Jetting a carb ('02 KTM 520SX)

[Ductard]

I took my sumo to a shop to have it set a little richer for the ice this winter.

Didn't switch it back and it ran like crap at NYST last weekend - was missing in the mid-high rev range and had a noticeable lake of power, even when it wasn't noticeable mis-firing.

It doesn't seem that the shop kept the old jet, or marked down what it was, but they said there is a 180 main jet in it now.

Anybody have advice on what jet to put in it?

When I search on amazon for jets for the bike, they're going for ~$13 each online, or they sell kits with an assortment of jets for ~$80....

I haven't jetted a carb before, I don't even know if I have to get jets that are specific to that bike/carb or if they're standard.

I have a new camshaft to put in the motor and the aftermarket muffler is pretty beat up (will probably replace with an SMR muffler I got my hands on), so I'm leaning towards just buying the jet kit so I can dial it in once I make those changes.

Any thoughts?

[Kurlon]

Buying jets one at a time gets expensive 'cause you get hammered with shipping and handling, etc. I've actually had good luck buying individual jets off eBay for cheaper than via Amazon or local dealers.

Visit: Jetting and Fuel Injection mapping for 2 and 4 stroke KTM's for suggestions on what to get for a jet. Alternatively, you could get a JD Jet Kit for your machine which includes hot and cold weather needles along with an assortment of jets and a guide for your bike, people seem to rave about them.

[Tricky Mike]

There *might* be notes in the manual from when I re-jetted... Main, pilot, needle, clip, etc. I know I wrote all of that stuff down as I went and it might still be in that three-ring bound manual. Pretty sure the spare jets, needle, etc are in the cover flap thingy. Might be worth a look there.

[Ductard]

(wrote this before I saw the responses from Tricky and Kurlon)

Just checked the service manual and it looks like the stock main jet is a 175.

Should I just pick up one of those?

[Tricky Mike]

I don't think I had a stock main in there. There's definitely a different needle (EMN taper) and I'm 99% sure that the main was changed as well.

[Paul_E_D]

They likely just raise the needle. I'd try raising the E-clip one or two slots on the needle and see where you are at. If 175 is stock, a 180 shouldn't make it run badly. The needle, on the other hand...

[Ductard]

I don't think I had a stock main in there. There's definitely a different needle (EMN taper) and I'm 99% sure that the main was changed as well.

Thanks, Mike!

On a related note, I mentioned possibly changing the muffler.

I got my hands on a stock muffler for like $50, the guy said was off of an SMR (even though it says "SX" on it )

I don't want to put something that's more restrictive than the aftermarket muffler that's on it now, but the exterior of the muffler that's on it has just given out.

So the way I see it, my options are as follows:
1. Install the new muffler as-is
2. Take apart the new muffler, if the internals seem to be more restrictive, install the baffle from the old muffler inside the new can
3. Go get a replacement aftermarket muffler (really not looking to lay out the cash right now).

[Tricky Mike]

For what it's worth, the can that's on there now is the stock SX can with a different end cap so that I could run a spark arrestor in the woods.

[Ductard]

For what it's worth, the can that's on there now is the stock SX can with a different end cap so that I could run a spark arrestor in the woods.

Ok, that's reassuring. Likely that I won't make it worse by installing the replacement I've got then. I wasn't looking forward to taking it apart b/c it's all rivets.

- - - Updated - - -

They likely just raise the needle. I'd try raising the E-clip one or two slots on the needle and see where you are at. If 175 is stock, a 180 shouldn't make it run badly. The needle, on the other hand...

They may have done that too, but the Repair Order definitely listed new jet installation, and charged for the part.

[Paul_E_D]

OK, so go back to 175. The main has very little effect on how the thing runs.

[Ductard]

OK, so go back to 175. The main has very little effect on how the thing runs.

Update:

A couple weeks ago I went from a 180 main jet to a 175, and raised the needle one slot.

That seemed to help, but it's hard to remember back to last fall and whether it's got as much power as it did.

The carb work wasn't *too* bad, but annoying enough that I really don't want to have to keep making changes until I find out what's optimal.

Is there an easier way to do this, besides sucking it up and taking it to the dealer? I think they make some gauge that analyzes the exhaust...if I'm right does anybody have one of those I could use?

I felt like I was getting pulled on the straight by some of the other motards at the classic...most notably ~5:15 in this video..
https://www.youtube.com/watch?v=4184VFjCvdE

Note
- checked the valves when I changed the main jet - they're in spec.
- exhaust is probably pretty leaky right now - haven't gotten the new can on yet, but wouldn't that cause more low-end problems than top- end?
- the pipe coming out of the new muffler is the same exact diameter as the header it's supposed to fit over, can an exhaust shop widen the pipe on the can a little bit, or do I need some sort of fitting?
- Somewhere on the agenda is to hook up this leakdown tester I got and see how healthy the top-end is. I expect it is OK, given a fairly recent rebuild

[Paul_E_D]

Getting pulled on the straight likely has little to nothing to do with your jetting. The only way you will "optimize" it is to get it Dyno tuned. The 520 is likely to get waxed by a new 450 SX bike regardless of what you do jet wise.

[Stromper]

If its a spigot you usually can rotate it to get at what you need

Usually the pilot is changed too because that is the super lean one for the EPA

If its a Mikuni look up sudco

Assuming its diaphram some times the drill out a hole for faster throttle response

Make sure you have finished your mods don't go chasing it
Start with a fresh plug to eliminate that problem

You will never use all that power

[Kurlon]

Getting pulled on the straight likely has little to nothing to do with your jetting. The only way you will "optimize" it is to get it Dyno tuned. The 520 is likely to get waxed by a new 450 SX bike regardless of what you do jet wise.

Hell, the current crop of 250s are freaking potent now.

[Ductard]

You will never use all that power

In the dirt, maybe....but on a motard I'll take all the power I can get.

Even on a "tight" track like NHMS, you spend a lot of time at WOT.

[Ductard]

Hell, the current crop of 250s are freaking potent now.

So.....the dyni runs of the 2015 450's are showing ~50 HP....the 520 has at least that...I've read 50-56 stock what am I missing?

I know values aren't directly comparable from one machine to the next but I was excited expecting to see the modern 450's much higher than the 520.....

[Kurlon]

It'd be interesting to see a current 450 vs ye olde 520 dyno shootout. Alas, as I have neither I'll just have to hope my even older 250 two stroke can still hang with everyone.

(Memories of my 32hp 01 520SX keep coming to mind...)