FCRMX39 -- Too Lean?


10 replies to this topic
  • mrgem

Posted January 27, 2014 - 07:36 AM

#1

I am trying to sort out this 2003 WR450F I just bought so that it runs well. When I bought it, the carb was completely gummed up from having sat in a garage for more than 3 years. I took it to the Yamaha dealer and he says  he rebuilt the carb and jetted it appropriately for the high-altitude riding we do here. 

 

Well, I got the bike home and it starts easily -- first kick -- as long as the choke is used (hot or cold), but when I ride the bike, it pops and backfires on deceleration. That suggests a pilot circuit that is too lean to me.

 

It's kinda obnoxious as the FMF Titanium exhaust (including a "Power Bomb" header) on the bike is quite loud (despite having added FMF's "Quiet Core" insert and their Spark Arrestor/End Cap). 

 

I live in the mountains at 8200 feet above SL and if anything, the bike should be on the rich side -- because the air is thinner here. At least that's how it seems to me.

 

Do I take the bike back to the dealer and ask that he put a bigger pilot jet in it -- or is this backfiring just the way these things run?

 

TIA for your input.



  • flyandride

Posted January 27, 2014 - 07:51 AM

#2

Did you try adjusting the fuel screw?  Also you may want to lower the clip a notch on the needle.  



  • grayracer513

Posted January 27, 2014 - 09:12 AM

#3

Inspect the exhaust for air leaks.  If none are found, try adjusting the idle mixture.  If that fails, you are either jetted lean on your pilot jet, or the pilot jet may be partly obstructed by a varnish deposit.  Adjusting the jet needle position won't help this problem at all.

 

http://www.thumperta...t/#entry2983539

 

http://www.thumperta...-mixture-screw/

 

http://www.thumperfa.../jetting.htm#PJ

 

http://www.thumperta...ircuit-setting/

 

http://www.thumperta...o/#entry6879695



  • Krannie McKranface

Posted January 27, 2014 - 12:52 PM

#4

The dealer probably did not clear the passage above the pilot jet, which is still partially clogged, making it extremely lean at idle.

...or they did not actually replace the corroded pilot jet, but cleaned it, which you should not do.

 

This is very common with a neglected carb.

This is a very common poor service issue with dealers.

Never leave the dealer without completely testing out all of their work.

 

You need the dealer to tell you what jets they installed. The probably went 'stock', which is WRONG.


Edited by TheKoolAidMadeMeSick, January 27, 2014 - 12:53 PM.


  • William1

Posted January 28, 2014 - 11:37 AM

#5

You also should check the ACV (decel valve) it adds a little extra fuel under high vacuum situations (like during high rpm/closed throttle) to help reduce decel pops.

 

You should only need the choke for cold starting.



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  • Krannie McKranface

Posted January 28, 2014 - 12:42 PM

#6

Yes, the ACV valve has a plunger/spring that can hang up if dirt gets in the carb.



  • grayracer513

Posted January 28, 2014 - 02:11 PM

#7

Things are so much simpler if you avoid the WR carbs altogether and run one from an '05 or later YZ450 instead.



  • mrgem

Posted January 28, 2014 - 05:17 PM

#8

You also should check the ACV (decel valve) it adds a little extra fuel under high vacuum situations (like during high rpm/closed throttle) to help reduce decel pops.

 

You should only need the choke for cold starting.

Hmmmmmm. Sounds like a ton of variables could impact this condition. 

 

Went back to the dealership and asked the tech which pilot jet he put in it. He said he'd only changed the main jet -- and that was changed from a 145 to a 168, as I recall. He remarked that the MJ in the carb was wayyy too small for Colorado.

 

When I told him that the bike was popping and backfiring on deceleration, he wasn't surprised, He said the problem was likely my FMF pipe. He said that WRs all backfire with low-restriction pipes in Colorado and that it was a basic incompatibility between these free flowing exhaust systems and the carbs on WRs at altitude.  Seems like a knowledgable guy, and has been more than willing to answer all my questions, but his explanation seems a bit counter intuitive. When I asked about adding an idle air adjusting knob, he said that they weren't needed, and that he'd adjusted the idle air when he rebuilt the carb. 

 

Now, I am sure this tech has forgotten more about this bike in the last 24 hours than I'll ever know, but everything I've ever learned suggests that popping and backfiring on deceleration are usually associated with a lean condition in the circuit that is in play when the throttle is closed. Going up 3000 feet in elevation (my house -- where I was doing the test riding -- is at 8200 ft and the dealer is at 5300) by my reasoning should make the bike LESS likely to pop and backfire -- because the lean condition is remedied somewhat by the thin air. But I may well be confused.

 

So now, I am more than a bit flummoxed. Do I chuck the pricey FMF pipe and replace it with an original Yamaha exhaust? Do I pull the carb apart myself and see if re-cleaning the idle circuit does any good? Should I add an external idle air adjusting knob? This shop and tech have an excellent reputation. But I cannot afford to toss a very expensive exhaust system -- only to find out that my problem is not fixed.

 

Can someone elaborate on the ACV valve a bit? I can recall one on an old Type 1 VW carb I rebuilt years ago, but I don't know anything about them on FCRs. 

 

My goal here is only to make this bike reliable enough to have confidence in -- no matter how far from civilization I happen to be. 

 

At any rate, thanks for all the input. 



  • Krannie McKranface

Posted January 28, 2014 - 05:30 PM

#9

The Automatic Coast Valve kicks in when the throttle is closed, bypassing the pilot circuit to provide a richer circuit, only when the throttle is closed.

It's just a check valve system.

You can by pass it by blocking it off and changing your main air jet, but I have never found that to make a difference.



  • William1

Posted January 28, 2014 - 05:33 PM

#10

The tech is pretty much correct about the exhaust. A muffler (the OEM WR one is very good) job is to muffle sound, this includes decel pops/backfires. Nearly all engine pop on decel, just a quiet muffler, you cannot hear it. As others states, make sure you have no leaks (test with the bike running, hold a wet towel over the end of the muffler, if you hear hissing, that is a leak) and ensure the packing is good. A more open muffler is not going to muffle well and is designed to achieve a certain max decibels at speed and not necessarily to make for a 'gentlemanly' riding experience.

 

Fuel screw adjustment is for getting a perfect idle. At the top of the forum here, is a sticky about fine adjusting the fuel screw. Bench adjustment only gets it close unless the person is lucky. The extended fuel screw makes it a super easy job.

 

The ACV is simply a little diaphragm valve on the side of your WR carb. Your owners manual covers checking it in good detail.



  • mrgem

Posted January 28, 2014 - 05:47 PM

#11

Fuel screw adjustment is for getting a perfect idle. At the top of the forum here, is a sticky about fine adjusting the fuel screw. Bench adjustment only gets it close unless the person is lucky. The extended fuel screw makes it a super easy job.

 

The ACV is simply a little diaphragm valve on the side of your WR carb. Your owners manual covers checking it in good detail.

 

Thanks, William to you, KoolAid and the others who've weighed in., It is good to know you and the dealership concur. Looks like I have some more homework to do. 






 
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