07 RM450 Bog and backfire

[Tapkawiman]

A friend brought his bike over to fix the above mentioned problem. It has a major bog off idle and I know that 4 strokes do have this problem hard off idle, then when you let go of the throttle it back fires with a rich condition(flames coming out)He claims that this started after landing fairly hard from a jump. I have checked vavle clearance the exhaust system for leaks the intake system for leaks. Changed the plug(didnt think that was it but?). Checked all electrical systems for connections and grounding. I just finished taking the carb off and making sure everything is still set to specifications. I have ran the adjustment screw in and out including setting it to spec for the power valve as well as changing pilot jet and clip positions on needle(of which none of these things have made even the slightest change(good or bad).


By the way the bike has low hours on it. And he seems to take good care of all his stuff. But he is one of the yahoo's that cant leave something alone he just has to mess with it.

One thing is that it will not surprise me that once I find the porblem and tell him he will be like oh yea I forgot I messed with that. If anyone has ever had anything even remotely close to this I would greatly appreciate any help.
:bang:

 

[ropethebike]

I don't have any idea what the problem might be.........but I do have a thought.........

Sounds to me like it might have jumped time? Is this possible? Is it possible to land hard enough, if the chain has some slack, for the timing chain to jump a cog or 2??

I think the valve clearance can spec but if the timing is off a little could create the symptoms that you describe....


Disclaimer: since I know nothing about 4t mx bikes......if this isn't possible or the rmz doesn't have a timing chain then please disreagrd my thought on the grounds of ignorance...ha

 

[Tapkawiman]

Bump

 

[hot125mod]

It is more than the pilot fuel screw and leak or bleed jets it is an fcr carb and they always seem lean on the bottom.