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XR200 XR200R Idle Issues

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I've tried for days to get this bike to achieve a stable idle.

  • Even after a 5 minute warm-up it will only idle with the choke on.
  • With choke off, it will die right away unless I keep tweaking the throttle.
  • The closest I've gotten requires the idle screw to be turned almost all of the way in.
  • I have tried various Pilot screw combinations from 2 turns out to about 3.5 turns out.

In general I think it is running way to rich.  I put an exhaust gas analyzer on it and the air to fuel ratio is around 11.  When running around the neighborhood, I get significant back fires when I go off-throttle which would indicate an over rich situation.

I got this bike from a remote family member and maybe they only rode it at low altitude.  I'm just outside of Denver Colorado at about 5500 ft.  I've avoided removing the carburetor to check the number on the main jet (stock is 138).  Perhaps I need to go to a 132 or so.

Is there any other suggestions or diagnostics I can do to troubleshoot?  Was thinking of removing the air cleaner temporarily to see what a slightly leaner mixture would do.  I did have the carburetor off the bike at one point when I was painting the frame and stuff.  It looked remarkably clean.  And I tried checking the timing but just got a face full of oil spray.  Could never see the timing mark.

I'm pretty frustrated.

BTW it is a 1983 model with the Keihin PD28A carb.

 

 

 

 

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3 hours ago, VortecCPI said:

Fuel/Float level way too high?

Perhaps slide way too high and slow/pilot plugged?

I will check those things.

Elsewhere someone stated that when you experience exhaust popping when you close the throttle and return to idle RPM, that it is an indication of a too lean situation.  I don't really understand the logic of this.  I thought that was indicative of too rich with the unburnt fuel igniting in the exhaust stream.  Can anyone explain the logic of this?

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7 hours ago, GeorgeBC said:

I will check those things.

Elsewhere someone stated that when you experience exhaust popping when you close the throttle and return to idle RPM, that it is an indication of a too lean situation.  I don't really understand the logic of this.  I thought that was indicative of too rich with the unburnt fuel igniting in the exhaust stream.  Can anyone explain the logic of this?

http://www.mikuni.com/tg_backfires_in_exhaust.html#:~:text=The%20popping%20is%20a%20result,system%20have%20rather%20open%20muffle

Old School Al

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Sorry for my lacking computer skills................google mikuni.com backfires in exhaust.   They explain how this happens very well.  

Old  School Al

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12 hours ago, GeorgeBC said:

I will check those things.

Elsewhere someone stated that when you experience exhaust popping when you close the throttle and return to idle RPM, that it is an indication of a too lean situation.  I don't really understand the logic of this.  I thought that was indicative of too rich with the unburnt fuel igniting in the exhaust stream.  Can anyone explain the logic of this?

Both overly-lean AND overly-rich conditions can cause Decel Pop.

In the case of HD MCs the Delphi ECU is mapped too lean at 0% MAP and the result is Decel Pop.  We get rid of it by lowering AFR making it richer.

Here is a great explanation from TTS:  http://www.mastertune.net/support_tuning_problems.html

image.png.67b7e8711fe07d6b84dc16496427ee7a.png

 

Here is another explanation from Dynojet:  https://www.dynojet.com/amfile/file/download/file/e1dbb467bb44428fdc637a1b1ff6d47f/category/115

Edited by VortecCPI

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I have a couple of PD28a's, one on a 218cc, and the other on a 233cc.  My house sits at 5000 feet, and I commonly ride up to 10000 feet.  This carb in good condition will work just fine on your bike.  They are fairly forgiving in nature and jetting doesn't need to be spot on to run decent.  I was lucky and found a new in box NOS 28a for my 233 engine, and I ran across a NOS Honda rebuild kit for the one in the 218.

If I had to guess I would imagine that your nearly 40 year old carb has a very worn needle and needle jet which can no longer control the flow of fuel properly.  No amount of jetting changes can fix that.  And clogged passages in the tiny ports for the idle circuit are almost always the reason why they wont idle.

You can order a PZ30 clone carb for cheap on ebay, the quality seems to be pretty good these days.  You may find a new PD97a which would be the Honda replacement for your factory carb, but they are kind of expensive and availability can be spotty.

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13 hours ago, GeorgeBC said:

I will check those things.

Elsewhere someone stated that when you experience exhaust popping when you close the throttle and return to idle RPM, that it is an indication of a too lean situation.  I don't really understand the logic of this.  I thought that was indicative of too rich with the unburnt fuel igniting in the exhaust stream.  Can anyone explain the logic of this?

The way the fuel circuits in these carbs work make it unlikely for it to be super rich when throttle is closed. If in good condition the needle will pretty much seal the main jet opening. This only leaves the pilot jet and fuel screw contributing. In practice it is much more common for these circuits to be clogged causing a lean condition than the other way around.

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