why does my bike backfire?


35 replies to this topic
  • Benno84

Posted March 26, 2009 - 05:26 PM

#1

My bike has started backfireing, there are no obvious exhaust problems, can anyone give some suggestions why it is doin this?

  • massip 138

Posted March 26, 2009 - 06:35 PM

#2

It could be lean. Turn the fuel screw out a bit and see what happens. Have you rejeted? In California the bikes come a bit lean from the factory.

  • jimdager

Posted March 26, 2009 - 07:17 PM

#3

sounds like lean to me, but wouldnt you turn the screw in? I thought it was an air screw, turning it out let in more air=leaner, turning it in meant less air=richer. I could be way off as I am just learning jetting myself, but someone please correct me if I am wrong as I have been doing things backwards.....

  • the white stallion

Posted March 26, 2009 - 07:35 PM

#4

On a 4stroke it's a fuel screw.Out is more fuel in is less.All bikes are jetted the same from the factory no matter what state it goes to.They wouldn't have different owners manuals for every state listing different stock jetting specs.Unless the op has more info to offer it's hard to tell whats going on and that it all of a sudden starting backfiring.does it backfire all the time at high rpms,is it a decel pop,did the temp rise signifantly outside. more than likely it's a decel pop which is the pilot jet.richen or turn out the fuel screw or go up to a 42 PJ.If he says he has no exhaust leak which if it all of sudden started backfiring and how bad this could be the problem.If it's hard to start and backfires bad it could be valve related.The MX bikes as opossed to an enduro bike does not come lean from the factory except most of them run a 40PJ and a 42 is better.The stock kxf is a 180 and most guy's go down on the main to a 175-172-170.Of course ones climate plays into all of this.

  • jimdager

Posted March 26, 2009 - 07:58 PM

#5

I guess I have been doing things backwards. I just moved from 2 strokes to fourstrokes about 2 weeks ago. can anyone explain some of the main differences in jetting and working on a carb between the two? I seem to be a little confused. as to the OP (Sorry for the thread hijack), it would probably help if you posted your currently running jets, as well as altitude, temps, stuff like that

  • Luigic81

Posted March 26, 2009 - 08:05 PM

#6

I guess I have been doing things backwards. I just moved from 2 strokes to fourstrokes about 2 weeks ago. can anyone explain some of the main differences in jetting and working on a carb between the two? I seem to be a little confused. as to the OP (Sorry for the thread hijack), it would probably help if you posted your currently running jets, as well as altitude, temps, stuff like that


not a hole lot of difference, on the 4t it's a fuel screw, so when you go "out" it lets more fuel in, it's the opposite on a 2 stroke, cuz it's an air screw.
The leak jet in the bowl on a 4t is called a "powerjet" on a 2t but they serve the same purpose as far as I know. You have the same other componets, needle, jets, float etc. On the newer 2 strokes you also had an adjustable powervalve more than likely.. ..

  • jimdager

Posted March 26, 2009 - 08:26 PM

#7

thanks for the explanation, is it about the same on where it should be out from lightly seated, about 2 turns out?

  • Luigic81

Posted March 26, 2009 - 09:15 PM

#8

yup, most guys run 2.5 turns out to start the day and then go in or out .5 turn if they need to throughout the day.

  • Jayson138

Posted March 28, 2009 - 02:32 PM

#9

I am also having this problem with my 09. It started out with stock jetting(40 pilot and 182 main). I switched the jetting to a 42 pilot and a 180 main with the fuel screw at 2 turns out. Before I switched the jetting I was getting some decel pop and it started to idle like crap. I took the float bowl of the carb and found some dirt and cleaned that. Than when I took out the pilot I noticed that it was clogged. I cleaned what I could and switched to the 42 pilot and 180 main and ran it for about 2 hours and really didn't see any change. Today I ran it for no more than an hour and was getting more decel pop than when I had the stock jetting. What is going on? Also, I noticed that it is pretty much impossible to get to the fuel screw without taking the carb off. Is there an easier way to get to it or should I try buying an aftermarket adjustable fuel screw? Please help me because this is really pissing me off!

  • Jayson138

Posted March 28, 2009 - 02:37 PM

#10

Also, I don't know if this is related, but I assume so. When I am sitting in neutral and go to hit the throttle hard there is a bog before it hits hard. What is going on?

