Hi. New the forum. New to Quadding itself, might I add. I must say, I am loving it. It has been some of the best days of the past 5 years!... And some of the more frustrating as well.

I purchased a 2000 Quadrunner 500 about 2 weeks ago. From the start, I experienced backfire from the quad.

First, there was water in the fuel. We addressed that, its good.

Then we proceeded to clean the Petcock Valve.

Then we cleaned the Carb.

Then we replaced the Carb Boot seals.

Then we checked the air filter.

Then we replaced the plug!

Between all this, it was pretty much fixed!

I went for a 75km drive, and it was pretty damn good. There was twice when the backfire returned, but only for about 2 minutes each. It then went away. Then I realized I burned 20 dollars of fuel in 70km! That's when I noticed it was spewing black smoke and reeked of fuel. Also, the throttle response at higher altitudes was almost.. delayed. Then we realized, it's too rich.

So we turned the air mixture down to what is optimal for this vehicle. Highest idle without throttle delay.

And then we turned it on for a little longer than when we were testing the air/fuel screw... And it still has no throttle response, and if you push the throttle too much, the excess fuel ignites in the loudest motherfucking backfire I have heard in my life.

So now we are kind of out of options.

The last thought we have is that the coil could be gone, and it just doesn't have enough spark to light the more fuel rich mixture which it receives when the engine is colder.

Thoughts?

 

I'd check the valve clearance.