhere? https://www.partzilla.com/catalog/honda/motorcycle/1981/cb650-a/carburetor-assy

Any guidance or recommendations would be greatly appreciated. Thank you!

[BenelliSEI]

Looking at the parts drawing those hoses are float bowl vents and both connect to “T” #20? It probably has another hose that just vents to atmosphere, laying over top of you air filter box. Do not plug them. I don’t have a cb650, so wait to hear what others have to say!

[fizzlebottom]

On the '81 CB650, there is no such thing as removing the vacuum petcock and replacing it with a normal one. There was already a normal petcock on the fuel tank, and a vacuum petcock attached to the carbs. Removing the vacuum petcock is just a bracket, plugging the #2 vacuum port, and routing a fuel hose directly from the fuel tank petcock to the joint between carbs 2&3. Nothing more.

NOW, what you're seeing in that diagram regarding hoses 10 & 11 are actually the hoses which lead back to the airbox through the T-joint #20. They do not touch fuel, only air. They are 100% unrelated to any petcock. They need to be connected to that T-joint, then the T-joint connected to the airbox.

[Backisen]

On the '81 CB650, there is no such thing as removing the vacuum petcock and replacing it with a normal one. There was already a normal petcock on the fuel tank, and a vacuum petcock attached to the carbs. Removing the vacuum petcock is just a bracket, plugging the #2 vacuum port, and routing a fuel hose directly from the fuel tank petcock to the joint between carbs 2&3. Nothing more.

NOW, what you're seeing in that diagram regarding hoses 10 & 11 are actually the hoses which lead back to the airbox through the T-joint #20. They do not touch fuel, only air. They are 100% unrelated to any petcock. They need to be connected to that T-joint, then the T-joint connected to the airbox.

Thank you for clarifying the setup of the petcocks and hoses on the '81 CB650. It was the previous owner who made the direct connection from the fuel tank petcock to the joint between carbs 2 and 3, without any filter.

Regarding the hoses numbered 10 and 11, I understand that they should connect back to the airbox via the T-joint #20. Could you please guide me on exactly where on the airbox these should be connected? The diagrams are a bit difficult to interpret.

Also, would you recommend installing a fuel filter in this setup? I'd appreciate your advice on this matter.

Thanks again for your assistance!

[fizzlebottom]

Let's refer to the air cleaner diagram here: https://www.partzilla.com/catalog/honda/motorcycle/1981/cb650c-a-650-custom/air-cleaner

The T-joint connects to the #8 hose. The other end of that hose connects to the barbed nipple in the lower right of the air cleaner housing (#5). Similarly, the hose #31 connects to another barbed nipple on the bottom of the air cleaner chamber (#11). All of it can be a little confusing and difficult to get together if they're straight up missing. The whole purpose of that system is to collect dirty blow-by vent air from the crankcase, allow the oil to condense and collect in the bottom storage tank (#17) and recycle the air. EPA stuff.

Regarding a fuel filter, the petcock actually should have one inside of the tank already. See #7 here: https://www.partzilla.com/catalog/honda/motorcycle/1981/cb650c-a-650-custom/fuel-tank

If you pull the petcock and find there isn't a filter, you can either spend the $ to replace it or you can install an inline filter by just splicing the hose between the tank and the carbs. I used to do that, then went back to the original configuration and haven't had any problems.

[Backisen]

Let's refer to the air cleaner diagram here: https://www.partzilla.com/catalog/honda/motorcycle/1981/cb650c-a-650-custom/air-cleaner

The T-joint connects to the #8 hose. The other end of that hose connects to the barbed nipple in the lower right of the air cleaner housing (#5). Similarly, the hose #31 connects to another barbed nipple on the bottom of the air cleaner chamber (#11). All of it can be a little confusing and difficult to get together if they're straight up missing. The whole purpose of that system is to collect dirty blow-by vent air from the crankcase, allow the oil to condense and collect in the bottom storage tank (#17) and recycle the air. EPA stuff.

Regarding a fuel filter, the petcock actually should have one inside of the tank already. See #7 here: https://www.partzilla.com/catalog/honda/motorcycle/1981/cb650c-a-650-custom/fuel-tank

If you pull the petcock and find there isn't a filter, you can either spend the $ to replace it or you can install an inline filter by just splicing the hose between the tank and the carbs. I used to do that, then went back to the original configuration and haven't had any problems.

Thank you so much for the detailed explanation and the links to the diagrams. They've been very helpful in understanding the setup.

I will check the petcock to see if there is an internal filter as suggested. Regarding the air cleaner, the setup on my motorcycle doesn't quite match the diagram on Partzilla. It looks more like the manual from this link: https://cosky0.tripod.com/imagelib/sitebuilder/misc/show_image.html?linkedwidth=actual&linkpath=https://cosky0.tripod.com/sitebuildercontent/sitebuilderpictures/page03-02.jpg&target=tlx_new. There are three connections to the air cleaner: one is connected to the cylinder head cover (#12 here: https://www.partzilla.com/catalog/honda/motorcycle/1981/cb650c-a-650-custom/cylinder-head-cover), another hose is plugged, and the third is open. I have attached images of the motorcycle where these connections are visible.

Thanks again for all your help!

Connection 2-3: https://imgur.com/YezsU2p
Connection 1: https://imgur.com/PqGDScb
Air filter: https://imgur.com/XUCqh8s

[fizzlebottom]

I definitely messed up a bit in all of my explanation by completely mixing up the valve cover connection. Let me clarify my mistake:

• Valve cover to airbox is the one that leads to recycling air and separating oil so it condenses and collects down in the little cube tank below the airbox.
• The carb T-joint to airbox, so far as I understand, is meant to provide atmospheric pressure to the carb bowls.
• The last hose leads down to the collector tank which then has a hose to drain.

Do you know which of your hoses or connections is plugged up? It seems for the most part that you've got the things you need.

[Backisen]

I definitely messed up a bit in all of my explanation by completely mixing up the valve cover connection. Let me clarify my mistake:

• Valve cover to airbox is the one that leads to recycling air and separating oil so it condenses and collects down in the little cube tank below the airbox.
• The carb T-joint to airbox, so far as I understand, is meant to provide atmospheric pressure to the carb bowls.
• The last hose leads down to the collector tank which then has a hose to drain.

Do you know which of your hoses or connections is plugged up? It seems for the most part that you've got the things you need.

Thank you for clarifying further, and no worries about the mix-up. Based on your latest description, here's what I see on my motorcycle:

- The blue hose in the pictures I previously uploaded is the one that's plugged.
- The red hose is currently neither plugged nor connected to anything.

Thanks again for all your help!