cthowards

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Feb 13, 2006
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I recently replaced the valves, springs, etc. along with a new top end (Wiseco 13:1 high compression piston) on my 2005 CRF450R. I initially put .6 hours on it during break in and then changed the oil and parked it for the winter. It was 50 degrees today here in Connecticut so I went riding. At about the 1.9 hour mark (per the hour meter) while riding today at a local MX track, the bike started to sound noticably different (lots of popping on decel) and began to run very rough. It was fine all day and ran smooth, no popping on decel, no overheating, nothing. After making it through a rhythm section of jumps, I pulled off and headed back to the trailer (I was spent and could barely hold on anymore anyways!). When I got the bike home, I checked the plug and it was very white (indicating a lean condition). I also checked the air cleaner and boot and both were spotless.

When I broke the bike in one day in November, I set the fuel screw to 2.25 turns out (that is where it ran the smoothest with the idle up high). Since it was slightly colder today, I backed it out another .25 making the screw set at 2.5 turns out. Other than the high compression piston, I also have a White Brothers Carbon Pro exhaust system. The jetting is stock. Any ideas what might have happened and what I should do to diagnose?
 

BSWIFT

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N. Texas SP
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Nov 25, 1999
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Recheck the valve clearences.
 

whenfoxforks-ruled

Old MX Racer
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Oct 19, 2006
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Merrillville,Indiana
Turn your carb upside down and check that smallest o ring ever made! When removing the mixture screw its replacement is hard and crucial, ap adjustment is also crucial, as it covers up jetting, I'm running a 48 pilot and 178 main for about the same weather(yes its fat). Try 1up on the pilot and at least 2 on the main and no air leaks allowed! Need to get to the rich side for safety and go from there.
 

cthowards

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Feb 13, 2006
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I will check the valves but I am hopeful they are in spec (only 1.9 hours on new Kibblewhite vales and springs, etc. - a local shop installed the valves and handled any lapping, etc. and set the clearances).

In connection with the rebuild, I installed a Ride Engineering fuel screw and it feels looser in general than the stock and it also vibrates out so I have it wired in place. I learned from prior threads that I searched that the spring should be stretched out to keep tension on the screw. In any event, I will definetly go 1 up on the pilot as I am near the end of the current adjustment (2.5 turns out which is very close to the max of 3 turns) and will also consider going up on the main. While I'm at it, I check that o-ring and spring. I may even put the stock fuel screw back in...just feels much better.

Do you think any damage was done to the motor (piston/rings)? I didn't see any flakes of metal on the plug even though it was very white.

Thanks for your replies.
 

Rich Rohrich

Moderator / BioHazard
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Jul 27, 1999
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cthowards said:
Do you think any damage was done to the motor (piston/rings)? I didn't see any flakes of metal on the plug even though it was very white.

Thanks for your replies.

Don't lose sleep over this. A) Plugs are supposed to be white when they are running at the proper temperature especially in a four-stroke engine & B) you can't hurt a big four-stroke by running it too lean around an MX track unless it starts going into serious detonation, and it's highly unlikely your CRF did that.

Here's what a properly jetted two-stroke plug should look like :

16241-Justaboutperfect.jpg


Without all that oil in the chamber a four-stroke plug will be much cleaner.
 

cthowards

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Feb 13, 2006
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lwsmithjr, you were correct! I checked the head pipe and both bolts were loose and the head pipe could be moved around if I pushed on it (definetly enough to cause a leak). It has been very cold here now in CT so I have not run the bike yet but I am assuming this may be the primary cause for the sudden change in how the bike ran. In any event, I will also consider some jetting changes as I think I am too lean.

Thanks for all the replies.
 

lwsmithjr

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Sep 18, 2002
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Most recommend you replace the crush washer between the pipe and cylinder every time you pull off the head pipe. Personally, I re-use them once or twice (because I forget to buy one while I'm at the dealer!), but they do wear out.
 

cthowards

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Feb 13, 2006
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After I rebuilt the motor, I put a new gasket in before installing the pipe. Everything was so clean when I started riding again that there was no natural thread lock (i.e. dirt and hardened mud) to hold the nuts in place!
 

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