carb jet size question

[misiek93]

I am runnig a set of cobra slip-ons currently on my bike however I bought a 4-1 holeshot exhaust recently especially since a lot of people on here told me full exhaust will make a much bigger difference.

I ordered a set of 147.5 jets from Sean for the cobra slip-ons (that was before I purchased the 4-1 exhaust). Here's my question (2 questions actually),

1. will I be fine using the smaller 147.5 jets with the full exhaust? (I have a stock airbox and filter now) I think this will work fine, since I've read on here that exhaust acts as a vacum and more exhaust coming out means more fuel is getting sucked in that's why the bike will run reacher with stock jets and aftermarket pipes- but if someone that knows more about carbs than I do can confirm that I would appreciate that, just for my piece of mind.

2. Also, what is the function of the jet on top of the carbs, it's circled in red in the picture below. The reason I ask is becuse I could tell my bike was running rich with the cobras but I didn't feel like taking my carbs out to rejet them (don't have a way of syncing them just yet) so I changed that top jet with the 147.5 and it seems like it's running a lot better now. Before it would have this weird (that's the best I can describe it) sound at about 3200 RPM and I could hear it being rich, now with that smaller jet that sound went away and the bike pulls really strong. Can I leave it like that? I think it is the pilot jet 2 but I'm not 100% sure plus I don't really know what exactly it does.

My elevation is 771 feet (not sure if that is relevant)
Stock air box
stock air filter

 

[maleko89]

You should be but might be a bit lean with a 4-1. With a Kerker 4-1 Sean had me try stock jets and it runs great.

That jet is pilot air jet #2 (PAJ2). It supplements air to the pilot fuel jet at small throttle openings. It's also closed off during closed throttle, high vacuum situations to richen the pilot circuit and prevent exhaust popping.

 

[SpecOps13]

I'm at Sea Level, running a set of Kerker 4-1's on my 89, now through a shortened Mark's Can.
I have 147.5's in and the plugs are golden honey colored... To get a good reading, you really
need to sync the carbs.

This one is great. I used PATMAX's at a tech day. http://www.carbtune.com/
I hate the ones I have with fluid. Good Luck.

 

[misiek93]

You should be but might be a bit lean with a 4-1. With a Kerker 4-1 Sean had me try stock jets and it runs great.

That jet is pilot air jet #2 (PAJ2). It supplements air to the pilot fuel jet at small throttle openings. It's also closed off during closed throttle, high vacuum situations to richen the pilot circuit and prevent exhaust popping.

I just finished installing the exhaust and put the original jets back in. We'll see in the morning how it runs. I must say the 4-1 look is growing on me :biglaugh:

 

[misiek93]

I'm at Sea Level, running a set of Kerker 4-1's on my 89, now through a shortened Mark's Can.
I have 147.5's in and the plugs are golden honey colored... To get a good reading, you really
need to sync the carbs.

This one is great. I used PATMAX's at a tech day. http://www.carbtune.com/
I hate the ones I have with fluid. Good Luck.

I'm thinking I'll leave stock jets in there for now get a new set of plugs and sync the carbs. Ride it for a while and see how the plugs look. I'll do the same with 147.5 jets and see which ones look better. Worse case I'll have to get some more jets.

The problem with Carbtune is that I have to wait to get it and I really want to get it dialed in for the long weekend. I'm going to try to make my own with some clear tubing and a yard stick, from what I've seen online it should work. I believe one of the members here made his own and had good luck with it. That should get me pretty close to where I need to be.

 

[SpecOps13]

RichWrench did make one and it worked.... Rich is better than lean so give the stock jets a try.. Should run alright but may be able to lean it out after the weekend.... Good Luck..

 

[dannymax]

You should be but might be a bit lean with a 4-1. With a Kerker 4-1 Sean had me try stock jets and it runs great.

That jet is pilot air jet #2 (PAJ2). It supplements air to the pilot fuel jet at small throttle openings. It's also closed off during closed throttle, high vacuum situations to richen the pilot circuit and prevent exhaust popping.

Mark-o....Isn't PAJ2 behind the diaphram? Think he's looking at PAJ1.

 

[misiek93]

RichWrench did make one and it worked.... Rich is better than lean so give the stock jets a try.. Should run alright but may be able to lean it out after the weekend.... Good Luck..

I rode it to work this morning and it seems like my bike really likes the stock jets now. I think I'll just sync the carbs and leave it alone for now, why mess with it if it's running fine. With the cobras I could smell gas a little thats why I got the smaller yets, but that smell is now gone now which tells me it's running leaner than before.

 

[one2dmax]

Usually they will make a little more power and run even better with the smaller mains

 

[misiek93]

Usually they will make a little more power and run even better with the smaller mains

Do you think the 147.5 will work fine? If not, what size would you recommend I get?

 

[06VMAXIMUS]

I split the difference between stock 152.5 and the 147.5, and went with 150's. I'm about the same sea level as you, and my plugs seem to like it. Runs good.

 

[mattmullins]

It ran best 4 me with 152.5 at 1000 feet above Sea level try adjusting your floats Sean's way and take off the air scoops off your air box lid should run sweet.