On Prime you have No reserve.
Why do you want to do that?

So I installed an aftermarket exhaust and took my bike to up the jets. Mechanic went up to a 155 on main jet and said the pilot was good at stock but bike seemed to want to die out so his suggestion was that the peacock wasn't allowing enough gas to get through so to try and run it on PRI and when I did it ran great and very little backfire like it had been before

If you have the stock petcock, it sounds like you have a very common problem for this bike - the stock petcock is known to fail. You can replace it with a genuine Yamaha petcock for the Raptor 660. It's manually operated but is reliable.

Running on prime won't hurt the bike. But if your carb float sticks then there is no way to turn the fuel off. It'll fill up your carb with fuel, run into your air-box, and possibly your engine.

The Raptor petcock is inexpensive and takes about 15 minutes to install. Just 2 screws.

One last question. Are you running an inline fuel filter? If so, it's not necessary and some people have problems with them.

What makes it weird is the bike is a 2017 with only 900 miles so when the mechanic said that same thing about the petcock not working properly it tripped me out. So yea I am running everything stock except for the Jardine Slip on that I put on

Mine's a 2016 and I had to change the petcock at about 2k.

Sosacj21 wrote on 08/15/17 at 04:32:13:


Poor fuel mileage, sooty exhaust, carbon build up on the spark plug.

If you apply full throttle in high gear and get the bike accelerating and pulling hard in the upper rpm's....slowly roll off the throttle and pay attention to what the engine is doing as you just begin to let off full throttle....if the bike runs smoother and feels like it runs better for a brief moment - that is an indication the main jet is too big.

Getting the right mix of fuel and air demands that the system be balanced.
Suddenly allowing exhaust to be virtually unrestricted creates a lean condition.

If it runs better on prime you have something going on petcock related.

If the vacuum can't open it up to flow enough to run it, that's screwy.

The petcock is a known issue and it's a variable. For about thirty bucks you can streamline the problem solving.


There are smart carburetor guys here, who can tell you what clues to look for, throttle settings, flat spots or power surges, etc.

If I thought it was lean I'd get rolling and cruising at steady speed, and click the choke a click.


Without a doubt you guys know what you're doing that's why I'm on here. I knew it was running super lean so I took it in and had a mechanic try two deifferent size jets and he felt the #155 main was a lot better than the #152.5 main. As for the pilot he said it was good at #52.5 so when I took it for a run it did sputter on lower and still was backfiring and eventually shut off after downshifting and trying to pick back up from first. He then tried switching it to prime because he thought the petcock wasn't allowing enough flow and once he did that it runs good and backfiring is small