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1986 XR600r Dual Carb problems

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I am helping out a buddy to get his bike running. As the title states, it is an 1986 XR600R, Dual carb, new foam air cleaner with air box baffle removed, stock header into what looks like a White Bros. E series exhaust. Also just replaced stator, exciter and spark plug.


Bike starts and idles smooth when cold, and choke full on. When the choke is moved to the mid point, it starts to pop a little. With the choke off, it pops, backfires and dies. It will start again off choke, but quickly pops and dies. Bike has an extreme backfire (loud) with choke off and whacking the throttle open and letting off quickly.

Stock settings were 45 pilot, 2 turns on the air screw and both carbs 122 mains with needle in 3rd clip down from top. Thinking that the bike had a lean condition and after some searching, we went to the following:

Pilot 48 (largest available for that carb)

both mains 128

Same needle position

Screw from 1 to 2.5 turns

The bike still starts fine on choke, but goes right back to the 'lean' sounding pops and backfires when the choke is off. In other words, no real improvement and it still acts lean on the pilot.

The carb has been cleaned three times, with no gunk or varnish showing, and the slides work smooth.

Any ideas or help is appreciated.

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Thanks for the response.

Yes, the seal seems to be okay, while the engine is running I sprayed WD40 around all the boots and plate, with no discernable change in engine speed.

I am really at a loss on what to check next, any ideas?


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I took the carbs COMPLETELY apart and blasted every orifice, pathway and nook and crannie I could find with carb cleaner and compressed air. Assembled it, put it back in and it fired first kick. Took the choke off and it started popping and backfiring again!

I truly believe the carb is clean. What are some other ideas?

Could a possible exhaust leak cause a lean condition? Maybe at the headers somewhere?

He is looking for another set of carbs to try out that angle, but I think it could be something else.

Any other ideas?

Thanks again for all your help, we appreciate it.

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with the choke on you are running rich. You turn it off and it starts having issues, it is most likely the carb running lean.

I would get some carb cleaner and spray around the intake boots. Then around the butterflies in the carbs or any place they can leak. check the choke plunger make sure it is not torn or leaking there.

How about the valves? are they in spec? if they are tight or if you have a chipped intake valve, it will give you the same type of symptoms.

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