1981 cb750 carbs and fuel lines

[Tooler]

I recently purchased a 1981 cb750 carb off eBay. I have a 1980 cb750c. Now the new carb has an external accelerator pump on top of the carb which my old one did not. The fuel line for it is 5/16 and my fuel petcock is 1/4. I'm thinking that the carb perhaps came off a cb750f because of the larger fuel line. I've seen fuel petcocks for sale on a website and they have one for 1980 cb750c and a different one for a 1981 cb750f and cb750k, however it doesn't specify fuel line size. I'd like to just purchase a new petcock if its a different size. Can anybody shed some light on this by chance? Thanks!

[Hoosier Daddy]

This should help... Petcocks 79-83 DOHC.html
There were a lot of bung size changes on the tanks in a very short time span on our DOHC's so chances are slim that a petcock from another years will thread right onto your tank. What I found most interesting in your post was the difference in carb racks from your '80 C model to your new '81. I'm not aware of any changes between the two years on the carbs, regaurdless of C, K, or F.
Can you post a pic? Here is a pic of my '81 C's carb rack from my build thread, this has the linkage circled but the accelerator pump is right next to it on the underside.

[Tooler]

Perhaps its not an accelerator pump...thats just my assumption. However, I have seen them on other carbs for sale on ebay. I'm guessing its some sort of after market deal. Anyways I'm at work now but ill post a pic when I get home....

[Lucky 1]

It is probably not any aftermarket deal.

It could be carbs from a CB750A which had some kind of vacuum device on the carbs?

The accelerator pump is on the bottom of the carbs and is part of the float bowl.
Only the #2 carb had the accelerator pump.


Why don't you just post a photo that has the carb right side up?
BTW...Many Ebay ads show carbs in the UPSIDE down position. Ads posted by auction companies.

[Hoosier Daddy]

Oh.. Oh .. I got it... Perhaps it's the "Spawn of Satan" (SOS) vacuum controled fuel valve. Those are often removed after they fail to flow properly....#2 in the attached microfische.

 

[Tooler]

Yes that's exactly what it is! So what is the function of it and if it sucks can I just take it off without having to do any mods?

[seestheday]

It's an "auto" petcock.  Basically a vacuum system that only allows fuel to flow into the carbs when the engine is running.

If it is working well, then it's convenient.  You might just have to crank the engine a few extra times before it'll start to get fuel into the carb bowls and into the engine.  It allows you to never have to manually turn the petcock off.

If it's failing it can be a huge headache.  

No harm in taking if off and running the fuel line straight to the carbs.

A lot of people just remove it and manually turn the fuel off when they stop their bikes (or make sure their carbs are always squeaky clean and won't get stuck float valves so they won't leak).

[Hoosier Daddy]

Yep, remove it, cap off the vacuum port (where hose clamp #9 is on the fische), and run the fuel line straight from the tank to your carbs.
It's nicknamed the "Spawn of Satan" for a reason.

[Tooler]

Cool! Thanks for the help guys!