1977 GS750 Carburetors & petcock

[janderson208]

New to the forum, and first time GS owner. I have owned several Honda CB's but this is my first GS.

I have a couple things I could use some REAL GS knowledge on.

1. Fuel cock, PRI? Should the bike be run on PRI or no? How does it work?

2. Currently the carbs are set up as follows. Number 4 has vaccum hose going to fuel cock. Carbs 2 and 3 are connected with a vaccum line.
(left to right 1,2,3,4 as sitting on the bike) Is this correct? The manual I have is very unclear about this.

3. When I disassembled the carbs I removed the screws in the bottom closest to the intake manifold, the manual simply says " Do not adjust these" lol, what is a good starting point for these screws.

4. Are the above mentioned screws the "air screws" or are the air screws the ones on the airbox side pointing to 10 o clock.

I picked this GS up from a friend of a friend. The rims, spokes, forks and shocks look like the day they were made. The inside of the HUGE fuel tank is spotless. the clock shows about 6,000. It came with a Dyna ignition installed. I really like the bike and would like to get it running as good as possible.

Any help or suggestions will be appreciated.

thank you

[hjfisk]

welcome to the forum.

[chuck hahn]

The petcock is vaccuum operated that means that it will shut itself off if the bike isnt running. The PRIME position bypasses this need for vaccuum in such situations that cause the carb bowls to be dry..maintenace, rebuilding, ran totally out of gas even on resereve,,whatever. If the bowls are fully dry you put it on prime for maybe a minute to prime them. Once they are primed and the bike starts you put the lever to on and leave it alone unless you need the reserve. You can also disconnect the fuel line and run a piece of tubing from the tank to a can if you need to drain the tank for any reason by using the prime setting as well. The little nipple on the petcock is the vaccuum port. That hooks to the nipple thats between the number 2 and 3 carbs.. The other two higher nipples that are on carbs 2 and 4 are simply vents to stop vapor locking.

[chuck hahn]

The bottom screw at the back of the bowls is the PIOLT SCREW.. This is set at 7/8 of a turn out from the bottom..seat all adjustment screws with care till they seat but you arent forcingbthem any further. The screws on the sides of the carbs just ahead of the slide towers are the AIR SCREW. These are set at 1 1/2 turns out. If the bike is stock the mainjet is supposed to be a 100 and the pilot jet is supposed to be a 15. the needle jet is a 5F21. The needle jet clip is on the center groove.. all that makes sense???

[chuck hahn]

I editted the firt poat..the vaccuum port at the carbs that hooks to the petcock is between 2 and 3..not 3 and 4..the bottom nipples on each bowl are overflows. None ofthe vents or overflows get tied together as you said they were tied together now...

[BassCliff]

Greetings and Salutations!!

Hi Mr. janderson208,

See Where Do These Hoses Go? Your petcock gets its vacuum from the #3 carb. The tubes between 1/2 and 3/4 are for vent hoses. The fuel inlet is right in the middle of #2 and #3. You'll see a picture of CV carbs on my website. The hoses will be similar except for the placement of the vacuum feed on your VM carbs.


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[Detroit Diesel Man]

Welcome to another 77 750 owner!!

DDM

[janderson208]

Thank you and just to be clear

I now understand the petcock now. Thank you. By the way the bike does not have the original air box.

I am clear about the fuel feed line to the carbs as well as the bowl over flow lines.

Still uncertain on the small diameter lines running from the carbs at about the 10 o clock position.

I am certain the previous owner messed this up (since the wires for the Dyna were switched around as well).

Carb number 1 has no nipple at 10 o clock. Should there be a T installed in 2 and 3 of and then to the petcock?

[Nessism]

The petcock hose is on the #3 carb on VM carbs. Check this photo below and note where the small hose comes off. These are VM22's but the hose on your carbs is similar.

Oh, and you need to rejet your carbs since it doesn't have the airbox.

[janderson208]

Thanks for the photo

Makes pefect sense. Thank you for your help. I plan on rejetting and running high quality pods. The bike has a very very nice stock exhaust, but I also have a 4 into 1 with a vance and hines. On the honda inline 4's the 4 into 1 with aftermarket muffler sounds good (I think). Not sure what I'm gonna do on the exhaust side. You guys are great, thanks again.

[chuck hahn]

IF you swap the exhuast for the 4 into 1s, may i have first dibs on the stockers? My 78m 750 E has a 4 into 1 thats not in very good shape. I am in the process of putting all my bikes back to stock and i would really like a shot at the 750 stock system.