Topic: Did a 78 Cb750K have a vacuum petcock stock?

[78whiteorbs]

Got a 78cb750 with a 75ish tank . The bike came with no tank and I just sourced this one which is not a vacuum type. The carbs which are the stock 78 PD carbs with the accelerator pump look to have a a small line for a vacuum petcock. If so I can just cap this off . Can anyone confirm this who has a stocker?

[flybox1]

Picture?

[redpine]

My K7 750 tank only has two lines running from it; the petcock (single line to the #1 carb) and an overflow line that's on the bottom middle. The K7 and K8 bikes are essentially the same.  I may be misunderstanding your question though. Got a pic of the carb port you're referring to? The carbs have fuel lines that connect them together as well; like a daisy chain #1 to #2 to #3 to #4.

[Bankerdanny]

I don't believe that any of the SOHC4's came with a vacuum petcock. That may be a vent, not a vacuum port.

[Shane72]

Don't remember ever encountering a vacuum petcock on any SOHC 750 I've either owned or worked on, both with and without accelerator pumps.

Have had plenty on other makes though.

All the best,

Shane

[750K]

77-78k's don't have a vacuum petcock, theres one fuel line from the petcock to the carb. You should have two vent nipples on the carb as well as the fuel line nipple, or possible like some of the 77 pd's the vent is part of the overflow through the float bowls

[78whiteorbs]

I will have to post a picture. There was one main fuel line form the carbs and one other smaller diameter line that appears to have what looks like a bb in it with a hose clamp wire on the outside pinching it . I try and get one tomorrow. thanks for the input fellas

[bollingball]

Got a 78cb750 with a 75ish tank . The bike came with no tank and I just sourced this one which is not a vacuum type. The carbs which are the stock 78 PD carbs with the accelerator pump look to have a a small line for a vacuum petcock. If so I can just cap this off . Can anyone confirm this who has a stocker?

There is only one fuel line from the tank to the carbs. On the #2 carb you will see two barbs the larger one is for fuel the smaller one is vent do not plug this one put a 8" long hose on it and strap it to the back bone. If you were to plug it it would be the same as a plugged gas cap. When you say you got a new tank was it a 77-78 tank if it is a 75 I don't know how the pet cock works
Ken

77-78k's don't have a vacuum petcock, theres one fuel line from the petcock to the carb. You should have two vent nipples on the carb as well as the fuel line nipple, or possible like some of the 77 pd's the vent is part of the overflow through the float bowls

No only one vent nipple. The screw on the bottom of the bowls does two things 1 it is a drain and 2 it is part of the over flow path but has nothing to do with the vent. When the screw is seated it allows the over flow to still work and if you open the screw it allows the bowl to drain. This system works well for the clear tube method to check the fuel level in the bowl that should be 2-3mm below the bowl gasket.
Ken   Tell me if I am wrong

[flybox1]

Got a 78cb750 with a 75ish tank . The bike came with no tank and I just sourced this one which is not a vacuum type. The carbs which are the stock 78 PD carbs with the accelerator pump look to have a a small line for a vacuum petcock. If so I can just cap this off . Can anyone confirm this who has a stocker?

There is only one fuel line from the tank to the carbs. On the #2 carb you will see two barbs the larger one is for fuel the smaller one is vent do not plug this one put a 8" long hose on it and strap it to the back bone. If you were to plug it it would be the same as a plugged gas cap. When you say you got a new tank was it a 77-78 tank if it is a 75 I don't know how the pet cock works
Ken

77-78k's don't have a vacuum petcock, theres one fuel line from the petcock to the carb. You should have two vent nipples on the carb as well as the fuel line nipple, or possible like some of the 77 pd's the vent is part of the overflow through the float bowls

No only one vent nipple. The screw on the bottom of the bowls does two things 1 it is a drain and 2 it is part of the over flow path but has nothing to do with the vent. When the screw is seated it allows the over flow to still work and if you open the screw it allows the bowl to drain. This system works well for the clear tube method to check the fuel level in the bowl that should be 2-3mm below the bowl gasket.
Ken   Tell me if I am wrong

Nope...you got it   

[madmtnmotors]

Got a 78cb750 with a 75ish tank . The bike came with no tank and I just sourced this one which is not a vacuum type. The carbs which are the stock 78 PD carbs with the accelerator pump look to have a a small line for a vacuum petcock. If so I can just cap this off . Can anyone confirm this who has a stocker?

