[VTXDobro]

SO! I was riding down the road the other day and decided to give it full throttle just for fun. About 20 seconds in, I lost all power as if I wasn't getting any gas (had full tank). I let up and the engine idled fine and worked normally till about 10 mph, then would cut out till I let off the gas again. I limped it about 2 miles down the road till it died altogether and wouldn't start at all. Got it towed back to my house where I discovered the little vacuum line feeding my petcock was totally disconnected and was in bad enough shape to replace. The new one's on there now, and it's back to idling okayish, and will rev up, but ONLY on full choke. Without choke, I'm lucky if it revs up to about 1200 RPMs before it gives that "I'm out of gas" noise and dies. I've hit the starter with the main fuel line disconnected and it spits plenty of gas, also there's what looks like a little window on the backside of the carb that looks like it's full of gas as well. For starters, I guess I have to ask, how does this vacuum system work? It doesn't seem like it'd be designed to feed grime into the carb if it comes unattached. I don't want to speculate at all, for fear I'll throw you guys off, so does anyone have any experience with an issue like this?

 

[SPARKY5151]

You might check the vacuum diaphragm in the petcock for cracking or pinholes. A how to test is included below and a how to disassemble the petcock is included below that.

Remove the vacuum line going to the petcock (the smaller line) take a piece of tubing the same size and plug it into the petcock port you removed the line from. Remove the fuel line from the bottom of the petcock and place a rag under the port. Now suck on the free end of that vacuum line and fuel should flow from the bottom port of the petcock into the rag, if not you have a leak (crack or pin hole) in the diaphragm. You should not have to suck very hard on the tube to get the fuel to flow and once the fuel starts to flow if you put your tongue over the end of the tube the fuel should continue to flow for some time on the trapped vacuum if not probably still a leak in the diaphragm. If you get a weak flow then you should go and see your urologist immediately!


Replace diaphragm assembly by:



Shutting petcock valve off.
Disconnect vacuum hose and fuel hose from petcock.
Loosen large nut that retains petcock valve to gas tank just enough that you can turn petcock valve .
Rotate petcock valve 180° so it is facing you and tighten nut by hand.
Remove 4 screws on rear of petcock valve and remove back of valve and spring (note orientation of vacuum port).
Remove intermediate assembly, containing diaphragms (note orientation of hole in assembly).
Replace new intermediate assembly, small diaphragm toward petcock valve and leak hole to bottom.
Replace spring positioned between large diaphragm and end assembly (vacuum port to your right)
Replace 4 screws and cross tighten evenly, just enough to prevent leaking (excess tightness will cut diaphragm).
Remove screws one at a time and apply blue LocTite on very tip of threads ( very little).
Return petcock valve back to original position and snug large nut to tank (valve has O-ring seal).
Replace vacuum hose on petcock valve.
Turn petcock valve to on position and bump starter to verify petcock valve is passing gasoline.
I USED A RAG OVER VALVE DISCHARGE TO PREVENT GASOLINE BLOWING OUT ONTO M/C
Reconnect fuel hose to petcock from carburetor.
Open fuel valve.
Get spouse to clean up worksite.
Crank engine and inspect very closely for any gasoline or vacuum leaks (correct if any).


To make loosening petcock nut easier you can remove the bolt holding the fuel tank down and place a small piece of 2X4 (on edge) between the tank and frame.

 

[TxVtxRx]

There is a very small filter screen in the inlet tube to the carburetor. Pull the fuel line from where it enters the carburetor and look. You should find the small filter in there and it might be part of the fuel starvation problem. It's easy and quick to do. Good luck