Not getting enough fuel from petcock when "on"? 78 XS 400

[400x]

Bike runs great when the petcock is on reserve but in the "on" position it will idle rough and if you try to rev it or ride it anywhere, it'll make it a couple minutes barely running and die. (fuel level in the tank is nowhere near low)

I'm wondering if the petcock is clogged in the "on" side or if the reserve has a separate filter element that could just be less clogged. Any ideas as to why this would happen? Its got a full tank so it might be a while before i take it apart.

 

[226dark]

might be clogged on the full side tube? You said it was FULL! I was tempted to say the tank is not full. Petcock is really great when working as it should. The 'on' side tube and then the prime tube alongside it (which works exact same as long as one is working with vacuum, both should be working).
I would simply take it out of the fuel tank and just clean it nice. Do it once and any future necessary removal and cleaning will be a cinch.
Might want to get used to that, and also removing vacuum line, fuel line and having the tank off the bike would be the best way of going about it.

 

[226dark]

And welcome to the forum

 

[US82XS]

I concur with Dark. Mine will only run on prime and I have yet to take it off and clean it because I'm too lazy to pump the fuel out and too busy to ride it dry as of now.

Once you clean your petcock, its advised by many to mount an inline filter between that and the carbs so that you can more easily monitor the quality of what's coming out of the tank.

And welcome to the forum.

 

[400x]

Coolness. I'll do that when I get the gas run down a bit.

...Unrelated question: Any advice on finding replacement intake boots for between the carb and motor? ...just got quoted $115 (after a $15 dollar discount) for a pair from the local yamaha dealer. Are they grossly marked up because they're dealer parts or are they just that hard to find?

 

[HoughMade]

Coolness. I'll do that when I get the gas run down a bit.

...Unrelated question: Any advice on finding replacement intake boots for between the carb and motor? ...just got quoted $115 (after a $15 dollar discount) for a pair from the local yamaha dealer. Are they grossly marked up because they're dealer parts or are they just that hard to find?

http://www.ebay.com/itm/YAMAHA-XS40...Parts_Accessories&hash=item43aab87254&vxp=mtr

 

[400x]

WOW. That price is much more "consumer friendly". I've never bought anything off of e-bay but I guess the time has come. Thanks much for the help!

 

[xschris]

Ebay is your friend when it comes to cheap parts!

 

[400x]

So... next question. There are two in the picture. Does that mean there will be two in the box when i order it?? i'm in a bit of a state of disbelief after hearing $130 per pair. haaa

 

[US82XS]

Two boots and gaskets, according to the post. And free shipping. Jump on it.

 

[OliverB]

So... next question. There are two in the picture. Does that mean there will be two in the box when i order it?? i'm in a bit of a state of disbelief after hearing $130 per pair. haaa

And they are of excellent quality. Installed sets on both '80 & '81 XS400s. Soon to order a set for my '79.

 

[HoughMade]

I have a set off eBay on my '79. They fit perfectly and work great. I paid $36 for both boots, the gaskets and bolts.

 

[400x]

Good to hear. Ordered a set last night.