GS450S- Mikuni BS34- Prime issue- Petcock Photo

[Guest]

This is the petcock. I turned screw to vertial, let it sit for awhile then cranked her over for a few seconds, let it sit some more and then cranked and cranked but no joy. Disconnected the hose to the carbs, cranked it a few turns and I have no flow at all. Selector set on RUN and screw turned vertical. I imagine that even though the petcock was working when I took it off, it must have gotten fouled over the past few months. Time to pull the tank and petcock and see what could be causing this...

[Nessism]

The petcock should have a Prime position. Find Prime and fuel should flow with the engine off. This is the setting you use to fill the float bowls. Be sure to turn the petcock back to ON when the bowls are full. With the level in the ON position fuel will only flow when there is vacuum pulling on the diaphragm. To test you can put a piece of tubing on the vacuum nipple and suck on it to see if fuel starts to flow. If the diaphragm has failed in the petcock fuel can backflow down the vacuum line so be careful of what you are doing. A piece of clear tubing is best if you are planning to draw the vacuum with your mouth. Consider this your warning.

[tkent02]

Prime on the 450 is different, you just back that screw out a few turns and fuel will be flowing. Don't take it all the way out, just loosen it a few turns. When the carbs are full, tighten up the screw and it's back to normal.

[T3nrec]

Working on my 1980 GS450s. Just learned some more about my bike thanks to this thread! Thanks guys!

[clark_sas]

Prime on the 450 is different, you just back that screw out a few turns and fuel will be flowing. Don't take it all the way out, just loosen it a few turns. When the carbs are full, tighten up the screw and it's back to normal.

Yup, that is the setup. Unfortunately, once I removed petcock and disassembled I found that the bladder/diaphragm inside was torn and the disk under the bladder/diaphragm was broken into four pieces... ordered a new one and will update once it arrives and is installed.

[clark_sas]

Yes, tons of info here. This forum is a goldmine!

[Nessism]

Yup, that is the setup. Unfortunately, once I removed petcock and disassembled I found that the bladder/diaphragm inside was torn and the disk under the bladder/diaphragm was broken into four pieces... ordered a new one and will update once it arrives and is installed.

I hope you ordered a new petcock instead of a new diaphragm. The problem being that Suzuki doesn't sell petcock parts and most of the aftermarket "kit" parts are poorly made and don't fit right.

[clark_sas]

I hope you ordered a new petcock instead of a new diaphragm. The problem being that Suzuki doesn't sell petcock parts and most of the aftermarket "kit" parts are poorly made and don't fit right.

Yes, purchased whole new petcock with all the peripheral parts (vacuum hose, clips, hose protector, o-ring(s) etc. Didn't want to chance one of the old parts not fitting with the new petcock.

[clark_sas]

OK, so new fuel cock arrived and.... it is completely different than the one that came out and the bolt holes do not line up with the holes on the tank. This tank must be from another bike is the only thing I can think of? To boot, it appears that the orientation of the fuel outlet is basically on opposite sides: the petcock that came out has the fuel outlet leading directly back towards the tank with the vacuum port on the rear of the petcock pointing up and right in the 2 o'clock position. Everything I see on eBay has the the fuel outlet and the vacuum port pointing to the right. I ordered the part using the diagrams for the OEM parts and clearly that didn't work out either. I have plenty of photos but haven't figured out how to upload them w/o starting a brand new thread. HELP!! Not sure how to proceed, this is my first rebuild...