How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Louie Your Own Question
Louie, Technician
Category: Motorcycle
Satisfied Customers: 12577
Experience:  25+ yrs. experience
Type Your Motorcycle Question Here...
Louie is online now
A new question is answered every 9 seconds

My new to me 2004 Suzuki KFX 400 quad is backfiring when

Customer Question

my new to me 2004 Suzuki KFX 400 quad is backfiring when giving it gas and running very rich but jetting has not changed. how to fix this?
It did not backfire when i bought it but it would not run long at full throttle and was acting like it was running out of fuel.
I don't know the history on the quad. I have replaced about everything.
The wire harness did take a chain strike but I fixed the wires.
What i have done.
Rebuild the carb and replaced everything.
changed the coil.
checked and repaired wires in the harness.
Checked the gear position sensor.
checked the timing chain and valve position.
Checked valve lash. It is out a small bit but not much.
New petcock and fuel.
filter new. checked the rubber boot for leaks.
Checked the choke for function. It moves but I don't hear any change while it is running.
Adjusted the fuel screw many many times.
New plug.
charging system looks good. 13.7v I believe
Tested the voltage going to the coil while cranking 130v +/-
Checked and cleaned wire connections.
Did not do anything with shut off on handle bar.
Submitted: 1 year ago.
Category: Motorcycle
Expert:  Louie replied 1 year ago.

Hi and welcome to JustAnswer,

Thank you for the question and your patience. Should you still need assistance for information purposes only...

Initially, proposed preliminary tries (if have not been done):

  • • carefully remove the sparkplug;
  • • check the burn color of the ceramic at the tip end;
  • • results would indicate:
  • - if black/sooty, then rich and/or vacuum leak issue;
  • - if oily, then an internal engine problem;
  • - if very light brown to whitish, then lean and/or an air leak concern.

Additionally, proposed repeat checks (if have not been done) on the possibility of over/under-voltage, regulator/rectifier and/or stator issue. With a volt/multi-meter, determine again the battery voltage when:

  • • ignition OFF and then when ON;
  • • when starting the engine;
  • • engine at idle;
  • • when attempting to rev the engine.

Please post back results / observations of the above once tried.

Customer: replied 1 year ago.
Sorry i have had nothing but problems getting on this site to answer. The voltage off is 11.95 running it was 13.5 and running the RPM's up it was 14.37v the plug is sooty as stated in the first part. I did find a missing spring in the carb on a diaphragm on the side of the Carb. I have ordered it. Its the spring that goes between the diaphragm#18 and the diaphragm cover#19 on the service manual. I also replace the new jetting with the old jetting and no changes. Perhaps I lost the spring when I rebuilt the carb. It will take a week to get the spring in.
Expert:  Louie replied 1 year ago.

Appreciate the post back.

And for some reason, the "... the plug is sooty (as stated in the first part)..." appears to be missing in the initial post.

Still, consider with a fully charge battery to try the quad with the voltage regulator temporarily disconnected.

Would appreciate an update when the carb + spring has been addressed.


Customer: replied 1 year ago.
Not sure how to disconnect the voltage regulator but i will look in the books. I did get the parts and the problem did not change. I cleaned the cab out again and it runs worse now. it want to die if i give it throttle and it back fires faster . Like it is lean. I dont know. I will test the power tomorrow with a charged battery and get back to you.
Customer: replied 1 year ago.
I could make a video and post it to youtube.
Expert:  Louie replied 1 year ago.

The regulator would look something like the part HERE and can be disconnected through its connector. With a fully charged battery the Suzuki should be able to start run and rev.

If problem persist, then an over-voltage condition would be the least likely cause.

A lean condition can either be carb or air cleaner issue. Is the air cleaner / box correctly fitted? air entry should only be through the designated air cover opening.

And yes please, a video when able.

Customer: replied 1 year ago.
The voltage not running was 13.32 running normal was 14.71 and with the plug un done like you asked it was 13.01
I pulled the older plug and it was black. I put in a clean plug and ran it for just a small time Recorded on the video and it was brown gray.
Customer: replied 1 year ago.
Expert:  Louie replied 1 year ago.


And the voltage appears to check good/acceptable.

Have you checked the air box / cleaner (filter-element) ? ...correctly mounted, sealed and air entry only through the designated opening?

Customer: replied 1 year ago.
I was running the engine in the video with the air cleaner off. It was the same with it on. I checked the rubber boot between the engine and the carburetor for air leaks, cracks and found none.
Expert:  Louie replied 1 year ago.

For the moment, let's say that mix is lean, there is a hesitation/flatspot when attempting to rev but there is no air leak; are you able to try raising the needle?

Depending on the carb (model) by lowering the clip or adding small washer/spacer(s).

...and with the air box / filter on please.

Customer: replied 1 year ago.
I can. I will tomorrow. That needle is not original with this carb. the old one was hot glued in because the slide was broke where it holds the needle in. That needle is new to this issue too. So it may be causing it .. or causing the new more backfiring than before.
Expert:  Louie replied 1 year ago.

Almost always, the needle would be tapering thus the fuel allowed is not equal at any given height. At 1 point, the fuel may still be not enough and the air is as required thus the lean but not constant and likely 1 of the reason the slight hesitation to rev and then the 'backfire' or delayed fuel in rush.

Yes please; an update when able.

Customer: replied 1 year ago.
I changed the height 2 times. Added another shim
it ran a little better. It would run much better if my hand covered 3/4 of the intake. But it would not rev up all the way. see video.
Expert:  Louie replied 1 year ago.

Thanks and very significant results:

  • • too little gas - increase main jet size 1 step up;
  • • too much air - if the same persist with the air box/original filter installed then consider replacing with a more denser filter element or over laying with add-on/universal air filter similar to the product shown HERE.
Customer: replied 1 year ago.
I opened the mail jet up to 150 and did not see much of any change. Still backfires. I will make another video today.
Expert:  Louie replied 1 year ago.

And have you tried further restricting air entry; i.e. replacing with a more denser filter element or over laying with add-on/universal air filter?

And yes please on the video though there may be a bit of a delay on my next post.

Customer: replied 1 year ago.
I have not yet restricted the air. I did buy a good used carb to swap out to make sure its not something else.
Expert:  Louie replied 1 year ago.

Appreciate the post back and would look forward to an update when able.