Roll-on break down
 

Folks,

Just recently, on my '00 SV650S, I am having trouble with engine revs breaking down when rolling on the throttle from part-throttle.

It occurs when I have been cruising at a constant speed and then open the throttle slightly, to overtake for example. When I open the throttle, there is a hesitation, revs drop briefly, then it picks up and carries on as it should.

Nothing has changed with the bike at all, same fuel, about 1/2 way between service intervals, weather is a perhaps a little cooler at around 10 degrees celcius.

Any advice on where to start looking?

Cheers,

Chris

Do you have- race system, slip-on, air filter? That's usually a ymptom of undersized main jets...

May I suggest a quick look at the spark plugs?

Not sure how long you were running at constant-throttle, but that can allow deposits to build up on the plugs (particularly if the engine is a bit worn and using a little oil, or if it is running rich for some reason such as air filter condition). These deposits then cause a misfire when you open the throttle, and it's made worse if the plugs themselves are old, not gapped correctly - or if you are using (for whatever reason) a colder heat range plug. Sometimes, running at the same speeds, but in a lower gear (ie higher revs), for a few miles will burn off those deposits.

However, a quick look at one of the plugs (the rear one....it's easier!) will usually give you some indication. Look for a rounded center electrode, widened gap, heavy dark brown deposits on the ceramic insulator (NOT the steel plug body).

The proper cure is to decide on, and fix, the cause but new plugs - espec. the use of "precious metal plugs" eg platinum, iridium, gold palladium etc - may also be beneficial.

Hope that helps.

Possibly slight carb icing?


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Weather is too warm for that. Sounds like leanness as Northy says. Try putting on a bit of choke and doing it, if it doesn't do it then it's running lean and needs sorting on a dyno (assuming you do have a non-standard exhaust).

Well he's saying it's only 10°c, which in the right (or wrong) conditions can certainly bring on a little carb icing from my experience. It might not be this of course, but riding for a fair distance on a constant throttle is definitely a criterion to cause ice build up if the ambient conditions dictate.


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Have you checked your choke cables? At that age they may well be a bit sticky and they don't always get checked/lubed on a service.

Gold star for Jonboy :-)

It didn't do it on the way home last night. This morning, I listened to the weather report on the radio before leaving home, and it was 2 degrees celcius this morning. The problem was worse than yesterday, the idle was really low, and the engine would stall at traffic lights if I didn't keep the revs up.

Hopefully this cold snap is nearly finished.

Thanks for your replies folks.

I think I ruled it out from sheer prejudice :) Nice one Mr Boy. Silkolene FST Pro is the daddy for carb icing, by the way.

Yes it works very well, but is hellishly expensive. Also, a higher octane fuel (98 RON plus) does a very good job too, and is a little cheaper.


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