GP shorty on my 02 600 Kat. Now it's running lean?

[SOS]

Hello I just had a shorty gp installed on my Kat and now she runs like a pizza ship! I've done some research and I think I need to rejet but I figured I'd come here and ask first. Thanks in advance!

[92xjunker]

Turn the a/f screws out to 2.5 turns.

[SOS]

I did that but once I start getting into riding it she bogs down like it's gonna stall.

[92xjunker]

That has nothing to do with the can. May just need to clean the carbs. Try seafoam.

[IanDMacDonald]

What kind of muffler is it? Hopefully not one of the cheaply-baffled variety . . .

The loss of backpressure from those makes tuning extremely difficult, usually leading to a ventilated piston from running lean

If it is indeed a cheapie, you'll need to look into going one size up on the mains to richen the mixture if your mixture screws won't get that bunson blue flame in the cylinder.

[92xjunker]

Many have run those cans and never went that lean, I'd recommend a jet kit. ALL US kats could use one, stock exhaust or not.

[cintidude04]

Factory Pro jet kit... *drops mic

[arsenic]

What kind of muffler is it? Hopefully not one of the cheaply-baffled variety . . .

The loss of backpressure from those makes tuning extremely difficult, usually leading to a ventilated piston from running lean

If it is indeed a cheapie, you'll need to look into going one size up on the mains to richen the mixture if your mixture screws won't get that bunson blue flame in the cylinder.

Check your facts there brother. Even if there is no can on the stock headers, it won't damage the engine from running lean. No headers, you'll have problems quickly. The back pressure comes from the OEM collector on the OEM headers. When you swap from stock to full exhaust, that's why you need a jet kit. When you swap the stock muffler to a slip-on, you don't. I'd recommend it, you'll have to make sure the A/F screws are at 2.5 turns out for sure, but you don't need a jet kit.

[IanDMacDonald]

Check your facts there brother. Even if there is no can on the stock headers, it won't damage the engine from running lean. No headers, you'll have problems quickly. The back pressure comes from the OEM collector on the OEM headers. When you swap from stock to full exhaust, that's why you need a jet kit. When you swap the stock muffler to a slip-on, you don't. I'd recommend it, you'll have to make sure the A/F screws are at 2.5 turns out for sure, but you don't need a jet kit.

Well, technically the backpressure (velocity) would be dependent on the whole system, i.e. (header, cat (w/o cat), mid-pipe, and finally the muffler. The muffler restricts the exhausts (unless it's a straight pipe with baffles removed).

The term "back-pressure" is over-used, and misunderstood on most forums. You want the exhaust gases to exit the chamber and move along at the highest velocity possible - you want a fast exhaust stream. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at.

And yes, even though the odds of "ruining" the engine are minimal from running the engine with a leaned-out muffler, it's still possible, as I've rebuilt my fair share of race engines running a lean mix at the track.

[SOS]

About $50 from a friend. Yeah not expensive I just wanted it for the sound. So I'm gonna seafoam the carbs tomorrow and see what I get. I'll look into jet kits after. Stage 1or 2?

[92xjunker]

Stage 1 should be enough with the exhaust.

[burninrock24]

Not trying to hijack but I'm reading mixed things on the forums here. When I search old threads, I see people saying that adding shorty slip ons will cause all sorts of issues, and now you guys saying it's just fine as long as they are still the stock primaries and collectors.

I've been thinking about switching to a shorty style, but I don't want to do a jet kit right now.

[92xjunker]

Do the jet kit first, it won't hurt the bike. It will actually help it.

[burninrock24]

Right, but I just spent a decent amount of money on the carbs at oem spec so I'd like to not do that. At least not right now. Maybe come next spring, was just wondering if I can do the shorty exhaust on OEM spec carbs.

[92xjunker]

Set the A/F screws to 2.5 turns out and hope for the best