Popping and Backfiring

I've been running YZ timing with the following setup all summer long with no problems until now:

Current setup: Main 172, YZ needle at 4th clip, 48 pilot and fuel screw 1 3/4 out. This setup was run from 6000 to 13,000 feet.

However, I recently began to experience popping on deceleration. So I set the fuel screw to 1 1/2 out. This seemed to help.

Then on my last two rides (6000 to 8500 feet), I have been experiencing backfiring on deceleration. Last night I changed my main from 172 to 170. Am I going in the right direction here?

Pulled my plug and it appeared to be tan (if not leaning a smidge towards white). It was black around the bottom of the rings though.

Any thoughts on this?

I have occasionally had decel popping on my YZF's, and it was always due to an exhaust leak at the head. Check to make sure your header is tight.

Scott F

The minor poping on decelleration is due to a slight lean condition. If the temperature drops makeing the air denser it will lean out the jetting. An exhaust leak at the header to head could cause it. A hot start button that is not fully closed (ie the rubber dust seal come off its seat not allowing the hot start button to close all the way). A lose clamp on the carb intake. A cracked hose form the Hot start button to the intake at the head (I asuming dougie still has a 99WR).

Dougie if you check all this stuff and everything is OK then start richening the mixture at idle to just off idle by opening the fuel screw 1/4 turn at at time and see if it stops. If this does not satisfy you then get the DVP needle (#4 starting clip) it is ever so slightly richer on the straight section and start with the fuel screw at 1.0 and repeat the opening of the fuel screw until it stops.

Or if it is truly minor in nature live with it and press on and enjoy riding. Remember as fall and winter approach and the air temperature drops jetting changes may be required to richen things up ever so slighly.

Dougie if you were running race gas or a combo of race gas and pump gas and then switched to straight pump gas this also will lean out the jetting and require jettting adjustments.


I had a similar problem when I first bought my 99 WR. The bike ended being back at the dealers for six of the first eight weeks that I owned it. Eventually they told me that the problem was caused by a blocked emulsion tube in the carb. I do not to this day know what that is, however the problem was gone when I finally got the bike back. Maybe Clark or someone else with more mechanical knowledge that me would be able to shed some light on this. I originally thought it was a jetting problem as I had the bike re-jetted when I bought with the YZ exhaust fitted. The shop was not able to fix the problem with jetting and eventually gave me this emulsion tube answer. By this stage I was just happy to be getting the bike back due to my frustration in buying a brand new bike that I had barely been able to ride.

Popping on throttle off is a lean condition.

Either by an airleak or most prob a poilet jet thats too small or fuel screw too lean.



**Ride it like you Stole it!**

Matt Porritt

99 YZ400F

Vist the Rubber Chicken Racing Online Shop

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