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SteveJC

XR600 Jetting Glitch?

8 posts in this topic

Hi all,

Just finished sorting out a 1998 XR600 that needed a bit of attention to get it road-worthy again.

Runs really nicely now apart from a lean-feeling hesitation when re-applying throttle after decelerating down to low-ish revs. Otherwise it pulls cleanly and strongly from idle to max.

New air-filter, carb stripped and checked, all in good order, right jets, valves OK.

Is there any known problems with this bike? - I suspect the coast-enricher, or at least that is the only thing I can think of that might be causing this.

BTW - I am comparing to my XR650 which has near-faultless jetting.

Thanks!

Steve

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How are you "re-applying" the throttle? Slow roll-on or fast crack?

Almost all the XR600s I hear and read about have a hesitation transitioning off of idle or closed throttle. My slide just stiffens up from vacuum until I open it a little. Other than that, I have to TRY to make it hiccup.

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The hesitation is when re-applying quite slowly, or at least in what I would call a normal way. It definitely isn't right. I know what you mean about the slide going stiff from the vacuum, but that is another issue - the XR650 is actually worse in this respect though on mine the throttle mechanism is a bit smoother operating being a newer machine.

I am guessing an air leak is the cause, either round the head/carb manifold join or via the air cut-off valve - these are the areas where air could get sucked in when decelerating with throttle shut or near-shut. And presumably when you accelerate with throttle more open enough air is going the correct route through the carb to counter-act any small leaks.

On a run last night it ran pretty crap - temperatures near freezing which doesn't help an air-cooled engine. But always accelerated as clean as could be from idle - weird! :cry:

Steve

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Leaks are definitely something that could cause problems, but out of curiosity, how many turns on the screw and what jets do you have?

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152 main, 62 slow, needle 3rd notch, and screw set at 2 5/8 turns out, but have turned it 1 turn either way without much effect. Got a fruity but not unsociable Arrow exhaust.

Jetting feels absolutely spot on when rolling it on from idle, right up to max revs, front wheel rising in 2nd under power from low revs. But at a steady 1/8th or when re-applying from deceleration its feels really lean and snatchy like a cold engine with not enough choke.

Stripped the carb down again last night - nothing blocked, broken, leaking. Float height at 14/15mm. Checked manifold and slapped a bit of sealant on cylinder head join for good measure.

Didn't get a chance to ride it but not confident much is going to be different!

Steve

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What's your plug look like? I'm guessing you're pretty lean. As I recall on my 600 I ran a 65 pilot w/ 165 main, needle in the center and about 2 turns on the screw w/ Uni filter and FMF Q. No hesitation, popping or backfiring and plug was slightly dark.

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YEESSSSSSSSSS!! :cry::cry:

Fixed it!!!! What a happy boy I am :cry:

I could have sworn I checked the needle position but must have mistaken it with a XR400 I was tinkering with at the same time. Anyway, checked the needle and its was set with clip in lowest position, ie very rich.

I guess this had been done by a previous owner trying to compensate for the too-small main jet (#152).

So I drilled out the main to give approx #160 (1.6mm) , dropped the needle to its middle position, reassembled and off up the road. 10:30 at night, pitch black and freezing cold, but, boy what a difference - smooth as could be.

Weird thing was why it felt like it was too lean previously, not too rich, and putting the choke half-on didn't make it worse. Looking back it makes more sense as the engine ran better for the first minute or so when cold and then got worse as it heated up.

So, next plan is to buy a couple of proper main jets and see what works best - the Arrow exhaust is straight-through but has a relatively small opening in the tip - approx 1 inch. I'll stick in a #165 and see how it goes.

Thanks for the replies BTW ! :cry:

Steve

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