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xr 350 backfires on decell

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I have been restoring this bike.

New piston, head re-done.

Starts up great, but back fires when I let off.

I'm stuck on tuning the dual carbs.

I've played with the jets and needles, mainly going leaner.

Is there a sequence for setting these carbs up?

The primary jet is @ 100 and the secondary is 105.

The manual calls out for p/112 and s/115.

I need help on this one.

Any help will be appriecated.

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What year bike? Backfiring in the exhaust can be caused by either a lean or a rich mixture. Check your plug color to see where you are. Assuming your air jet is set correctly, this setting is different for different years.....If the manual says you need larger jets, that would be the first thing I would try. Set up the primary carb first. It is the one you use the most and the secondary comes on at 1/2 throttle or better depending on how you have the linkage set up.

Edited by bstig60
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The bike is a 1983 xr 350r

I have an '84 XL350R. Same engine! But different carbs. Assuming you are still using the stock PD01A carbs; Main jet sizes are #112 for the main carb (left) and #105 for the secondary carb. Pilot jets should be #45. The air screw setting should be 1 1/4 turns out from a soft seat. Needle jet clip positions are (p)2nd groove; (s)3rd groove. Floats settings are 18mm.

If you are running a #100/ main jet you are running way too lean unless you are running at really high altitudes and that is probably what is causing the popping on deceleration. These engines run hot with the factory settings; running leaner will make the bike run hotter and could cause you some problems running in off road conditions. My suggestion would be to go back to the factory main jet on the primary carb. You can leave the secondary at a #105 since it is the factory jet according to my manual. As long as you are running under 6000 or so feet, you can leave the jets at the factory settings. If anything, I would go one step richer to aid in engine cooling.

FYI: I run my XL350R, my XR200R and my XR650L at altitudes up to 8000+ feet and all have factory settings on the carbs without any problems.

Bill

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Thanks, I went back to the stock jets and needle position.

There is no backfire. Bike runs great......

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