Mystery Exhaust, to Jet or Not to Jet?

[Heeltoeclutch]

Hello fellow Katriders,

I have a '91 600 Kat I bought as a project from a guy who wasn't the smartest, and who was not the original owner.

I'm cleaning the carbs now, and trying to determine if I should re-jet while I have them apart. My bike has a Vance & Hines 4-1 full system on it, but I have not been able to find a pre-98 exhaust like this. This leads me to believe it is a post-98 exhaust, which I have read bolts up fine(?)


(Sorry for shitty pic I am away from the bike currently)

When I bought it, it ran shittily - the PO had removed the airbox, told me the carbs needed to be synced, and turns out one of the slide springs is kinked so the carb slide doesnt open properly.

I read that the post 98 aftermarket full system exhausts actually run pretty well on the pre's with no jetting. But while I am in here, do y'all think I should do it anyway?

So:

1. What exhaust do I have,
2. Rejet or leave as is?
3. Maybe adjust needles?


Thanks!

[92xjunker]

Doesn't really matter if you have an aftermarket exhaust or not, ALL KATS can benefit from a jet kit but, an aftermarket exhaust will require one to run correctly so, yes, you need one.

[maniac1886]

1. Vance and Hines exhaust, probably a post 98 header.
2. Definitely re-jet stage 1 factory pro
3. The jet kit will have adjustable needles and directions explaining how to use them.

[shpielers]

+1 get a jet kit from factory pro

[maniac1886]

The way to tell for sure if they are post headers is to look at the oil pan. It differs between the pre and the post. The pre oil pan gets in the way, so if those are post headers, it's should also have a post oil pan. My kat had the elusive yoshimura headers so I had a 4-1 with a pre oil pan. But you don't see that very often.

[Kreylyn]

Vance n hines made full systems for the pre kats. I have several. The can is the SS2r. No larger jetting is needed, but the ultimate determination is how the bike runs. Get a factory pro kit and follow the directions.

[Heeltoeclutch]

Thank you so much for all the responses guys; I will have a look at my oil pan and post some pictures. Although if it is a pre- exhaust, as Kreylyn says, maybe the mystery is solved.

Vance n hines made full systems for the pre kats. I have several. The can is the SS2r. No larger jetting is needed, but the ultimate determination is how the bike runs. Get a factory pro kit and follow the directions.

Kreylyn, I am curious why you say no larger jetting is needed; the general consensus around the forum seems to be if you go full system, you need to jet. Could you explain?

(If I can get away without rejetting, I wouldn't mind saving $120!

Thanks!

[92xjunker]

Most just need an A/F adjustment. But, as stated it all depends on how it runs. Popping threw the exhaust on decel is a sign of being lean.

[Heeltoeclutch]

Sorry - my carb needles do seem to be adjustable; although am i correcr thinking most stock carb needles have multiple slots for the clip?

[92xjunker]

Sorry - my carb needles do seem to be adjustable; although am i correcr thinking most stock carb needles have multiple slots for the clip?

Nope, you have a jet kit. 6 grooves Dyno jet. 5 groove Factory Pro. Which explaines the odd jet sizes you had. YOU CANNOT USE MIKUNI JETS WITH Dyno Jet NEEDLES!!!! Factory pro uses stock Mikuni jets. Looks like you have Dyno Jet.

[Heeltoeclutch]

(Sorry put the text into a new reply)

WOOT! That is awesome! This bike is now a much better deal!

I guess I'll give the existing jets extra special attention and put it back together and see how it runs.

[arsenic]

V&H DID make a 4-1 pre 98 exhaust, 100%. They made 2. The SS2R and the Super Sport.

[Kreylyn]

Kreylyn, I am curious why you say no larger jetting is needed; the general consensus around the forum seems to be if you go full system, you need to jet. Could you explain?

There are many factors that determine the jetting. High flow race exhaust is one, if you have other mods to back up the change. You can't flow more exhaust with out also increasing the flow through the intake and engine. You would need to make more changes than just the exhaust.

Additionally the change to the V&H exhaust are not high flow race exhausts. They have very little effect on the over all flow actually, through the entire system. The change is very minor.

They sound a lot different though, and people tend to associate sound with performance. Just look at all the stock Asian cars with those horribly bad sounding loud cans.

This doesn't mean that adjustements are not needed. A jet kit would be needed. A jet kit allows you to tune the bike. You don't need larger jets. New jetting is not needed.

Tuning the bike is needed.

A jet kit allows you to tune the bike.

Stock bikes are lean and limited in ability to tune them due to the stock needles. Jet kits resolve that issue.

So to clear that up... Jet kit yes, larger jets no.

Krey

[Heeltoeclutch]

There are many factors that determine the jetting. High flow race exhaust is one, if you have other mods to back up the change. You can't flow more exhaust with out also increasing the flow through the intake and engine. You would need to make more changes than just the exhaust.

Additionally the change to the V&H exhaust are not high flow race exhausts. They have very little effect on the over all flow actually, through the entire system. The change is very minor.

They sound a lot different though, and people tend to associate sound with performance. Just look at all the stock Asian cars with those horribly bad sounding loud cans.

This doesn't mean that adjustements are not needed. A jet kit would be needed. A jet kit allows you to tune the bike. You don't need larger jets. New jetting is not needed.

Tuning the bike is needed.

A jet kit allows you to tune the bike.

Stock bikes are lean and limited in ability to tune them due to the stock needles. Jet kits resolve that issue.

So to clear that up... Jet kit yes, larger jets no.

Krey

Got it. Thanks for the clarification!


It would seem my carbs do have a jet kit installed; as such, the adjustments I can now make seem to be:

• Adjusting needle (e-clip?) position
• A/F screws
• Idle Speed

Hopefully this will be enough to get the bike running well (stock air filter and V&H exhaust). I do not know what size pilot and main jets were installed with the kit.