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Honda XR650R: Excessive Popping on Deceleration, Fine Tuning and "Uncorking"

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I purchased a 2007 BRP last fall. The bike was purchased from a previous owner who said that the bike had been uncorked, and jetted for higher elevation(8-9k ft) in southern Wyoming. I never rode it at that elevation or heard it run. I brought the bike home to a lower elevation in Wyoming (5k ft) and noticed that after 100-75% throttle to 0-25% throttle (quick decel) the bike had an excessive amount of popping and a continuous pop all the way through the decal (the faster the speed the longer the popping). This is only when using the compression to decal as well. There is no popping on acceleration. It feels to me that it has plenty of power all the way through each each gear with wide open throttle. The PO said that he never experienced any popping on decel (he seemed to be an honest person but take with a grain of salt). At that time I tried replacing the main jet which was a 165 to a 168-172. Each one after 168 progressively made it hesitate on acceleration especially on top end and pop just as much on decel. Maybe a little less in frequency but louder. I left the 168 in due to it performing a touch better.


I recently moved to North Dakota (1.5k ft) and the popping has stayed the same. Today I did a full investigation for myself of the bike and the mods that were and were not done. 


They are as follows:

main jet = 168

pilot jet = 65s

Intake = non-restricting (no D) 

muffler= i think stock with a different tip. Maybe drilled out? see pictures. number on muffler is Honda HM MBN A1



Throttle needle = stock. only 2 clip positions, not 5 like 2006 and before. 

Air Intake = removed restrictions


If you would like any other basic information please request.


My question is where to go from here. 


I just purchased a 175 main jet, a B53E needle which I will put to the 3rd position from top, a 68s pilot jet, new header gaskets, and all new gaskets for the rest of the exhaust system (just in case it's a leak). 


I have not put the items in because I have not received them and I am curious as to what this forum thinks. I have not adjusted the mix screw and do not plan on doing so unless needed later ( need the special tool).


I am a beginner when it comes to working on bikes, but I am pretty mechanically inclined. My goal is to have the bike run well with minimal to no popping on decel. I hope I provided adequate information, and if you want to know anything else please request. 


Any help would be greatly appreciated, especially from Moses the admin. Seems like an awesome guy. I can upload any pics as well. Thanks in advance


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Hi, jtompkin1...You do have an "uncorked" BRP, and the tailpipe opening is part of that package.  The PO had this engine jetted quite rich for 8K-9K feet riding, and the tailpipe insert shows a lot of rich and sooty carbon buildup.  (You need to clean up that insert, it's currently restrictive.)  Your tuning plans are okay for 1,500 feet, I'm running a 172 main, 68 (not S) pilot and a recommended, uncorked needle position...At 4,400 feet elevation, this is acceptable, slightly rich but not enough to foul the plug or fuel-wash the cylinder, a primary concern.  A 165 main would likely work well, I ride to 7,000, even 8,000 feet elevation at times.


Before performing any further tuning, I would perform a cylinder leakdown test as I talk about at our General Repairs forum (click to http://forums.4wdmechanix.com/topic/164-how-to-make-an-inexpensive-engine-cylinder-leak-tester/?hl=leakdown) and in a video at the magazine site (http://www.4wdmechanix.com/How-to-Cylinder-Leak-Down-Test-on-Honda-XR650R-Motorcycle-Engine.html) when I performed a leakdown test on my XR650R's engine.  A cranking compression test is very difficult with the OE decompressor mechanism.  A leakdown test is performed at TDC on the compression stroke with all four valves fully seated, which may require loosening the valve adjusters.  The leakdown test is more conclusive anyway.


Note: I eliminated this decompressor stigma by installing a Hot Cams Stage 1 camshaft.  I start the engine with the manual lever and tickling to just past TDC before kicking through.


The point to this is that tuning makes no sense if the compression is suffering from valves that are carbon coated and not seating fully.  From a tuning vantage, you have the right formula and approach.  The 168 main should be working at your elevation.  You need to start with a leakdown test to at least confirm normal seal.  Check out the resources at the forum and the magazine.  There are also my two streaming how-to rentals at Vimeo On Demand for adjusting valves or rebuilding the top end of the XR650R engine.  See trailers at: www.vimeo.com/ondemand/hondaxr650r.


Objectively, you need to eliminate the possibility of too much carbon in the combustion chamber and make sure valves are seating properly.  A simple test of how the engine runs and the fuel mixture is a spark plug coloration test.  This is a time-honored tuning tool.  If you need feedback on spark plug readings, take a photo(s) and post it here.  I'll reply...


Glad to assist at your getting this great motorcycle in top form.



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