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Huskqvarna SMR 570 Won't Rev/"Accept" <1/2 Throttle Opening

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Please just technical/focused/knowledgable answers.  I am a decent mechanic, so anybody's "guesses" have probably been considered.

I have a 2001 Husqvarna SMR 570.  While riding on the freeway it began to make an intermittent loud noise from the engine, first every 5 seconds or so, then every 3 seconds or so, and had a loss of power.  Exiting and quickly returning home, it made the noise at idle as well.  Running it a week later, now it does not make the noise.  Judging by kick starting it where the engine feels as though it has locked up until the compression release has bled off a little air, it has the usual extreme compression.  It idles slightly lower but pretty well, and has some power loss at lower throttle positions, then "will not accept larger throttle openings"; refusing to rev over about 5k RPM where it booooogs until the throttle is closed a bit.  I did once hear a pop while closing the throttle.

I opened up the carb to see if a fuel passage had become blocked of if the main jet had fallen out causing a lean mixture, with pre-ignition being the previously heard noise, but it all looked fine.  I then rode it with the choke on, and alternately with the fuel off, to see if a richer mixture, change in float level, or leaner mixture (as it almost ran out of fuel) might change the running, but the condition of not accepting throttle existed exactly the same under those conditions.

My only guesses are that the noise was from a temporarily-failed timing chain tensioner, causing the slack timing chain to jump a tooth and change the valve timing, or that somehow the ignition timing has changed (maybe the computer not advancing it?).  I have not checked either of these.

Any ideas?



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Oops, I meant > (greater than) 1/2 throttle opening.  Also, If an engine is bogging I can generally tell by the sound whether it is too rich or too lean...and in this case I couldn't say if that is the issue, much less which of those it would be.

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