Idle Issues/Major Backfiring

Clark and others, please help. I have posted this on the YZ board and gotten some insight, but cannot remedy the problem. :)

I recently bought a 98 yz4 and have ridden it about 10 times. Love the bike, but every time I ride it, after about 1 hour in the saddle, the bike refuses to idle and backfires like mad. Starts fine w/o choking, but won't idle. The idle screw is all the way in. Any ideas of how I can increase it? I am just above sea-level and the pipe is a FMF Power Core 4 (stock head pipe.) Several other knowledgeable YZ riders say it sounds like there is an air leak. The hot start button is definitely not out, and the hot start hose is hooked up.

I have tried altering the air screw anywhere from 1 to 3 turns out. I don't know anything about carbs, so I couldn't tell you the jetting setup. The bike starts fine cold & hot, runs great - holds consistant speeds well, and accelerates really smoothly (makes me not want to dick with the carb.) The bike idles fine when cold, but stalls when hot. If I open the idles adjust screw at all - forget it - no idle ever. :D

Thanks guys.

Go to the technical section and under the maintenace section read the Throttle shaft adjustment procedure and complete it.

Clean all dirt from the outside of the carb. When you have the top off clean all dirt around the slide area with contact or carb cleaner spray use lots to make sure its all clean.

While you have the carb off the bike clean the accelerator pump area above the diaphram (be careful not to loose any of the O rings and make sure you put the diaphram back on correctly the sealing ring goes up and the longer metal contact button in the middle goes down.

Record all jets in the carb:

Start jet stock is 65

Pilot jet stock is 45

Main jet stock is 175

Stock needle is a DVR with the clip in the number 4 spot from the top. The letters are on the side of the needle.

make sure the floats are free and that the needle and seat are clean.

Carefully reassemble everything put in a NEW plug make sure the plug cap is seated all the way down on the plug and try it out.

Then give the forum feedback along with the current jetting setup of the carb. I and others will try to help you out.


I'm not really sure but it may be the tps if that model came with one the factory manual tells how to fix it

Alright Clark, I finally got the time/courage to yank the carb off the bike. I followed your instructions, and here are a couple of observations....

1) Jetting is close to stock

Start jet 65

Pilot jet 45

Main jet 180

Needle is a DVR #4 position

2) The top lid of the carb has a hairline crack in it. I don't know if I did it, or it was there before, but I am going to replace it.

3) The accelerator pump was a mess. The metal contact button on the diaphram as well as the rod that pushes it are very corroded and rusty.

4) When I turn the throttle dial, the spring does not push the rod into the accelerator pump the way I would other words, it comes off of it's stop and doesn't really seem to push the diaphram in very much...could be because it is so corroded.

Can you recommend any jetting changes so that I can pick them up when I get the new lid? Should I be worried about the corrosion in the accelerator pump and rod? Any ideas about #4 above?

Again, I am running below 500ft and have a FMF PC IV - stock pipe.

Thanks so much, Clark or anyone else who can help me get my bike running correctly.


Power core IV, sea level to 3500' Jetting for starters.


Fuel Screw 1.5 turns out

Needle DVR #5

Main Jet 180

Make sure you clean up the mess in the accelerator pump area. You may want to get a new diaphram. Make sure you reassemble the pump correctly per my previous post.

Then set the pump rod so it floats in the activation hole. This is done by adjusting the little fork shaped stop on the back of the black plastic actuating arm that the pump pushrod end is in. Spread the fork stop with the blade of a screw driver to raise the top of the hole on the end of the pushrod and squeeze the fork together to lower the top of the hole on the end of the push rod. YOU WANT THE END OF THE PUMP ACTIVATING PUSH ROD TO FLOAT IN THE CENTER OF THE PLASTIC HOLE WITH THE THROTTLE CLOSED.

After you get the bike running and adjust the idle and are happy with it recheck the postion of the end of the pump activating rod and make sure it is still floating in the hole with the throttle closed. Re adjust if required.

The pump actuating arm only has a stroke of about 0.100" to 0.125". It does not move much at all.

Keep us posted on your progress and the forum members will help

Make sure you fix that carb top and have a good seal and gasket for the top of the carb. No air leaks or you will have problems and a crack or bad seal on the carb top is aprime air leak.

Also make sure the hot start button dust seal is properly seated on its boss, and that the hose from the Hot start button to the intake manifold on the engine head is in good shap and does not have ANY CRACKS.


[This message has been edited by Clark Mason (edited 05-17-2000).]

Thanks Clark, I really appreciate your advice. I went to the stealer today to get the 48PJ (not in stock) and a new carb top. They told me I had to buy a whole new carb. :) The lid isn't sold separately. I am going to try some JB Weld to seal the crack. Will let you know how it goes. Thanks again.

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