The age old Wr450 backfire 2004


3 replies to this topic
  • Gofres

Posted August 28, 2015 - 04:14 AM

#1

Hi guys,

I know this topic has been covered to death and I've read all the posts but still going round in circles trying to resolve this.

Since I bought the bike it backfires when snapping the throttle shut from med /high revs.

I've just reshimmed the valves and rebuilt the carb using the 6 sigma jet kit. The bike starts, ticks over and runs very nicely after much tinkering but I seem to be swaying between too rich (to the point of fouling the plug) to too lean where it back fires.

There is also some bog when snapping the throttle open from idle but not noticeable on a ride (it just wheelies like crazy)

It has a yamaha exhaust but I'm guessing it's from a yz as there are no baffles and no signs there ever was one?

Airbox has been modded with side panels cut out.

Carb is currently set with mixture screw 1 3/4 turns out, pilot jet is new from the kit, main jet is 172 (up from previous 165), needle is on second ring from top, leak jet has the blocking screw, and pump mod o-ring in place. I also have a 175 and 178 mj to hand.


My question is, in what order should I make changes to the settings? I have tried so many combinations and now starting to lose my marbles!

  • Krannie McKranface

Posted August 28, 2015 - 05:23 AM

#2

1. Those jets are junk. Unless you use genuine Keihin or JD jets, you have NO idea what the actual jet value is, as they are not flow tested. The 'good' jets will be stamped with a brand name.

2.  Closed throttle popping is a function of being too lean on the pilot circuit, which includes the ACV system, the hot start, the pilot jet, the exhaust junction seals, and the muffler packing

     Until you address all those areas (the ACV is probably fine, so skip that) you can't know for sure what is causing it. Since the ACV is designed to ELIMINATE  this issue, you must have a MASSIVE air leak.

     Most likely, your hot start plunger is not sealing, due to corrosion of the plunger and cavity, or the cable. This is very very common. Remove, clean, polish, and anti-sieze everything. Replace the cable nut with an alloy

     one, as the plastic ones crack the first time you rotate the carb.

3.  175 is too large a main jet unless your head is ported.

4.  The stock accelerator pump set up is tamed down on purpose. You need to upgrade the parts to bring it to life: The link in this post are to the parts needed.

      http://www.thumperta...ew-and-jetting/

5.  The slide plate and seal must be fresh, and the needle jet (emulsion tube) must not be worn out. They are typically worn out at about 200 hours. Replace them all.

6.   Fouling a plug (black) is not a function of 'rich' jetting on a four stroke. If it were that rich, it woud 'gurgle' and bog constantly. You have oil burning issues. Probably the valve stem seals. A leak down test

        would narrow it down, but there is no doubt of a head or ring wear issue somewhere if your plug is black. 


Edited by KRANNIE, August 28, 2015 - 05:25 AM.


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  • Gofres

Posted August 30, 2015 - 11:07 PM

#3

Hi Krainne, thanks for your reply although it's not what I wanted to hear.

I rode the bike this weekend, it was running nicely on sat, but had the occasional pop on throttle off. On the Sunday it was a bugger to start, power was way down and after a couple of hours it died completely. Checked fuel flow, and spark which were fine but the plug was black. Also noticed soot deposits in the exhaust.

As you suggested, it seems it may be burning oil.

Do you suggest scraping these jets then?

All I can see is a big bill coming my way :-(

  • Krannie McKranface

Posted August 31, 2015 - 05:09 AM

#4

Hi Krainne, thanks for your reply although it's not what I wanted to hear.

I rode the bike this weekend, it was running nicely on sat, but had the occasional pop on throttle off. On the Sunday it was a bugger to start, power was way down and after a couple of hours it died completely. Checked fuel flow, and spark which were fine but the plug was black. Also noticed soot deposits in the exhaust.

As you suggested, it seems it may be burning oil.

Do you suggest scraping these jets then?

All I can see is a big bill coming my way :-(

 

It's all about motor hours and preperation.

 

The motor will run 500+ hours with little or no prep......or 50, if not prepped correctly to begin with.






 
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