Stop the backfiring



11 replies to this topic
  • jbratton

Posted June 25, 2001 - 05:37 PM

#1

I have a 2001 426 and I have to admit the bike ran great while the temperature was in the 60’s to 70’s. Once the 80’s and 90’s came around I have had to make some major changes in the pilot, main, fuel screw and clip position. I am riding this in Colorado currently with a 68 pilot, 152 main, 3rd clip position and the fuel screw at ¼ turn out. I can have the screw all the way in and it runs alright. I have good power throughout the low end, mid range and top end. It’s only on deceleration that I get an annoying backfire. I run the bike with a White Brothers R4 with the stock header. It seemed the backfiring started after putting on the R4. I went back to the stock pipe and its doing the exact same thing. Both pipe run the best with the same settings. I would have thought for sure one would run better one way and the other another way. I have replaced the CDI unit, cleaned the carburetor and checked for any leaks. Any ideas what would cause this? I always was told that the fuel screw should be 1 to 2 turns out. I can’t seem to get the bike to not backfire as much unless I have in fully closed to ¼ turns out. I am frustrated at the least.

  • mrstooge1

Posted June 26, 2001 - 02:08 AM

#2

The "backfire" you are refering to is actually after-burn in the exhaust. The bike is so lean during decel there isn't enough fuel in the cylinder to ignite. When the unburned fuel gets into the exhaust pipe, it meets up with red hot carbon and more fresh air and it explodes and you get the popping you are hearing. Turn your fuel screw out to 1 turn and see if it reduces it or stops it completely. If it stops it then I would suggest leaving it or turning it until the popping starts back up and then back the other direction until it JUST quits. Go 1/4 turn at a time. If it doesn't cure it by adding more fuel screw, it's jetted too lean in the pilot circuit and you'll need to go one step richer and start over with the fuel screw.

Tony

  • Roostn67

Posted June 26, 2001 - 03:57 AM

#3

I am also in Colorado and run a 152 main on a FMF pipe. My pilot, needle, and fuel screw are all on stock settings. Every now and then I get a pop back but not as often as you. Try changing back to stock settings and try it. Maybe try a 155 main as well.

Roostn in Golden

  • Dave_S

Posted June 26, 2001 - 05:51 AM

#4

Jbratton,

Some of us have had similar experiences by installing the metal plate on the carb slide upside down.

I dropped mine when I took it out and looking at it made me guess the wrong way to install it. The manual has a picture of everything and shows clearly how it should go, there is also a recent post on here somewhere that links to fiche that shows it as well.

Maybe?

Dave S

  • James_Dean

Posted June 26, 2001 - 06:50 AM

#5

Do you have the stock pilot jet? (#42) Have you developed any air leaks?

Sometimes the backfiring comes from a leaking headpipe at the flange by the cylinder. Did you loosen it recently for an oil change or when fitting the muffler? You may want to loosen and retighten it making sure it fits up and seals.

-The 68 pilot is normally the "starter jet" for the choke circuit.

James

  • emcdan

Posted June 26, 2001 - 07:16 AM

#6

Hey Mr. Dean

Is there anyway to insure a good seal at cylinder/pipe joint and at the pipe/canister joint. I have used high temp silicone at the canister joint but any suggestions for the cylinder?

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  • mikeolichney

Posted June 26, 2001 - 07:24 AM

#7

Jeff, you have a 38 pilot in last I checked. You have not removed the slide, right?

James Dean, can a header to muffler exhaust leak cause these symptoms?

[This message has been edited by mikeolichney (edited 06-26-2001).]

  • mikeolichney

Posted June 26, 2001 - 07:32 AM

#8

From a previous post about this problem:

I have a friend who has an R4 muffler on his 01YZ426, it is otherwise stock including the header. We are in CO, 5000 ft ASL, 70-100 degrees F. My stock 01YZ426 runs great with 155 main, stock needle, 40 pilot, 1.25 turns out on the fuel screw. I had severe plug fouling problems this winter but a new CDI fixed it.

His bike runs strong in mid and top but it has bad decelleration backfiring. Throttle response is not great at low throttle openings (I dont think he notices because he is faster than me and doesn't use the low as much). Usually this would mean too lean, but when we go up on the pilot or fuel screw the backfiring gets worse. He has swapped his CDI, no big change. His air filter is pretty
dry, pretty sure its not the filter oil in the carb issue. I can't find any carb boot air leaks. We took out his accel pump diaphragm and cleaned it, no visible dirt. His plugs are sooty black everywhere, and he occasionallyfouls one. He is a fast rider on an open MX track, the bike is not being lugged at low RPM.

Right now the bike has a 152 main, one clip position up on the stock needle, and a 38 pilot. We have tried a 45, 42, 40 and 38 pilot with various fuel positions, no improvement. He thinks it runs best with the 38. I am bugging him to put back on the stock muffler, he is reluctant but we will do this next time.

Any ideas? Could an air leak in the muffler to header cause this problem? Anyone else with an R4 having this problem?

  • jbratton

Posted June 26, 2001 - 07:37 AM

#9

Originally posted by James Dean:
Do you have the stock pilot jet? (#42) Have you developed any air leaks? Yes, I have a 42. I haven't noticed any leaks in that area. I will double check though.

Sometimes the backfiring comes from a leaking headpipe at the flange by the cylinder. Did you loosen it recently for an oil change or when fitting the muffler? You may want to loosen and retighten it making sure it fits up and seals.

-The 68 pilot is normally the "starter jet" for the choke circuit.

James



  • jbratton

Posted June 26, 2001 - 07:41 AM

#10

Originally posted by mrstooge1:
The "backfire" you are refering to is actually after-burn in the exhaust. The bike is so lean during decel there isn't enough fuel in the cylinder to ignite. When the unburned fuel gets into the exhaust pipe, it meets up with red hot carbon and more fresh air and it explodes and you get the popping you are hearing. Turn your fuel screw out to 1 turn and see if it reduces it or stops it completely. If it stops it then I would suggest leaving it or turning it until the popping starts back up and then back the other direction until it JUST quits. Go 1/4 turn at a time. If it doesn't cure it by adding more fuel screw, it's jetted too lean in the pilot circuit and you'll need to go one step richer and start over with the fuel screw.

Tony


I have used the fuel screw out as far as 1 and 1/4 turns but not any further. That is due to the backfiring getting worse as I screw it out past one turn. I will try and go even further out. This is what Colorado Powersports told me what they thought the problem was. Let me know what you think.

Hi Jeff,

The only way you can be getting this type of reading is if the bike is getting fuel from a fuel level that is too high or a choke that is allowing fuel flow. I recommend bringing it in to have the machine looked over by one our technicians.
Thanks for the inquiry

Joe Denning
Service Director

  • mrstooge1

Posted June 26, 2001 - 02:17 PM

#11

Did you have the carbs off to make the changes or did you make them with the carb on the bike? Make sure the carb is completely seated in the manifold and there are no air leaks. Make sure that the connector for the throttle sensor is plugged in correctly and there are no pins bent over or pushed out of the connector. If it appears to be correct and the exhaust is sealed to the cylinder with a good gasket, you need to take the bike to the dealer and have them set the throttle position sensor and give it a good goin over. It can also be the junction at the head pipe/silencer connection doing that but, it would less likely be affected by fuel screw adjustments.

Good luck.

Tony

  • mikeolichney

Posted June 29, 2001 - 05:24 AM

#12

Thanks to everyone for the suggestions. Jeff took off the carb and exhaust, cleaned all surfaces and reassembled it. We jetted it like mine and now it runs great. Must have been a air leak somewhere.





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