HIGH-TECH: Husqvarna Motorcycle Won't Rev Above 5k RPM, Won't "Accept" More Than 1/2 Throttle


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?

2 strokes are notorious for coking up the exhaust ports and mufflers. And the mufflers are tuned for a specific back pressure and flow rate. Even a slight restriction could cause some extreme performance problems at higher rpms.

Rod Knox,

It is an ultra-high-performance single cylinder 4 stoke engine.
I was going to include that info in the original post, but forgot to.

(Yes, I understand this isn’t “Bike Talk”, but engines are engines, so I’m trying out this forum.)

Thanks though…

Hitting an rpm wall is a symptom of a restricted exhaust on 4 strokes also but it’s usually the catalytic converter on them. Having only one cylinder always makes diagnosing things more difficult but if there is no indication of an exhaust restriction you can check the valves for a bad spring. A week spring will allow a valve to float at high rpm. And while I am not familiar with timing on late model motor cycles there is some system to advance the spark and if it fails the engine will just wimp out as rpms increase.

A Husqvarna forum might jump right on this question though.

When did Husqvarna move from 2 strokes to 4 strokes? I recall the Zundaps and DKW 2 strokes that raced in the fields near me long ago and occasionally there was a Husqvarna.

Pretty much all the dirt bike makers now have added 4 strokes, sometimes eliminating the 2 strokes.

2 strokes are HISTORY…Emissions laws have eliminated them…A few marine outboard engines use direct injection to allow 2-stroke operation…

Thanks all for the input. I’m not sure how a broken valve spring would affect it, and it hasn’t seemed to be spitting back out of the intake (if it were an intake spring broken), but that is an interesting theory.

Four-strokes have taken most of the market for about 10 years now, in dirtbikes weighing about 10-15 pounds more than an equivalent two-stroke, but with a more usable broad power range. In motocross, the 250/450cc four-strokes compete with the 125/250cc two-strokes, twice the engine displacement in four-strokes is roughly compensated by twice the power strokes in the two-strokes. The two-strokes are still sold, though.

Somewhat of an update: I’ve been reminded to try the simple things first, an approach I usually strictly adhere to, but have been waiting for a direct approach to a logical solution before buying parts. It has been suggested that I try a new spark plug, so I have one on order, and should wait until about Wednesday to try the new plug before thinking of anything else.

Re broken/weak valve spring

floating valves seemed to be engineered into Ford 6 cylinders in the 60s. The result was limited rpms. Intentional or not they resulted in a great governor. If the motorcycle has 2 springs on each valve and one is broken the spring tension will not allow the valve to operate at high rpm.

Thanks again for the input. Valve spring is a possibility, but as that article noted, one valve spring in a single cylinder engine will still allow the engine to rev. I do understand the probability of the improbable, :wink: but the chances of all four springs losing tension at one time seems unlikely.
I did enjoy reading that article, thanks for forwarding that.
I did find a new plug yesterday, and had planned to try it out today but it snowed last night, so I am on a weather hold.

Take Care!

One weak spring will cause the problem…

So will a restricted exhaust…

ok thanks. I’m pretty much at the point of opening the engine up to investigate, but as noted, have tried to wait until I have a focused plan in mind before doing so. I imagine if I were to look around at the components (including the CPS pickup which could’ve moved, shifting the timing) I would find something amiss. I just want to avoid doing unneeded work, buying gaskets, etc…

Thanks again.

Also, does the bike have a carburetor? If so what type? The dashpot on CV carburetors can cause a problem such as yours.

did somebody put something in your exhaust pipe?

The issue arose when riding down the freeway, so unless someone is pretty accurate with a potato gun I don’t imagine that something in the tailpipe is the issue…but I had thought of that…and would check it if it were easy to do so…but it idles so well and all I’m doubting that that is it…

I tried the new plug today and no change. It starts on one kick and idles perfectly, just doesn’t like wide open throttle and has the RPM ceiling. Tomorrow I plan to try some new gas just to be sure that isn’t an issue, but I think it is time to open up the valve cover to check for broken valve springs, since that is the next-easiest thing to check.

I don’t know what kind of carb it has, but could look it up if it is important. I don’t see how a dashpot would affect it in that way or that it would have one–doesn’t a dashpot just slowly close the last little bit of the throttle slowly, for emissions reasons?

Many motorcycle carburetors are constant velocity/variable venturi. You might check your carburetor for make and model and investigate whether it is a CV type. A sticking dashpot can be quite simple to correct. But if the problem suddenly materialized while driving I would suspect a restricted exhaust or a valve problem, likely an exhaust valve due to no back fire.

Clogged spark arrestor?

Surprisingly enough it isn’t a CV (constant-velocity) carb. The slide in it is pulled up the old fashioned way, not by engine vacuum.

It has an aftermarket exhaust which is quite loud…just a huge inside-diameter glass-pack muffler. I stuck a stick down it just to be sure it didn’t have something that came loose inside and lodged itself in there, but it was wide-open. I suppose that it could always be plugged somewhere upstream…but I think that that is pretty unlikely. It does idle perfectly.

I think that I should be able to actually just remove the smaller valve-adjustment openings and push down on each valve/rocker arm to feel if there is consistent pressure in all of them, rather than removing the valve cover itself which holds the rocker arms on this single-cam engine.

I plan to try that soon. Otherwise, it is a long-shot and I don’t know how it would explain the engine noise that it initially had (but it doesn’t do any more after cooling and restarting a week later), but I think that included in all of the goofy things a bad coil can cause is a weak spark/rev ceiling like this…so may explore that option as well. They can be tough to test accurately when not completely failed.

I would definitely take the carburetor apart and clean it, and then clean it and perhaps clean it again. It takes very little to create a too rich or too lean condition, and especially too lean can cause the performance issues you describe as well as overheating, which can result in all sorts of very bad noises. Don’t go tearing into the engine until you absolutely know for certain that the carburetor is clean and doing what it should. If there is a filter in the fuel line, replace it. If the fuel lines are original equipment, replace them. All these things are cheap and easy, and they should be done first.