XR 400 popping

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Sep 22, 2000
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#1
I have a 96 XR 400 with a mukini pumper carb and XR's only exhaust. When I let off the gas, while riding, the exhaust pops quite a bit. The plug looks fine so I don't think the bike is running lean. Is this just a fact of life for some 4-strokes. I've heard other 4-strokes make the same sound while off the gas. Is this something I should be concerned about? The bike runs strong other than the annoying popping noise.
 
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#2
Decreased backpressure from your XR's Only exhaust, and also the pumper carb (sometimes) will cause this decel pop.

It's usually nothing to worry about - in fact, it's normal with four strokes set up for really high rpm performance - like a dragster - they don't use fuel as efficiently at low engine rpms, and on decelleration.

As long as it's popping out the pipe, and not coughing back thru the carb, you're okay. Little pops are fine - one big pop is not so good, and can be caused by a loose timing chain or incorrect ignition timing.
 
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#3
Hey,
Funny you mentioned it. I used to ride an 87 XR200 (no pumper) with XR's Only pipe and it would pop like that on deceleration all the time. I never thought much of it, just thought it was the pipe. This summer, I bought a 96 XR400 (no pumper, yet) and also put on an XR's Only pipe. It does not make the pop at deceleration. What's the difference? It's a fact that my 200's valves need adjustment, and even though I tried to adjust my chain, it still rattles. My 400 was serviced right before I bought it, so the valves are adjusted good, and the chain doesn't sound loose. I believe DualSportr is correct when he said it could be cause by timing chain, ignition, and in my opionion, valves out of adjustment. Looks like you should check your timing chain and valves.

So what's you take on that Pumper carb, Biker? I've been strongly considering one for my bike when I get the extra money. Good luck.
 

Vic

***** freak.
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#4
Make sure you have no exhaust leaks and that the valves are adjusted properly. You might try adjusting the fuel screw for a richer mixture. Turn it counter clockwise in 1/8 turn increments and see how it runs. If you richen it enough to completely get rid of the popping, you might end up with an off-idle stumble. It's also possible that you need a larger pilot jet and/or richer needle position. You just have to experiment. Let the fun begin!:)
 
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#5
TT600 doing same

I have an 1985 TT600 that is doing the same thing. Does anyone know if there is a mixture screw on the dual carb for this bike. The backfiring goes away if I pull the choke knob just a little so I think if it has a mixture adjustment that would do the trick. If not I will go 1 richer on the pilot.

Jim
 
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#7
Snaggle,
I really like the way the bike runs. It's a totally different machine than how it was stock. I put the pipe, pumper, unifilter, and opened up the air box all at the same time. The whole package made a big difference. It has good throttle response to lift the front end over logs, no hesitation. I road a new DRZ, totally stock, and my XR had better throttle response off the bottom. This surprised me after all the reading about the DRZ, although it was nice and quiet. I do wish the XR had the slim feeling of the XR. I would spend the money again on the pumper carb. It's the best aftermarket item I've ever bout. I'll mess with the air screw this weekend and see if that makes a difference on the detonation.
 
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#8
DS-the TT600 has a dual 24mm carb, don't know if it is CV or not (don't think so). I think it just needs to be richened a bit because if I pull the choke just a tad the popping is just about gone. If there is a mixture screw on the carb it is not in an obvious spot. The bike runs well and I would prefer not to start changing jetting if I don't have to.
 
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#10
I think it just needs to be richened a bit because if I pull the choke just a tad the popping is just about gone.
If it's a cv style carb, the popping is due to the slide staying open, even after you let loose of the throttle - putting the choke on will increase the 'suction' and make the slide drop faster.

If it's a regular carb, with a butterfly style choke, then reducing air and increasing fuel sometimes don't achieve the same goal (increasing pilot jet size may not solve the problem). If the bike is completely stock - you might look for adjustment issues, or worn carb parts that could cause the popping problem.
 
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#11
Originally posted by SlowJack
BINGO! Valves in nearly all cases!
I just had my valves adjusted about 2 months ago. Thats about 5 or 6 rides since the adjustment. It runs the same as it did before the valves were adjusted. I'm not compaining about how it's running, just curious about the popping.