  • Benno84

Posted March 28, 2009 - 04:01 PM

#11

it only backfires when im decelerating, and i also have the problem as jayson138 described.

  • Luigic81

Posted March 28, 2009 - 04:58 PM

#12

do a search :thumbsup:

  • Jayson138

Posted March 29, 2009 - 05:27 AM

#13

I did a search and didn't find much, so I could really use some insight from you guys.

  • Chokey

Posted March 29, 2009 - 05:44 AM

#14

Also, I don't know if this is related, but I assume so. When I am sitting in neutral and go to hit the throttle hard there is a bog before it hits hard. What is going on?

If you can't cure the bog with the fuel screw, then you need to adjust the accelerator pump linkage, or possibly change the leak jet.

Externally adjustable fuel screws and adjustable leak jets are wonderful things. :thumbsup:

  • Jayson138

Posted March 29, 2009 - 07:15 AM

#15

I know this is a stupid question, but is a bog the result of too little fuel or too much?

  • Chokey

Posted March 29, 2009 - 07:42 AM

#16

I know this is a stupid question, but is a bog the result of too little fuel or too much?

It's a result of a slight delay in fuel getting to the engine when you crack the throttle open. Four-strokes have a much weaker vacuum signal to the carb than two-strokes when the throttle is closed. When you snap the throttle open, there is a slight delay before fuel begins to be drawn up through the jets. That's why four-strokes use an acclerator pump, as a mechanical means of getting fuel into the engine before the vacuum-induced flow can react.

This is why the pilot circuit of four-strokes is different than two-strokes. The four-stroke carb is basically two carbs in one, with the pilot circuit being a seperate, more sensitive carb. The fuel screw controls this seperate fueling system. The pilot circuit on a four-stroke has to be slightly rich to provide decent throttle response.

Very minor bogs can be cleaned up with the fuel screw adjustment. But a significant bog will usually require adjustements to the AC pump linkage and/or the leak jet.

  • Jayson138

Posted March 29, 2009 - 08:20 AM

#17

Chokey,

What do you think of putting an R&D flexx jet remote fuel screw on so I can make adjustments without having to take the whole carb off the bike. Also what about an adjustable leak jet to also speed up the testing process in between adjustments.

  • Luigic81

Posted March 29, 2009 - 08:32 AM

#18

if you change the leak jet to the recommended 50, and then set your fuel screw and install the proper pilot jet, you will not have any more big problems, atleast at from closed throttle to just over 1/4, which is were most people have them. If your having bog at 1/2 or higher throttle openeings then you need to adjust your needle and/or main jet

  • Luigic81

Posted March 29, 2009 - 08:34 AM

#19

I did a search and didn't find much, so I could really use some insight from you guys.


go tot he main kx250f page, and then on the right, click on "search this forum" them type in the box, backfire, or jetting. There will be a million threads

  • Chokey

Posted March 29, 2009 - 12:29 PM

#20

Chokey,

What do you think of putting an R&D flexx jet remote fuel screw on so I can make adjustments without having to take the whole carb off the bike. Also what about an adjustable leak jet to also speed up the testing process in between adjustments.

Yes to both. I have the Flex Jet and the R&D Power Bowl on my YZ250F. They are worth every penny of the purchase price. The additional tunability of the adjustable leak jet is priceless. :thumbsup:

You also have to make sure the accelerator pump linkage is adjusted correctly. If it isn't, the squirt won't be timed correctly, causing bogs or rich stumbles.





Related Content

Reviews

Kawasaki KX250F 2015 by Bryan Bosch


Kawasaki KX250F 2015
  • - - - - -
  • 0 reviews
Forums
Photo

What is the best size big bore for 250f ?.. by Johnnywadd739


Dirt Bike   Make / Model Specific   Kawasaki   KX250F
  • 2 replies
Reviews

Kawasaki KX250F 2014 by Michael499


Kawasaki KX250F 2014
  • * * * * *
  • 2 reviews
  • Last review by Michael499
  • On October 13, 2014
Forums
Photo

Cylinder measuring by Dan Paull


Dirt Bike   General Dirt Bike Forums   General Dirt Bike Discussion
  • 13 replies
Forums
Photo

Durability of the 2013/2014 kx250f how has yours held up? by Tony477g


Dirt Bike   Make / Model Specific   Kawasaki   KX250F
  • * * * * *
  • Hot  88 replies
 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.