Popping on Decel


20 replies to this topic
  • angrybob

Posted April 12, 2009 - 11:54 AM

#1

After a long bout of denial, I am finally addresing my 'non-normal' popping on decel. Its bad. Of the three things that are common culprits:

1. exhaust leak
2. hot start valve leak
3. pilot jet circuit

I am leaning towards the hot start circuit.

It runs beautifully with minimal pop on decel (but still a little) with the choke on.

I have adjusted the hot start cable to maximum slack with no change. I can hear a 'contact' sound when I release the lever as if it is bottoming on something. That said, the valve had some soot on it when I checked it this morning. The assembled valve/spring looks fine. On the hot start vavle lip seal in the installed position, should the lip be on top or bottom?
Mine is on top.

How can I confirm that the hot start is closed completely during normal operation? How can I confirm the cble is not FUBAR? When the choke is on, the idle speed increase when I pull in the hot start and returns to normal when I let go.

Yesterday I installed a new exhaust gasket btwn the header and the head. I also sealed the mid-pipe joint and confirmed this morning that its still good. I do not think its an exhaust leak.

Finally, I don't think its the pilot circuit. It starts pretty easy and will idle fine. With a 45, 48, and 50 pilot jet, it still pops - all at various screw settings using the zip-ty fuel screw.

I don't think its TPS because there was no difference with the connector unplugged. Granted, I have not checked the TPS but it may be the next step if the hot start is not the root cause.

I am hoping the lip seal is upside down :p

Any help would be appreciated.

  • grayracer513

Posted April 12, 2009 - 12:11 PM

#2

The sealing lip on the hot start plunger should face up toward the nut.

To verify the operation, start the engine cold, and leave the choke on. Pulling the hot start while running in this mode should raise the rpm a little, as if the hot start were a thumb throttle. The rpm should drop again as you release it.

  • angrybob

Posted April 12, 2009 - 12:43 PM

#3

The rpm increased when hot start pulled and dropped back down when released.

Just checked resistance on TPS - 890 ohms at fully closed (0-2k spec = OK) 3640 ohms at fully open (4-6k spec = not OK).

Crap ($200). Problem solved?

Would the bike run OK with it unplugged? I unplugged it an there was no change...still popping.

  • grayracer513

Posted April 12, 2009 - 02:01 PM

#4

The TPS provides the information that allows the ignition to be advanced during part throttle operation to make it run cleaner and sharper in that mode, and to boost fuel economy. The bike will run "OK" with it unplugged, but not the best. The TPS won't make the bike pop on decel.

  • REDBULLRICK

Posted April 12, 2009 - 04:00 PM

#5

did you ever change the main jet?

  • angrybob

Posted April 12, 2009 - 05:44 PM

#6

Main is 165, but it feels great at full throttle. I can't imagine that bumping up to a 168 would affect off-throttle/decel popping but I could be wrong.

Next steps:

Carb cleaner to check for leaks around carb isolator and airbox boot.

While its apart, I might as well check the valves since it been a year.

I'm going to have the carb in hand. Besides blowing it out and cleaning, anything in the idle circuit that I should pay attention to? I have read about cracked slides but where?

At this point, by process of elimination, it seems to be a air leak somewhere.

  • angrybob

Posted April 12, 2009 - 05:45 PM

#7

The TPS provides the information that allows the ignition to be advanced during part throttle operation to make it run cleaner and sharper in that mode, and to boost fuel economy. The bike will run "OK" with it unplugged, but not the best. The TPS won't make the bike pop on decel.


Is this a close counts part or does it need to be replaced? Would you replace this TPS?

BTW - thanks for all the input.

  • grayracer513

Posted April 12, 2009 - 06:00 PM

#8

Well, the TPS is out of range, but isn't producing any symptoms that an out of range TPS ordinarily would. For the moment, I would leave it.

Now, I assume from your opening post, this had been going on for a long time, and has perhaps always done this. So let's go back a second.

