Irregular Choke/Idle issues plus heavy decel pop???

7 replies to this topic
  • stevemorgan728

Posted November 10, 2011 - 04:18 PM


So my buddies 06 yz450f is having issues.... There is a huge difference in idle speed between choke and no-choke first of all. The idle screw needs almost constant adjustment to keep it idling with no choke. Also, he has an obnoxious decel pop, and it seems to have been running a bit rich. Where would we start with the re-jetting? What's up with the choke thing? Any help here would be awesome, Thanks!

  • LemonDrizzle

Posted November 10, 2011 - 08:48 PM


Well.....To the best of my knowledge, I thought there should be a difference in idle speed from choke to no choke. I have an 06 450 as well and have always had a pretty big difference in the two speeds, especially when it is first started. Then as she warms up the rpms will come down a bit and the choke can be turned off. As for the erratic idle speed and popping on decel, the erratic idle could be trash in your pilot circuit which could also be causing your popping on decel which is usually a result of a lean condition.

I know that you said you thought the bike was running rich, but just out of curiosity, what makes you think that it's rich and not lean? I'm certainly not an expert, but what you have described sounds like lean to me.

If I were having those problems, especially the erratic idle, the first thing I would do would be to give that carb a good cleaning before I did any rejetting.

  • baxterj787

Posted November 11, 2011 - 05:18 AM


Check the hotstart (HS) plunger too. With time, they can hang open a little bit, causing slight air leak (erratic idle, popping, etc...).

Lube the HS cable, pull the plunger out and clean it, CAREFULLY reinstall. The nut holding the carb end of the cable is plastic and is easy to crack if overtightened. I learned the hard way...

When you clean the carb, don't lose the little washer and o-ring around the fuel screw.

I' sure someone will weigh in with "check your valves" sooner than later.

  • grayracer513

Posted November 11, 2011 - 07:37 AM


Your symptoms indicate a lean condition, not rich. Start by trying the pilot screw adjustment:


If it won't run right by the time it's a half turn out, there's something wrong. Assuming the hot start isn't hanging open, the most likely thing would be a varnished and partly obstructed pilot jet. Note that just spraying carb cleaner and compressed air through these will not usually clear them.

If that isn't it, look for air leaks at the mounting sleeve, be sure there's nothing wrong with the vacuum release plate on the inside of the slide, and check the clearance of your valves.

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  • stevemorgan728

Posted November 11, 2011 - 09:23 AM


I know it's rich because we just did a piston and rings replacement(as well as valve check), and the spark plug was dark and wet as well as the top of the piston. We've also cleaned the carb a couple of different times, and it still runs the same. The hot start seems to be functioning properly as well, I suppose there could be a very slight air leak. I will inspect further. Also, the fuel screw is intact and I even turned it in a half turn(1 1/2 turns out). His exhaust system is suspect. It's a Dr.D system with a strange headpipe connection to the head. It's like a two piece floating fit with springs, could definitely be leaking at the head. Could that be it?

I have an 08 yz450f and the difference between choke and no choke is WAY less then my friends bike. It also has almost no decel pop either. I suppose this could just be the difference between an 06 and 08.

  • grayracer513

Posted November 11, 2011 - 09:32 AM


I know it's rich because we just did a piston and rings replacement(as well as valve check), and the spark plug was dark and wet as well as the top of the piston.

It will be dark and wet whenever it doesn't fire correctly, including when it's lean. Your engine is telling you what the problem is, listen to it.

As I said, carb cleaner and air won't fix a varnished pilot. Read:


Cold with the choke on at idle, the hot start should change the RPM significantly, almost as if it were a thumb throttle. If it doesn't, it may not be sealing. Also, the possibility of a missing pilot screw O-ring that was mentioned is worth looking at.

  • stevemorgan728

Posted November 11, 2011 - 09:54 AM


Hmmm... wet and dark even when it's lean, yikes, that throws a kink in my diagnoses. Well, we will definitely pull the carb and clean the piss out of it. Probably even replace the pilot jet with a new one. The hot start is fine and the o-ring is still in place. It running lean is hard to comprehend, seeing it was originally jetted for close to sea-level, and we ride 6000ft+ (lake tahoe area).

Thanks for your help Grey!

  • grayracer513

Posted November 11, 2011 - 11:02 AM


You have to realize that the metering orifice of a #45 pilot is .45mm (.017"). A drop of gas can easily dry to a crust of varnish .006" thick, making the effective jet size something on the order of a #27. Think that might be too lean for 6000 ft?

Check the number drill chart in the post I linked to and give that a whirl, or even a carefully inserted piece of fine wire.

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