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Jetting advice for XR600R stock carb with 99mm piston needed please

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Hi all

 

Just rebuilt it, and its way lean, and I'm looking for advice on jetting

 

Specs:

 

99mm piston (615cc or so), stock compression

Stock everything else engine-wise

No snorkel

 

It has 155 main and  I think a 59 pilot jet

Needle 3rd slot from the top

Pilot screw 2 and 5/8 turns out

 

 

 

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How lean is it?

 

No one will be able to give you exact jet sizes; trial-and-error on your part will be necessary. Fuels vary across the world, air density, and even individual bikes. I would start somewhere around 62/162 and see what it wants from there.

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Yeah, looking for a starting point, but its super lean at idle, if I rev it almost dies, and that with the pilot screw 4 turns out, and its lean at low throttle openings/rpm

 

Thanks for the reply!

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Double check your pilot jet.  I've never heard of a #59.  But try a #62 or #65.  #62 will give crisper jetting, but #65 will give less bog on acceleration.

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Double check your pilot jet.  I've never heard of a #59.  But try a #62 or #65.  #62 will give crisper jetting, but #65 will give less bog on acceleration.

I found it odd as well, I must have mis-read it when I took it off. It might have been a 55, pretty sure about the first 5. Will try getting a 62+65

 

What size main jets should I have in hand? (I will return those not needed after adjusting)

 

Seal level, about 25C

 

How did it run before you rebuilt it?

Lean as well, but not so much (air filter was stuffed and had lost quite a bit of compression, 98mm), and the pilot screw was tightened by the previous owner for some reason

 

 

 

Thanks for the replies!

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I checked today, it had a 65 actually, but I swapped it with a brand new 65 (honda OEM) and got a considerable improvement

 

So I'm thinking going to 70

 

I forgot to mention it has a loud exhaust

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I checked today, it had a 65 actually, but I swapped it with a brand new 65 (honda OEM) and got a considerable improvement

 

So I'm thinking going to 70

 

I forgot to mention it has a loud exhaust

 

Was the old 65 pilot jet a real Keihin part? The fact that replacing it with a new one of the same size made things better tells me that it was either clogged or not the same 65 (0.65mm) size as a real Keihin.

 

I have a 68 pilot in mine and it is nearly too big for ~1000m above sea level and all the mods I have, so I tend to doubt that you will need a 70 pilot. Stranger things have happened, however.

 

Since you mentioned that it has aftermarket exhaust, 65/165 might be a more suitable starting point.

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It has the keihin star, and then 65 (does it have to have an S at the end?)

 

I had it cleaned, and checked in light, and didnt see any obstructions, but it was quite old, makes me think that it was the original one

 

Could this be float-level related? I think I set it at around 13.5-14

 

When I rev it in neutral, it drops below idle, which means its lean

 

I got the best result, with the pilot screw at 3.5 turns (didnt go more, because it felt loose), and the fact that the pilot screw affects it, makes me think its the pilot jet

 

I guess it wont hurt to try a 70 and see what happens

 

Thanks for the advice

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The 65S has smaller emulsion holes which make it run slightly richer than a normal 65, but no, you do not have to use a 65S.

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Your pilot jet may not need to be changed.  But it has been my experience that, when you go bigger with the bore, go smaller with the jet sizes.   If you only changed the bore size, and no cam, intake, or exhaust mods.  The increase in cylinder volume, creates more air velocity pulling more fuel out of the jets.   

 

If you just changed exhaust, or air filter you would increase jet size.  These mods increase air flow but not velocity.

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Maybe try moving the needle up (clip down) one more position and see if it affects things at idle before you go to a 70 pilot. If it lowers your idle speed, you will know your on the right track. Depending on how exactly your carb is tuned you may end up having to go down one pilot size after moving the needle. Or it may not affect idle and you may need the bigger pilot.

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Your pilot jet may not need to be changed.  But it has been my experience that, when you go bigger with the bore, go smaller with the jet sizes.   If you only changed the bore size, and no cam, intake, or exhaust mods.  The increase in cylinder volume, creates more air velocity pulling more fuel out of the jets.   

 

If you just changed exhaust, or air filter you would increase jet size.  These mods increase air flow but not velocity.