You are proceeding as if it is definitely a fuel issue. I am going to guess then that you have ruled out the exhaust leaking air as a possibility. True? You're certain?

If so, what size is the pilot jet? Your bike came new with a 42, and I have never seen a 42 work well in an '06 below about 6000 feet. I have a 45 in both mine, and IMO, it's what it should have been shipped with. The main jet and needle have almost nothing whatsoever to do with decel backfire. It's an idle circuit issue.

If it's a 45, look for any blockage or lacquer deposits in the pilot jet (including the cross holes), or in the circuit as a whole. Also, be sure, perhaps before you tear into the carb, that the O-ring on the pilot screw is in place, and is not torn or cut.

  • angrybob

Posted April 12, 2009 - 06:53 PM

#9

Yeah, its been going on for a while.

Blowing out my spark arrestor screen within five or so rides finally convinced me to fix it...sad, but true that I let it go for this long :p . In fact, when I heard how it ran with the choke on with limited popping I said to myself "oh shit...now that sounds good/right".

Pilot is a 45. I have had a 48 and a 50 in there also just to check the extreme, all with popping on decel.

Absolute on the exhaust? No. However, yesterday I replaced the crush washer btwn the header and the exhaust outlet on the head (I did not do this when I installed a new MRD Racing full system). I also sealed the header to tail pipe connection yesterday by using high temp 650F RTV. The seal/bead was intact after the ride. I am 'guessing' that its not exhaust based on this latest effort.

Checked the idle screw o-ring yesterday (zip-ty fuel screw) and it was fine.

My gut feel is that the hot start valve is not fully seated, but it performed as you stated it should. I guess I could shim it with something like alum foil so that its known closed.

The head valve check was simply because its due for a check and its likely coming apart enough to check them.

Hmmm...plug is a year old. Might as well change that too.

  • Weapon X

Posted April 12, 2009 - 06:55 PM

#10

happened on my '08 all I did was adjust the fuel screw

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  • REDBULLRICK

Posted April 13, 2009 - 07:08 AM

#11

Main is 165, but it feels great at full throttle. I can't imagine that bumping up to a 168 would affect off-throttle/decel popping but I could be wrong.

Next steps:

Carb cleaner to check for leaks around carb isolator and airbox boot.

While its apart, I might as well check the valves since it been a year.

I'm going to have the carb in hand. Besides blowing it out and cleaning, anything in the idle circuit that I should pay attention to? I have read about cracked slides but where?

At this point, by process of elimination, it seems to be a air leak somewhere.




iv got a 170 in my crf and my dads yz450 it used to pop on decel pretty bad until we put a 170 main and a 48 pilot in his yz

  • angrybob

Posted April 13, 2009 - 03:55 PM

#12

Thanks. I'm not sure how a main jet bump would alter off throttle decel pop though. It probably got fixed with the 48 pilot.

One other thing...

When I had the exhaust header off the head, I noticed that the exhaust port was gray'ish. I have always seen this as sooty black. Obviously a lean condition somewhere, but does that eliminate an exhaust leak at least at the tail pipe / header pipe joint?

I was going to put on the stock exhaust tonight just for shits and grins and also check for leaks with carb cleaner. Heh - quick and easy stuff first I guess!

  • jmalkames

Posted April 13, 2009 - 05:23 PM

#13

I didnt read the whole thread but i have had my bike for 7 years and there is a few things that i had happen to me. 1 dirty air filter will cause it to pop on decel, 2 over oiling the air filter will make it pop on decel, 3 i had a sheared flywheel key that made it pop on decel, hard to start, and made my spark arrestor screen glow (very hot exhaust). Just some things for you to check but sounds like the flywheel key to me.

EDIT: i just read that its still starting fine so it may not be the flywheel key, but its worth a shot.

  • angrybob

Posted April 13, 2009 - 10:08 PM

#14

Hmmm...I have never had this carb apart enough to where I had the slide in my hand. I'm pretty sure the cover that goes on the slide was installed upside down. I bought this bike used and have no idea of the history except a 48 pilot was installed.

