Just a preface: I don't know very much about working on bikes, so please don't assume I know anything. Basically, talk to me like I'm an alien that doesn't understand Earth transportation mechanics.  

My Georgie is a 2007 S40 that's got 8,323 miles on it (660 of which my  husband and I put on since mid-March). It seems like the bike had only one previous owner and I really don't know what he did to the bike, other than superficial stuff (wrapped exhaust, flipped the steering stem upside down). I honestly can't tell if it's got the stock petcock or not. The bike is mostly reliable, especially after I refilled the battery with distilled water (I hope that was the right thing to do!).

So, for the last work week, my husband has been riding my bike due to a flat on his bike (he doesn't have a car). He's noticed some funny things about Georgie. His belt seems squeaky, for one, but I've seen some threads on this and I think it just need to be realigned. Hubs seems really concerned about the popping/backfiring on acceleration, as well as the sputtering/loss of power over 60 mph. I know the backfiring on decel is due to the lean mixture of the carb, but what about on acceleration? I've never messed with the carb and I don't know if the previous owner ever did maintenance on it.

A failed petcock can cause all sorts of performance problems, especially the bike not being able to run smoothly through the full rpm range. It's always a good place to start when there are performance issues.

I wanted to add that the usual decel backfiring has basically stopped and now it's backfiring on acceleration alone.


Gary_in_NJ wrote on 04/26/19 at 16:30:34:


So what are the most common symptoms of a faulty vacuum petcock? I don't know that I have the skills or expertise to test mine or change it out.

Justin is right.

Get a genuine Yamaha petcock to replace the stock petcock.

When you get around to changing it be sure that the tank is almost empty. You don't want fuel running all over everything. It is only two screws, no problem really.

We can talk you through it. Very easy!

Have a gas can handy
Pull the fuel line from the carb
Put the fuel line in the gas can
Put Petcock on prime
Drain the tank
10 mm socket
Remove old Petcock
You can just snip the vacuum line up by the carburetor
We'll get to that later
Inspect the bottom of the tank and look at how the Petcock seals.
If it's not smooth where the seal goes. Fix that..

Got mine locally for$28.00.

Putting it on prime just allows fuel to flow freely, without the vacuum diaphragm being involved, which is how the Raptor Petcock works.

If fuel freely flowing to the carburetor is a problem
The carburetor needs attention.

I didn't mean to suggest a Raptor isn't needed.
I totally forgot about the vacuum line getting gas.

If your Petcock is that screwed up, you better check out the oil.
Might have gas in it.