post 600 factory pro main jet sizes

[charlie2000]

trying to determine optimal main jet size set-up to use with my factory pro kit. Kit came with two 117.5 mains, two 120's, and two 122.5's. The most specific information I could find so far is that Facotry pro recommends different main jet sizes for the inner and outer carbs. If this is the case, can anyone out there tell me from experience specifically what sizes they're running, and in which carbs. Are carb 1 and 4 supposed to have the bigger two, or is 2 and 3 ? I'm running the stock airbox with a V&H slip-on.

[arsenic]

use the stock sizes which are 115 1 & 4 and 112.5 2 & 3. Factory Pro doesn't "recommend" that. That's how they come. 2 & 3 are stock 112.5 and 1 & 4 are 1 size bigger 115. Mikuni goes in 2.5 increments so 115 is 1 size bigger then 112.5.

[charlie2000]

Thanks. So I just got some helpul info from Astro 4x4. He ran the two bigger stock jets (115) swapped into where the 112.5's were, and used only the 117.5's from the kit. So, in the end, you only need two of the six jets they send you in the kit. Which is what I think is confusing to a lot of people (judging from the many threads I've read through) who buy the Factory Pro kit. Its taken me all day to figure this out.
Thanks Arsenic and Astro 4x4!!!

[arsenic]

not exactly, for a full exhaust, you'd go one size up. So 117.5 1 & 4 and 115 2 & 3. For extreme modification you'd go 120 1 & 4 and 117.5 2 & 3, and I have no clue but for something or other you'd go 122.5 1 & 4 and 120 2 & 3. Get it? And I disagree, you need NONE of the main jets that come in the kit with stock exhaust.

[charlie2000]

OK, I think I've finally figured it out. I definitely want to follow your advice, as your name seems to be everywhere carb related, and especiaaly with Factory pro. I'm going 115 and 117.5 with V&H slip on (from p.o.). Does this jive with you?

I realize a slip-on is not a full exhaust, but you don't think I should use any of the main jets from the kit?

[arsenic]

No, a slip on is NOT a full exhaust. You're still using the OEM 4 into 1 header. Not a 4 into 2 into 1 aftermarket full system. Therefore, jetting does not change. A slip on is just for sound. use 112.5 in and 115 out. Really, you spent all day figure that out? Just ask me next time. No, I don't think you should use any main jets in the kit. Did you even read the Factory Pro instructions? Cause they tell you EXACTLY what I just did.

[charlie2000]

Yes, I agree. and would not have personally wasted my money on a slip-on, but you think I'll be going backwards with one size up? (beating a dead horse). I've just heard so many people espouse the factory pro throttle response, elimination of flat spots, etc. over factory. Should I still adjust the float height and needle jets, or what?

[arsenic]

How many times do I have to tell you to use 112.5 in and 115 out, honestly

Yes, I agree. and would not have personally wasted my money on a slip-on, but you think I'll be going backwards with one size up? (beating a dead horse). I've just heard so many people espouse the factory pro throttle response, elimination of flat spots, etc. over factory. Should I still adjust the float height and needle jets, or what?

[charlie2000]

LOL!!! I know, its sad, isn't it. I'm like the guy in the showroom trying to get the salesman to tell him what he wants to hear about a car thats really no good for him. Too funny. But seriously, I appreciate your help. Keeping the mains the way they are. Shiny big jets staying in their shiny little plastic box. And thats final!
Until I find the proper exhaust...
then I'll be back to drive u crazy all over again

[Kreylyn]

Not a throw a wrench in what Chris stated, but I believe the post factory header can be modded slightly to increase the flow a bit. Grinding out the welds on it has been shown to increase the flow.

That might warant going up 1 size on the jets instead of OEM.

Krey

[arsenic]

no it doesn't, cause I've done that, but way to throw useless information in there to confuse people. The thing that controls the air flow more than anything is the collector. Weather or not you grind the excess weld seems, you're not changing the collector at all.

[charlie2000]

I love it! Now you're giving candy to the baby. I'm all for low buget solutions. got any links?

[arsenic]

Katana Header Flow & Grinding Page:

NOTES ON JETTING

Having opened up the exhaust flow some, engines which were set particularly lean may be pushed into a "too lean" condition. This will be noticeable by difficulties starting after this modification, and possible after-fires (exhaust popping) during deceleration or starting. To correct for this, there are two basic options:

(Option 1): [use with the stock OEM jetting if needed:] Modify the existing jetting by increasing the turns outward on the pilot screws for each carb by one-eighth to one-quarter of a turn (be consistent on all of them), plus adding an 1/2 height shim under each diaphram needle. See Jet kit installation guide to help understand what I am talking about here.

(Option 2): [use in place of the stock OEM jetting for additional power:] Install an aftermarket jet kit to improve fuel delivery and increase engine power. For the 98+ Katana's, I recommend specifically using the Ivan's Jet kits. For the 1988-1997 Katanas, I recommend the FactoryPro brand Jet kits. These recommendations for the 98+ models are based on personal experiences comparing the Ivan's and DynoJet (sometimes sold under the K&N brand name) jet kits directly to each other. For a general installation guide to jet kits on the Katana, see Jet kit installation guide.

I rest my case on the main jets BTW

[charlie2000]

no it doesn't, cause I've done that, but way to throw useless information in there to confuse people. The thing that controls the air flow more than anything is the collector. Weather or not you grind the excess weld seems, you're not changing the collector at all.

I was just being silly anyway. there are too many used exhausts out there to get that silly. Hey, I've thought about doing home-grown port-jobs with the old grinder on my car, but really when it comes to airflow (intake or exhaust) its probably easier to do more harm than good in the way of horsepower and torque when you don't know what you're doing. You've got to spend the money.

the pics on that link are pretty "convincing". you say you've grided the welds smooth on a factory collector, with no improvement in high rpm airflow?

[arsenic]

No, that's not the collector number one. Number two, I said I have done that and I did not go up a main jet size. I did not say it didn't improve air flow.