2006 Honda CRF450R Decel Popping

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6/12/2018 9:53 AM
Edited Date/Time: 6/12/2018 9:57 AM

Hello, and thanks in advance for the advice. I just finished a rebuild of the above mentioned bike, new piston, rings, cylinder, intake valves, carb kit, yoshimura full rs2 exhaust system, and a bunch of other stuff not relevant the the issue. Set everything up according to the manual, took into account the recommended jetting and carb settings for elevation and tempurature. Bike started right up, idles well enough, ran strong, nothing unusual except the popping on deceleration that happens when you let off from about 1/2 to 3/4 throttle. (I have had a 2002 CRF450r and a 2004 CRF250R that did not do this)

I have tried reading lots of forum posts from different sites, tried some easy fixes, i took the pilot screw out 1/2 turn and it had trouble idling, so then i went in 1/2 turn from base and idles great and did reduce some of the popping. I noticed my hot start cable isn't sliding as well as I think it should so I will look into that and I have even read that it could be caused by the seal on the flat slide in the carb being installed upsidedown. I cannot find any air leaks in the air boots or fittings.

Ok so for the actual question. Since I have read both, is the decel popping a bad thing or not? Also, If you had a similar bike and problem, what was your fix?

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6/12/2018 10:09 AM
Edited Date/Time: 6/12/2018 10:10 AM

You have a "fuel screw" not a "pilot screw". Turning it in leans and turning it out makes it richer.
Popping can be caused by a few things....
Too lean in lows peed jetting. ( either jetting or fuel screw setting)
Exhaust gasket between head and pipe reused and not replaced or leaking.
Bad exhaust gasket at the head pipe to muffler connection.
Leaking Hot start will lean out the fuel mixture.

Paw Paw

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6/12/2018 10:24 AM

Check your pilot jet. Remove the nut on the bottom of the float bowl, remove your pilot jet, check the size, go up one size(probably need a 45 or 48 depending), throw new pilot in. If that doesn't fix the issue, you have something else going on.

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6/12/2018 11:01 AM

Paw Paw 271 wrote:

You have a "fuel screw" not a "pilot screw". Turning it in leans and turning it out makes it richer.
Popping can be caused by ...more

Thanks for the response. New exhaust gasket was used for the head and pipe, but there are no gaskets or clamps for connecting the head pipe to the mid pipe and mid pipe to the muffler, just springs, it is a titanium pipe. Should i try sealing those joints with something? I feel like they are pretty solid though, so I should probably start with the jetting as you and Matty have mentioned. I will check back in once its looked at and see where we need to go from there, thanks!

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6/17/2018 9:59 PM
Edited Date/Time: 6/17/2018 10:00 PM

Ok I had a 42 slow jet in (I believe that is the stock sized jet) went up to a 45 and with 3 full turns out on my air screw it was still popping. Bumped up to a 48 and 3 turns out still popping.

Double checked intake air boot, no leaks detected
Double checked head to pipe gasket, looked good, although now that I've taken it off I'll probably need to get a new one.
Fuel is brand new
I even removed the slide in the carb to make sure my plate was interested the correct direction, it was
Bike runs strong, idles and starts perfect....

So I've got a few more questions.

1. I'm really not seeing/feeling any difference in performance, idle, throttle response, or any notable change at all with the new larger pilot jet. Is that normal? Even with adjusting the air/fuel screw. I guess I expected I'd get to a point going up 2 jet sizes that I either made it run noteably better or worse.

2. I'm running a titanium yoshimura head pipe with a larger diameter than the stock pipe. I've heard people attribute the decel pop to the exhaust allowing air to get sucked back in and detonate easier than in the stock exhaust.... Any credibility to that thought in this application?

3. I am skeptical of my hot start. When the bike is off, I can hear what I think is the plunger returning to a fully seated spot, almost like a small click if I just pull the lever and dump it and let the spring pull it back in. But if I use this video as a guide to set my jetting and air mixture screw (https://youtu.be/9Ib1SyAHPRs) when I pull the hot start the bike dies. This video would guide me to an even richer jet setting.... But seems like I'm pretty rich already. Here is a photo of my spark plug, pulled after I put in the 48 jet. Black soot, no oil residue.Photo
Photo

Next step is pull the hot start and verify it's working well, other than that, any other paths I can look into?

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6/21/2018 7:36 AM

Sounds as though the mid squirt is not working. Wedge the front of the carb out of the boot and push it slightly sideways so you can pump the throttle and see if it is squirting like it should. When the mid squirt clogs it will backfire because to lean. You can us a tiny wire from a wire brush and some hemostats to poke the hole clean. Use safety glasses, if the brass hole is clogged and you push the wire in to clean it fuel can be under pressure and squirt you in the eye.

The little brass tip sticks up out the back of the carb. They clog like the pilot jet does.

Medical hemostats hold the tiny wire brush wire well so you can manipulate getting up in the carb to poke the hole.
Use the smallest wire you can find, do not force it and do not break the wire off in the hole. It works, I have done it many times.
Photo

Photo

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7/31/2018 2:44 PM

Just checked the mid squirt/accelerator pump squirt. Here is a video, it seems to be spraying good and strong every time.... hmmmmm.

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7/31/2018 3:48 PM

The squirt should not hit the slide, so the timing of the squirt is off. That is adjustable at the cable location of the carb . It is called the accelerator pump adjustment.

Paw Paw

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7/31/2018 6:57 PM

I fix lots of decel backfire on the current four strokes.

Usually it is to lean of pilot jet, although I use an Exhaust Gas Analyzer to find out quickly.
2006 CRF450R my guess after doing many carb rebuilds and rejet is 48 to 50, maybe even 52 to 55 pilot, try the 50 at 3 turns out? The worse the fuel gets the leaner the engines run.
Or, the muffler or mid pipe has a big leak you can feel with your hand.

Lack of mid squirt also causes lean backfire, but you checked it.

If the decel backfire disappears with the choke pulled out 1mm or so then it is just lean. Slightly pull the choke and clip a hemostat on the plunger rod, test ride and is the backfire is gone?
If the pilot jet did not fix it I would check the needle clip and maybe lower the clip, raising the needle and making it richer in the middle.





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