.038 slow jets

[pancho]

I want to purchase and tune in a set of .038 jets. I am wondering which keihin jets these are that fit our bikes and where is the best place/price to purchase them from. Any pertinent information or experiences related to the subject will be appreciated. My bike is running well right now with CObra pipes, but I want to see if it will be better with the 38s.

[big poppa pump]

Here you go.

https://www.rockymountainatvmc.com/p/930/24375/Pro-X-N424-21-Series-Keihin-Pilot-Jet-?term=pro+x+n424-21+series+keihin+pilot+jet+38

[pancho]

Thanks brother,,,  that's a pretty good price,, my kind of shopping!

[98valk]

I want to purchase and tune in a set of .038 jets. I am wondering which keihin jets these are that fit our bikes and where is the best place/price to purchase them from. Any pertinent information or experiences related to the subject will be appreciated. My bike is running well right now with CObra pipes, but I want to see if it will be better with the 38s.

unless u open up the air box some way it will run rich and mpg will go down.
any PJ is good to up to 4.5 turns open. if it needs to be opened more than the next size PJ is needed. open up your mixture screws to 3-3.5, then go for a ride.

I have extensive air box mods and ceramic coated viking exhaust system. I just dyno'd with a 4 gas analyzer and 38 PJ is perfect for my setup with a 112 main jet and a Dial-A-Jet. These bikes don't need big jets, cylinder head is pretty efficient.
remember a lot on here go to 38s and then run mixture screw 1.5-2.5 to run right. use stock 35s mixture screw to 3-maybe 4  and it will run right.

from factorypro.com
Question:

Greg Johnson wrote:
 
> I have installed one of your 1.0 jet kits on my R6 and I am coming to the final parts of tuning it.

> It runs well and passes most of the tests as provided on your tuning guide. It does, however, appear to be lean low down, as there is a "hole" when cracking the throttle wide open at below 2500rpm and at high rpm/part throttle, it shows some surging. I have though set the fuel screws, so that it idles correctly and returns to the correct rpm, when the idle is set below 1000rpm and the throttle is blipped.

> To fix the lean patch, is it better to go the next step up in pilot jets, rather than adjust the fuel screw?
> Will upsizing the pilot jet, require re-adjustment of the fuel screw and thus negating some of or all of the gains made by the larger pilot jet?

> Thanks,
> Greg J


Answer:

Greg - Good diagnosis on the pilot jet size -

As far as the pilot... Look at it this way...

There are 4 outlet holes for the pilot mixture. ~3-4 at the butterfly and one "downstream" of that (for 75% of the idle mixture).

1 hole is controlled by the mixture screw and with the other 3 or 4, max flow is limited by the size of the pilot jet.

At idle, 1 hole (metered by the fuel screw) and 1 un metered hole are open.

At cruise, when the "butterfly" is just "cracked", all 3-4 unmetered holes + the 1 metered one are uncovered -

So.....

At cruise, you get those 3 unmetered holes + the metered hole... So, at least 75% of the fuel delivered at cruise is limited by the size of the pilot jet.

At idle, you get 100% of the trimmed 1  hole + a butterfly valve-trimmed amount of the other 3-4 metered holes.

Rule of thumb....
If you go 1 size larger or smaller on the size of the pilot jet, you will change the fuel screw ~1.5x richer or leaner to retain the original idle mixture -

Example:
You have a #40 pilot jet installed (with the proper main, needle height and fuel  level already done)  and to get best idle, you are 4.0 turns out (from lightly bottomed out).
Cruise seems lean....

So, I'd expect that I could richen the cruise with 1 size larger (42) pilot jet (size of pj is 75% of cruise mixture) and the "trim" the fuel screw "in" for best mixture for best idle (size of pilot is 25% of idle mixture).

To go back to ~ the same idle mixture, after going from 40 pj/4.0x to the 42........ try 2.5x

40 pj / 4.0x = our reference idle mixture
42 pj / 2.5x = ~ same idle mixture
38 pj /5.5x = ~ same idle mixture (this is a "stretch - after 4.5x, not much changes)

(Using another "rule of thumb", if you have the correct pilot jet, the fuel screw will end up at between 1.5x and 3.0x when set for best idle.)

Marc

I have extensive air box mods and ceramic coats viking exhaust system. I just dyno'd with a 4 gas analyzer and 38 PJ is perfect for my setup with a 112 main jet and a Dial-A-Jet.

[pancho]

Thanks CA,, great information on the operation of our carbs.

 I am going to dial in the 38s as well as I can without doing any airbox mods,, just to get a feel for what the overall performance is compared to now,,,  the long term plan is to return to a set of factory pipes ,,, unless I get suprised by the 38s.  When I get this done,, it may be awhile,, I'll report on what I find

[98valk]

Thanks CA,, great information on the operation of our carbs.

 I am going to dial in the 38s as well as I can without doing any airbox mods,, just to get a feel for what the overall performance is compared to now,,,  the long term plan is to return to a set of factory pipes ,,, unless I get suprised by the 38s.  When I get this done,, it may be awhile,, I'll report on what I find

if u still have the stock 35s u will get better crisper throttle response and better mpg with them just by opening up the mixture screws. u might get less performance with the 38s than the 35s opened up.
go to the factory pro site and read the carb tuning. u always jet from the mj down to the pj. make sure the mj is right first.
my dyno showed I was pig rich from 50% throttle to WOT. it ran good but not all the time. so I just jetted down from 120 to 112 mj. runs so, so much better all of the time. even my pj and needle adjustments fit right in with the 112s.
trying to tune pj first is backwards and u will never get it right.
u can always call factory pro, sometimes Marc the owner picks up, I've had the pleasure to talk to him twice, a wealth of knowledge. He did the dyno tune for the valkyrie with cobras, he knows exactly what it needs.
Good Luck.

[John Schmidt]

FWIW, I installed the #38s a few years ago when I kept getting signs of blockage from the ethanol gas. That's the sole reason for installing them. I got real good at pulling my carbs and finally installed the 38s and have had no trouble since, the slow jets are the only change made. Doesn't seem like much of a change in size, but overall the only difference in operation is I no longer get plugged slow jets. My mpg is unchanged, performance unchanged, frustration is changed....no blockage. I set the pilots at 2 out and it runs fine, giving mpg in the mid to upper 30's, depending on load....and wrist action. I run a K&N w/o the prefilter, an I/S ICM & carb springs, aftermarket pipes with slightly less backpressure from stock,  and a 4 deg. T/W. All of that was on the bike prior to changing out the slow jets. I sync'd the carbs at the time and haven't touched them since.

[WintrSol]

if u still have the stock 35s u will get better crisper throttle response and better mpg with them just by opening up the mixture screws. u might get less performance with the 38s than the 35s opened up.

That's why I'm returning mine to stock #35s; PO put in #38s to keep them from plugging during storage, but I don't expect I'll have that problem.