There is only one fuel line from the tank to the carbs. On the #2 carb you will see two barbs the larger one is for fuel the smaller one is vent do not plug this one put a 8" long hose on it and strap it to the back bone. If you were to plug it it would be the same as a plugged gas cap. When you say you got a new tank was it a 77-78 tank if it is a 75 I don't know how the pet cock works
Ken

77-78k's don't have a vacuum petcock, theres one fuel line from the petcock to the carb. You should have two vent nipples on the carb as well as the fuel line nipple, or possible like some of the 77 pd's the vent is part of the overflow through the float bowls

No only one vent nipple. The screw on the bottom of the bowls does two things 1 it is a drain and 2 it is part of the over flow path but has nothing to do with the vent. When the screw is seated it allows the over flow to still work and if you open the screw it allows the bowl to drain. This system works well for the clear tube method to check the fuel level in the bowl that should be 2-3mm below the bowl gasket.
Ken   Tell me if I am wrong

Nope...you got it   

I concur. One fuel line to carb from petcock. Tha other (smaller) connection at the carb (between 1 and 2 I believe) is your bowl vent that must be open to atmoshere *DO NOT PLUG OR CAP*. My 750 K8 has this vent tube routed up to the top of the air cleaner where there is a little bracket on top of the air cleaner to hold this tube in place. Could be routed just about anywhere, even zip tied to the underside of the frame tube as suggested.

[lucky]

No. No vacuum baloney.

One line into the carbs for fuel supply.
One line next to it is the float bowl vent tube.

[750K]

Got a 78cb750 with a 75ish tank . The bike came with no tank and I just sourced this one which is not a vacuum type. The carbs which are the stock 78 PD carbs with the accelerator pump look to have a a small line for a vacuum petcock. If so I can just cap this off . Can anyone confirm this who has a stocker?

There is only one fuel line from the tank to the carbs. On the #2 carb you will see two barbs the larger one is for fuel the smaller one is vent do not plug this one put a 8" long hose on it and strap it to the back bone. If you were to plug it it would be the same as a plugged gas cap. When you say you got a new tank was it a 77-78 tank if it is a 75 I don't know how the pet cock works
Ken

77-78k's don't have a vacuum petcock, theres one fuel line from the petcock to the carb. You should have two vent nipples on the carb as well as the fuel line nipple, or possible like some of the 77 pd's the vent is part of the overflow through the float bowls

No only one vent nipple. The screw on the bottom of the bowls does two things 1 it is a drain and 2 it is part of the over flow path but has nothing to do with the vent. When the screw is seated it allows the over flow to still work and if you open the screw it allows the bowl to drain. This system works well for the clear tube method to check the fuel level in the bowl that should be 2-3mm below the bowl gasket.
Ken   Tell me if I am wrong

Well Ken here's the funny part, I had a discussion with lucky about this last year. He insisted my 77k's Pd41a's had vent nipples, it turns out some of the 41's don't. The atmospheric vent on mine happens through the float bowl overflow tubes. I took pics of the 77 41a's and my spare set of 78k 42a or B's which have the external vent nipples. So yes you are right but some of the PD's vent differently, all my 77 carbs have is a fuel inlet and nothing else.

Here's the thread it was all from
http://forums.sohc4.net/index.php?topic=107640.0

Found the pics I posted for lucky to see
Here you go Lucky, I had my tank off tonight and my PD41A's do not have vent tube nipples. Where the set of PD's off a 78 motor I have do have them.




Here's the 78 carbs

[bollingball]

At 750K very interesting
 I have two racks both 78 750K&F 42a &42b both are the same other than jets. I know the 77 had needle adjustment slots but have never read about different vent systems. HondaMan is good about changes made during mid year maybe he can chime in. Also maybe http://forums.sohc4.net/index.php?topic=105640.0;all knows about this. Thanks for the heads up. Would like to hear from others.
Ken

[bollingball]

As far as to the OP title. No.
Ken

[750K]

Yeah Ken I found it interesting, I thought I was blind or something when I was looking for the vent openings haha. The pd41's do have the adjustable needles like you mentioned.

[78whiteorbs]

Wow, good info- mine are 78 carbs and I do believe that it is a vent tube as stated above now. Somebody had it plugged as you can see in to photo, must have thought it was a vacuum line for a petcock as I almost did- here are the pix.

[78whiteorbs]

DSCN8127

[78whiteorbs]

and one for the road-

DSCN8128