Fuel Screw/ Popping Backfire


14 replies to this topic
  • Dwood11

Posted April 28, 2005 - 10:03 AM

#1

Ok my 99WR400F seems to run strong even in a little bit of elevation. I have noticed that when I am crusing like down a hill or grade and I let off the gas it pops and backfires just a little bit. Two of my friends, who know a little about 2 strokes and nothing about thumpers say that I need to adjust the fuel screw, and that I am running too lean. I know that I hear other bikes do the same thing so I am not to worried about it. I have a pipe, headers, and rejet. Throttle stop gone, air box cover removed, but grey wire still intact.

Am I running too lean? Should I spend the money and buy a new fuel screw so that I can change the mix on the go? I always heard a little backfire pop is not such a bad thing. Am I missing something?

  • RichBaker

Posted April 28, 2005 - 03:15 PM

#2

A properly jetted 4-stroke will "cackle" a bit on decel....it's normal. When you close the throttle, it causes a high-vacuum in the intake which pulls more fuel into the cylinder, since most of this fule doesn't burn in there(not enough o2), it goes out the tailpipe and when it hits the fresh air, the unbunt fuel ignites. Don't worry about it unless it sounds like a machine gun....then you need to lean her out a touch.

  • Dwood11

Posted April 29, 2005 - 09:11 AM

#3

Thanks for the info. Thats what I wanted to hear. I appreciate it. I kinda knew it was normal, and thats why I never really worried about it. Now I know.

  • Oasis_Flame

Posted April 29, 2005 - 10:41 AM

#4

Hey Rich, are you shooting from the hip on this one? A well tuned 426 will not "cackle" on decel. If it's poping, open the fuel screw a 1/4 turn. If the fuel screw is more than 3 turns out you'll need to reduce pilot jet by one and readjust fuel screw. Nothing to it. :naughty:

  • Math

Posted April 29, 2005 - 11:18 AM

#5

. If the fuel screw is more than 3 turns out you'll need to reduce pilot jet by one and readjust fuel screw.


I thought you would get a bigger pilot in this case... not a smaller :naughty:

I think you are too lean on the pilot circuit. Try to adjust the fuel screw out 1/4 of a turn at a time. If you get over 2 1/2 - 3turn out, go with another pilot jet.

What are your jetting and your riding conditions?

  • RichBaker

Posted April 29, 2005 - 11:26 AM

#6

Hey Rich, are you shooting from the hip on this one? A well tuned 426 will not "cackle" on decel. If it's poping, open the fuel screw a 1/4 turn. If the fuel screw is more than 3 turns out you'll need to reduce pilot jet by one and readjust fuel screw. Nothing to it. :naughty:


Actually, no I'm accurate and correct! i'VE BEEN TUNING 4-STROKES FOR OVER 30 YEARS....A PROPERLY JETTED PERFORMANCE ENGINE WILL CACKLE ON DECEL.....oops, hit cap lock accidentally...but apparently at the right time.
Note I said"cackle" it's a synonym for laugh....I'm not talking machine gun-like popping and explosions here, simply some slight, barely noticeable laughter, like she's having fun! If you watch road racing, you'll see it when the bikes(or cars) back off the throttle on turn-in, you'll see some flames in the tailpipe....The Foggy-Petronas bikes had flames shooting out the exhaust several feet for a while, til they got it figured out!
You really should have known this before calling me out..... :naughty:

  • RichBaker

Posted April 29, 2005 - 11:28 AM

#7

Correct, Math....a carb tuner's rule of thumb is: more than 2 turns out, richer pilot, less than 1 turn out, leaner pilot. Ideally the fuel screw will be between 1 and 2 turns out...at least, that's how the pros do it!
If you're tuning a 2-smoke air screw, it's just the opposite.

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  • Dwood11

Posted April 29, 2005 - 02:13 PM

#8

I think I might be a little lean. They are not loud pops at all. Just very frequent almost to the point of a small machine gun but not quite. If anything you are saying it would might be a little to lean? I ride in about 4-5000 feet in elevation, and to tell you the truth I am not sure what click the Jets on. I might play with the fuel screw a little on Saturday. Anyone going to Dove Springs?

  • MorrisWR

Posted May 02, 2005 - 09:46 AM

#9

Most of my 4 strokes did this stock because they were too lean. When jetted leaner, they really backfire. Richened up on the pilot and it goes away (as well as racing/inconsistent idle when it's lean). Easiest way is just to try adjusting the screw out like people have said, if it clears up then that's it. You can always change the pilot jet if the screw is out of the 1-2 turns.

  • Math

Posted May 03, 2005 - 08:41 AM

#10

a carb tuner's rule of thumb is: more than 2 turns out, leaner pilot, less than 1 turn out, richer pilot. Ideally the fuel screw will be between 1 and 2 turns out...at least, that's how the pros do it!
If you're tuning a 2-smoke air screw, it's just the opposite.


Guys, you're getting me mixed up.... I think you just said the opposite of what you meant Rich.

With an air screw, the more you turn out, the more you let air come in the pilot circuit. If you are more than 2 turns out, it means you need to use a leaner pilot jet i.e. a smaller one.

With a fuel screw, the more you turn out, the more you allow the air-fuel mixture access the engine. If you are more than two turn out, you need to go bigger on the pilot (let some additional fuel go into the engine).

  • RichBaker

Posted May 03, 2005 - 02:03 PM

#11

Guys, you're getting me mixed up.... I think you just said the opposite of what you meant Rich.

With an air screw, the more you turn out, the more you let air come in the pilot circuit. If you are more than 2 turns out, it means you need to use a leaner pilot jet i.e. a smaller one.

With a fuel screw, the more you turn out, the more you allow the air-fuel mixture access the engine. If you are more than two turn out, you need to go bigger on the pilot (let some additional fuel go into the engine).



Sorry 'bout that.....after 12 years riding 2-smokes, I still mix it up sometimes.The numbers are correct, though.

  • Math

Posted May 03, 2005 - 03:55 PM

#12

No problem Rich :) LOL!

It gets confusing real quick :) .
I had gotten what you meant.
I just wanted to rectify so things could be absolutely clear.
:D

  • Dwood11

Posted May 05, 2005 - 10:08 AM

#13

Thanks for all the info guys. I am heading up to Dove Springs on Sat. I will let you know how it all works out. Leaner smaller jet. Got it. Thanks again

  • Seabass

Posted May 05, 2005 - 02:23 PM

#14

What year is your bike? I've got an '05 and I ended up taking off the AIS and now there is no more backfiring!!!! :)

  • grayracer513

Posted May 05, 2005 - 04:28 PM

#15

RB is right in that some exhaust popping on decel is normal and of no concern. In fact, there are any number of people running their idle circuits unnecessarily rich to get rid of it.

But another common cause that should be checked first is an air leak in the exhaust system, usually at the midpipe/header joint, or at the cylinder head/header. As Rich said, you already have fuel and heat in the exhaust. Add a little air, and it goes bang.

Incidentally, idle Fuel screws as opposed to Air screws are a recent phenomena even in four strokes. It never surprises me when anyone gets it backwards.




 
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