Would this cause the bike to pop on decel?

  • REDBULLRICK

Posted April 14, 2009 - 05:11 AM

#15

we had a 165 in it it still popped so we put a 170 in it problem solved its just a shot its like 5 buks for a jet couldnt hurt to try my dads 06 yz did the same thing



Thanks. I'm not sure how a main jet bump would alter off throttle decel pop though. It probably got fixed with the 48 pilot.

One other thing...

When I had the exhaust header off the head, I noticed that the exhaust port was gray'ish. I have always seen this as sooty black. Obviously a lean condition somewhere, but does that eliminate an exhaust leak at least at the tail pipe / header pipe joint?

I was going to put on the stock exhaust tonight just for shits and grins and also check for leaks with carb cleaner. Heh - quick and easy stuff first I guess!



  • TeamTech

Posted April 14, 2009 - 06:14 AM

#16

As mentioned above, check your air filter...make sure you keep it nice and clean.

  • grayracer513

Posted April 14, 2009 - 07:30 AM

#17

I'm pretty sure the cover that goes on the slide was installed upside down.

Would this cause the bike to pop on decel?

No. When the slide plate is inverted, it will idle, and run OK at more than 3/4 throttle, but crappy rich in between.

  • angrybob

Posted April 14, 2009 - 09:15 PM

#18

No. When the slide plate is inverted, it will idle, and run OK at more than 3/4 throttle, but crappy rich in between.


Ah. Thanks.

Tonight's findings with the valve clearance check - one tight valve.

On the left side (clutch lever side) intake, the clearance was probably 0.085mm. I say probably because I could force a 0.089mm shim stack through barely. Knowing this needs to be fixed, could one tight valve cause popping on the off-throttle decel...enough to be blowing out spark arrestor screens?

Rubber carb ro intake joint looked fine although it did have some fuel moisture in the circumferential grooves of the rubber.

  • grayracer513

Posted April 14, 2009 - 10:14 PM

#19

I could force a 0.089mm shim stack through barely.

If you take an oily feeler gauge, set it on a metal plate and turn a 7/8 or so open end wrench on its side so the weight of the jaw rests on the feeler, the force you feel resisting you as you drag the gauge out from under the wrench is the most you should feel when checking clearances with a feeler.

If you force a gauge under a cam, you can actually lift the valve off its seat.

Assuming it's .08mm, that's less than .001" under spec, and not enough to really produce symptoms.

  • angrybob

Posted April 15, 2009 - 08:34 PM

#20

Good news and bad news.

Good news is that its FIXED! The bad news is that I'm not exactly sure why :p

Last night I checked the valves, cleaned out the carb (again), and wiped down the mating surfaces of the carb joint. Tonight I put it all back together with a 165 main and 48 pilot. I could tell that thing had improved dramatically when I started it up. That said, I could turn the pilot screw in all the way w/o stalling.

I stepped down to the 45 pilot, did some tuning, and no pop on decel. No surge at idle, smooth, and starts on the first kick.

Here's what changed from the last time it was running (poorly with popping on decel):

1. the slide cover was inverted - reinstalled correctly
2. detached, cleaned mating surfaces (had 'some' raw fuel in the mating grooves), and reinstalled the carb joint btwn the carb and the intake
3. blew out the carb everywhere possible
4. prayed to the motorcycle gods to help a brutha out

My guess is as good as anyone's because all of them passed the normal 'checks' to one degree or another, but I am still thinking there was crap in the carb.

All is well except for that tight valve.

Anyway, thanks to all who chimed in. I really appreciate it.

Non-forum folks were beating me daily to "take it to the dealer". Sometimes that is a good idea and sometimes not. I have my own opinions on dealerships but I'll leave it at that.

My main problem is that outside of an electrical problem, there is no diagnostic tool they have that I don't...especially the fresh out of school newb who probably would have drawn my job! Seriously, with the process guidance found here, there is noting they would have done differently except at $60/hour.

Thanks again TT! Now I can go to the Virgina City GP next weekend. Woohoo!





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