Thats normally the case, but from experience, this is more valid for the main jet than the pilot jet, In a DRZ that went from 400 to 440, had to increase the pilot jet as well

 

 

I ordered a 70 pilot and a 165 main, but I'm still with the 65S and 175

 

At the moment, it behaves very well at idle, because when you rev it, it quickly drops back to the idle revs

 

But when you go steady 1/6th of the throttle, it misses, if you open more its ok

 

I checked the pilot screw, and its slightly bent, straightened it as much as I could

 

I get the best result, with the pilot screw fully closed

 

Previous owner says he had this issue, but to a lesser degree

 

 

Maybe try moving the needle up (clip down) one more position and see if it affects things at idle before you go to a 70 pilot. If it lowers your idle speed, you will know your on the right track. Depending on how exactly your carb is tuned you may end up having to go down one pilot size after moving the needle. Or it may not affect idle and you may need the bigger pilot.

 

I think its a good idea, I will try it next, its 3rd from the top, should I go 4th or 5th?

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Hmmm, now you have me confused. I thought you were lean at 4 turns out on the pilot, now you say you have it screwed all the way in? I dont want to give you bad advice.

Lets start fresh, the bent pilot screw could be your issue but lets try going through the routine anyway.

So starting with it idling good with the screw all the way in, what happens when you turn the screw out 1/2 turn? Do the rpm's increase or decrease?

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Hmmm, now you have me confused. I thought you were lean at 4 turns out on the pilot, now you say you have it screwed all the way in? I dont want to give you bad advice.

Lets start fresh, the bent pilot screw could be your issue but lets try going through the routine anyway.

So starting with it idling good with the screw all the way in, what happens when you turn the screw out 1/2 turn? Do the rpm's increase or decrease?

 

You are right, I did have mixed results

 

But at the end, best idle behaviour, and when reving in neutral, was with 65s, and all the way in

 

Idle increased more as I screwed it, till all the way ( not much, but it increased) and it idled very smoothly

 

I tried needle from 3rd to 4th groove, it was a bit worse at 1/4 throttle

 

I tried it 2nd groove, it was unridable, misfiring everywhere, could hardly increase revs

 

I didnt try the pilot screw when I moved the needle to 4th groove, but I dont think that would have helped

 

Doesnt make sense, if its lean at 1/4 throttle, going 4th groove from the top, should have helped

 

I tried from 175 to 155 main jet (still waiting for 165 and 70), no change in 1/4 throttle

 

 

Thanks for the replies all, open to suggestions

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It sounds as though you are rich and not lean at idle. Typically the instructions for pilot jet selection are as follows.

1. Turn pilot screw out 2 to 2 1/2 turns.

2. Start the bike.

3. Use the idle adjustment to set proper idle speed.

4. Turn the pilot screw either way until highest idle rpm is achieved.

5. Once highest speed is found, turn the pilot screw out until idle speed is reduced by 100 rpm.

6. Use the idle adjustment to re set proper idle rpm.

If after this process, your pilot screw is less than one turn out, you are rich and need a smaller pilot jet.

Since your idle speed increases all the way in i assume that to reduce rpm by 100, you would be way less than one turn out. This says to me that you need to go one (or maybe two) sizes smaller on your pilot jet.

As far as the needle, did you try adjusting the pilot screw when you had it on the second groove?

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Doh! accidentally had the choke on when I moved the needle to 2nd from the top

 

I tried this:

 

60 pilot, 155 main, needle 2nd from the top

 

I got the pilot screw to 4.5 turns (as you said idle increased), but still quite lean at idle, so gonna try 62/65 tomorrow

 

but now, the misfiring at small throttle opening/steady throttle is gone!

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Glad you got the steady throttle thing worked out. I think you were actually rich at idle from having your needle too high and that is why it was running better with the pilot screw all the way in. With that said, I would not mess with the main jet or needle any more because it sounds like you have those pretty close to right.

Maybe just toss in the 62 pilot and go through the tuning process i posted again. I think you are close.

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Bomber, I took your advice

 

I did use a 62, i think idle behaviour is 95% right, I am 4 turns out

 

After 2.5 turns out, I couldnt see an increase in idle, but it was a bit lean, so I went to 4

 

No problems at all now. But the throttle sticks now from the vacuum

 

(I did polish it and oil it, but the oil quickly washed away)

 

I did make a 2mm hole 23mm from the top

 

Anyway, running fine now, for a 15 year old carb

 

Using a 160 main now, which shouldnt be far off

 

Any bolt-on carbs I can use from other models?

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