17 replies to this topic
  • Mehdi

Posted October 11, 2006 - 11:00 PM


When I roll fast on the throttle the bike bogs down and sometimes dies... I tried messing with the fuel mixture screw and had no luck... My bike was sitting there for a while and the gas had gone bad and turned in to varnish... So took the carburetor out and cleaned it and cleaned all the jets, needle, and most of the passages. The bike turns on fine with 1-2 kicks. And it runs well at mid range throttle. It's just when I roll off and accelerate from when it's idling...Also I noticed it Boggs down when I do hill climbs...

Any ideas?

Elevation is about 6000ft
Main Jet - 165
and needle position is on the 4th one...

any help would be appreciated.


  • grayracer513

Posted October 12, 2006 - 07:11 AM


Year, make, model?

  • StPeters21

Posted October 12, 2006 - 07:58 AM


The boysen quick shot helps cure this...also there is a piece that you can wire together to help the accelerator pump there is a plastic tab that you wire to the metal one(sorry I do not have the technical names for these pieces). All the factory teams do this. It causes a hesitation when you have a quick acceleration.

  • Mehdi

Posted October 12, 2006 - 09:34 AM


My bike is a 04 YZ450F.

  • Ga426owner

Posted October 12, 2006 - 09:53 AM


Did you clean accelerator pump and diaphram? Varnish is bad....clean entire carb...and remeasure squirt must be off. The Quickshot helps a little (I have it) but I would do the RedBeard/JD Jetting accel pump mod first as well as JD Jetting needles to add more power and better throttle response

  • 426_kid

Posted October 12, 2006 - 10:04 AM


id you have an acelerator pump that could not be working that could cause the trouble... or if thats dumpin to much fuel that its flooding... idk but check it out

  • Mehdi

Posted October 12, 2006 - 12:00 PM


Actually I took the carb apart and cleaned out all the components with Part wash Spray... I also took the diaphram out from the accelerator pump and cleaned the through holes... Do you think it may not be clean enough?
Whats the best way to clean the carb? I was thinking of using some carb cleaner and running it through the carb... Maybe it is flooding it, what should the squirt duration be?


  • bg10459

Posted October 12, 2006 - 02:10 PM


Post your jetting specs. It's likely that this can be cured with simple leak jet and AP timing adjustments.
Also, IMO, you should get your jetting right before you start doing mods. There is no reason you can't get it real good without any mods.

  • grayracer513

Posted October 12, 2006 - 04:04 PM


There is no reason you can't get it real good without any mods.

I agree in the case of the '03 or '04 450. Mine has no carb mods at all, and I doubt it could be any more responsive.

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  • Mehdi

Posted October 12, 2006 - 04:29 PM


Thanks guys... Yeah I would really like to get it run perfectly with out doing any mods. Does anyone know the timing? How do I adjust the duration?

Thanks a lot for you help guys...

  • grayracer513

Posted October 12, 2006 - 06:41 PM


Ignition timing is fixed, and will not be incorrect unless the flywheel key shears or the CDI unit fails.

The procedure for cam timing is outlined in the manual.

  • bg10459

Posted October 12, 2006 - 07:00 PM


Read here for AP Tuning

The ThumperFAQ has a wealth of info. It's normally found at the top of the 250f forum :devil:

  • Mehdi

Posted October 15, 2006 - 11:48 PM


Guys, I took my carb apart and cleaned it completely with carb cleaner spray… However its still bogging when I rev too fast… I tried adjusting the fuel mixture screw and noticed that I do not see any difference in the fuel mixture screw… I set the idle high and screwed in the fuel mixture screw and backed out while seeing if the idle changes and if the sound gets sharper, however that did not happen….

So now I believe it’s my jetting, which I’m surprised. It was professionally jetted when I installed the E2 pipe and it ran fine before the varnish problem….

Here are my jetting info: 04 YZ450F
Main Jet = 165
Pilot Jet = 72
2nd Jet = 100  the one on the side of the carb…
Needle = 4th position
Pipe = E2

Any ideas guys?


  • grayracer513

Posted October 16, 2006 - 07:57 AM


I'll bet you a quarter it doesn't have a 72 pilot. Probably a 42.

  • StreetbikePimp

Posted October 16, 2006 - 09:26 AM


If you have 42 pilot jet...your jetting looks good to me. Fuel screw should be between 1 - 2 turns out.

What RPM are you running your idle at? Your should be at about 1800-1900. I wrestled with my jetting for a year to solve a bogging and stalling issure before I finally discoverd that I had a bad ignition coil. I mentioned the RPMs because with the bad coil on my bike, my idle RPMs could not drop below 2500 without the bike stalling. With the new coil, I could dial the idle down to 1500 and the bogging and stall issue dissapeared completely.

Yamaha sells a hour meter that also reads RPMs that I found really helpful.
Check out the link.

  • bg10459

Posted October 16, 2006 - 11:04 AM


That #72 could well be the starter jet. The pilot jet is right next to the main jet, accessible from the plug in the float bowl. As mentioned, maybe you had the right one and just mis-read it.
Anyway, I think your problem is with the AP (except you only mentioned it's a new problem in your last post). Find out which leak jet you have in; it's inside the float bowl. Look into the throat of the carb and see what the AP squirt looks like when you wack the throttle. Some debris may have worked its way into the very small passage to the nozzle causing a weak stream. Then adjust the timing per the link I provided and check the jetting D-base for someone with the same bike and similar conditions to where you ride. This should give you an idea which leak jet you might want to try.

  • Mehdi

Posted October 17, 2006 - 11:43 PM


Guys, thanks for all the replies... Really appriciate your help...

I think you guys are correct... right next to the main jet there are 2 jets... one that is a very small jet and that is set to 72, and the other which is the longer jet which also has spray type holes on the bottom (Pilot jet) = 42... Also the one that is on the side (near the air filter) = 100...

I'm riding at low elevation... 3000ft...

I remember I looked at the fuel spray when I turned the throttle, it looked pretty clean and sprayed pretty far... I think I will start playing around with the adjustment rod. Maybe it's getting too much fuel and it's bogging?

Thanks guys...

  • motosv

Posted October 18, 2006 - 03:14 PM


If your bike ran well before the varnish then there is no reason that jetting would solve the problem. I think you are on the right track with the air mixture screw. Having worked on a lot of carbs you really have to be sure that all the carb circuits are clean and flowing properly. Try cleaning all the circuits one more time, use the carb cleaner with the straw attached and also use low air pressure to make sure that each circuit will allow a good clean flow. Don't attempt to use wire or drill bits as this cause big problems. A lot of times people will not find the varnish under the o-ring in the air screw hole. Oh yeah be sure and use your safety googles.:devil:

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