Knocking sound, i think i've found it. Now what ?

[errorcauser]

This has blew my mind for the last few days, as I was thinking my motor was toast. I have a knocking sound that sounds exactly like a rod knock in a car. I took a screwdriver, stuck one end to my ear, and the other end to every part of every place on my motor with no luck in locating the knocking sound.

Well just now, I went out and started it up to do some more hunting. Low and behold I'm sure I've found it, but I don't understand why. The knocking, it's coming from .....of all things....my gas tank. I leaned over and laid my ear against the tank, and there it is KNOCK KNOCK KNOCK KNOCK !!! What in the world would be causing that ? I unplugged the gas hand connector because I thought the thing was bouncing up and down in there. I also checked the voltage at my battery, because I thought maybe to much voltage was bouncing the gas hand up and down inside the tank, but that's not it. All my rubber mounts for the tank are good and inplace.

What the heck is this, and has anyone else noticed this with their bike ? Maybe if I fill it with gas it'll muffle the sound ?
I'm happy that it's not my motor making the sound ! But, now I'm confused as to why it's knocking from the gas tank, and would like to stop it if anyone knows what to do for it.

[Draketh]

Make sure your tank is fastened tight to the frame, in addition try adding some rubber grommets to the bolts are whatever hold your tank to the frame, it could just be some vibration or force coming up into the tank through the frame.

[errorcauser]

Make sure your tank is fastened tight to the frame, in addition try adding some rubber grommets to the bolts are whatever hold your tank to the frame, it could just be some vibration or force coming up into the tank through the frame.

I pulled the tank up about an inch above the frame while the bike was running, it still made that noise.

I've sat here and gave it some thought. It has to be the petcock. I figure every time the engine hits it with vacuum, it's working the diaphragm in and out causing the sound. But then again, I'm not sure. But I can't see anything else that it could be.

[smagnusen]

My money is on the petcock, probably a broken metal backing on the diaphram. On every intake it sucks the diaphram and knocks the parts against the interior wall. Take it apart and inspect it.

[errorcauser]

My money is on the petcock, probably a broken metal backing on the diaphram. On every intake it sucks the diaphram and knocks the parts against the interior wall. Take it apart and inspect it.

Yeah, it's got to be something in the petcock. I'll pull it apart after while and maybe I can stop the noise.
If not, I guess either a replacement petcock or a rebuild kits in my future. It's not leaking nor running gas though the vacuum hose though. Works like it should best I can tell.

[earlfor]

I pulled the tank up about an inch above the frame while the bike was running, it still made that noise.

I've sat here and gave it some thought. It has to be the petcock. I figure every time the engine hits it with vacuum, it's working the diaphragm in and out causing the sound. But then again, I'm not sure. But I can't see anything else that it could be.

The only other thing in the tank that can move is the float level arm for the gas guage. As for it possibly being the petcock opening and closing with vacuum cycles, to rule that out, I would disconnect the vacuum line to the petcock and stick a pencil in the end of the vacuum line so the carb isnt drawing vacuum on an open line. Then switch the petcock to prime.
This will delete the vacuum function from the petcock and fuel will flow by gravity. Start the bike and see if the sound is gone.

Earl

[highmileage]

It's someone knocking on the gas cap from the inside. Let 'em out!

No seriously, is the noise there with the engine running and the gas cap off?

[errorcauser]

It's someone knocking on the gas cap from the inside. Let 'em out!

No seriously, is the noise there with the engine running and the gas cap off?

Yep done checked it like that too. I went out, started it up just now and stuck the screwdriver to the petcock and to my ear. Sounded like a Tommy Gun . So that's the problem.

All the air box sealing and new intakes has made this baby have all sort a vacuum ..lol

[errorcauser]

The only other thing in the tank that can move is the float level arm for the gas guage. As for it possibly being the petcock opening and closing with vacuum cycles, to rule that out, I would disconnect the vacuum line to the petcock and stick a pencil in the end of the vacuum line so the carb isnt drawing vacuum on an open line. Then switch the petcock to prime.
This will delete the vacuum function from the petcock and fuel will flow by gravity. Start the bike and see if the sound is gone.

Earl

It's the petcock. I can handle that kinda knock a lot better than i can a rod bearing knock.

[Suzuki mad]

Going to sound MAD here. Have you checked your horns??

If I put my tank on slightly wrong the horns rattle the tank.

Suzuki mad.

[Dogma]

Yeah, it's got to be something in the petcock. I'll pull it apart after while and maybe I can stop the noise.
If not, I guess either a replacement petcock or a rebuild kits in my future. It's not leaking nor running gas though the vacuum hose though. Works like it should best I can tell.

You ride the exact same thing as me. I had to replace the petcock, though mine didn't fail in the same way. Mine wouldn't hold vacuum long enough to keep running at high speed. The rebuild kits notoriously fail to fix things like that, so the usual recommendation around here is to replace the whole unit. $42.25 shippped from Z1, and you never have to worry about it.