P-4 NJ with -50 needle is almost spot on stoichiometric at low throttle openings, at around 1/4 or so.
P-6 probably puts the mixture closer to 13 - 14 which is little better for power.

For steady throttle cruising the P-4 gives little better MPG without loss of driveability.

For pilot jet I think either 15 or 17.5 is fine, you just end up with different setting on the pilot screw.

And as for the MJ, it's all about the airbox configuration. "No lid" requires easy few steps larger MJ than "with KLX snorkel". Your 130 is likely just fine without the lid.

In general I like my carb jetted a tiny bit on the lean side of perfect (what ever that means) at the the sea level. That helps with jetting when playing in the mountains.

--
Mikko

 

I tried a P-4 but didn’t get the same power as the P-6. Same with the Main jet I tried 125, 127.5 then the 130 and got the best power out of the 130

I tried 12.5 and 15 pilots before I settled for the 17.5. 15 was ok but I couldn't get rich stutter when winding the fuel screw out. With the 17.5 I got the lean and rich stutters when winding in and out and set the idle in the middle (around 1.75 turns out I think).

Had the stock needle but was getting some revs staying up high when chopping the throttle in the mid range. Changed to the richer needle and all good + pulls better.

Fuel consumption at current jetting is around 180klm (not sure the range in miles and I am to lazy to convert) per tank mostly on black top.

 

This is a small place I don't agree. If the pilot screw can be turned out until the bike sputters or stalls, the pilot system is too rich. There is not a fine enough adjustment for the screw. The bike runs on the pilot system after its off idle and the adjustment screw has little to do. Yet its still running off the pilot JET and air bleed or pilot air jet. So it can be too rich just off idle and still be right at idle.

The mixture screw is adjusting the amount of PRE mixed gas and air going past the screw and into the engine. Once the throttle is opened a little, up to 1/4 its just getting the pre mixed gas and air supplied and metered by the pilot jet and pilot air jet.

ONLY my opinion.

I have the 351. My settings are not relevant in this case.

David

 

The pilot circuit is composed of the pilot jet, adjustment screw, and several ports into the throat of the carburetor.

If you look closely at the adjustment screw you will see that there is a machined step below the tip of the screw. Fuel is drawn into this area. With the throttle closed, only one port into the carb throat is opened. That particular port is the only one that is adjusted by the tip of the adjustment screw.

The other ports are connected to the well below the tip of the screw and the machined step. As the throttle plate is opened, the additional ports are sequentially uncovered and provide additional fuel to the engine.

The distance from the tip down to the step (and the shape of the step) determine the mixture ratio going to the transfer ports. If the manufacturer of the screw got this distance right, the correct mixture will be provided to the transfer ports as well as the idle port as the screw is adjusted. All of the fuel for the pilot/transfer ports comes through the pilot jet so a change here affects both idle and transfer.

Some aftermarket adjustment screws have this distance/shape wrong. The Kouba screw has it right.

BTW, nice work, Mikko.

Ron

 

I just went with what i have learned + what the mauanl talks about for pilot jetting.... bassed on the rich and lean stutters when winding the screw in and out.

If your best idle is less than on turn out your too rich go down one jet size.
if you best idle is more than 2.5 turns out your too lean (this is how I was with the 15 pilot) so go up one jet size.

 

Jetting update on my stock 250 + TM36.

Smaller pilot air jets are not easily available (stock is 1.00, Niche Cycle didn't have 0.9 or 0.8) so I went one up on pilot jet. Now I'm at:

PJ 17.5, stock 1.0 pilot AJ.
9EBBY-50 needle, P-4 NJ
MJ 115.

A/F = 13 at idle and at full RPM WOT.
At about 1/4 steady cruise A/F = 14 - 15
Throughout the rev range when ever throttle is wide open A/F = 12 - 13.

There's one spot around 2/3 throttle where A/F dips to 11.5 - 12.0.
I might try to fix that by dropping the needle down by one notch. It looks like the second taper in the needle kicks in just little bit too early.
Then again, I can only see the dip in A/F on the meter, it feels great as it is.

Fuel mileage on today's test ride was 50 - 60MPG. That included lot of riding at WOT and full-throttle accelerations from dead stop. More than two tanks of gas total, 30 miles on freeway at > 70MPH. Total average was >50 MPG.

This carb works.

--
Mikko

 

Thank you FlyingFinn. This is with stock snorkel correct?

Neilapples or everyone else, I never said anyone was wrong, its just how I think it should be done.

Another reason, My bike was at 1 1 /4 turns out on the pilot. I went down a size jet to end up at 2 1/2 turns out. This gives more resolution to the adjustment. In other words, 1/4 turn on the jet makes LESS of a change so it can be fine tuned.

Yesterday I had the TM36 off because when I open the throttle full, the slide goes up past full, so I adjusted the stop. I also stripped the head on one of the screws. Replaced them with 4mm X .7mm X 12 mm allen head. Changed pilot and main jets. Put it back on. I should be almost there. I also removed the backfire screen. Don't attempt this unless you have a lot of time.

David

 

Yup, stock snorkel on a KLX300 airbox.
Stock 250 motor, stock header and a Q4 muffler.

On that note, I don't much care for the Q4. It's too loud for my taste.
The noise get tiring when riding all day and is embarrassing when riding through quiet residential areas.

Next project for me is to make up a new exhaust. I have a used Muzzy header and a stock KTM 450 muffler. The muffler should flow plenty good for a 250 motor while not being obnoxious. It's also very light weight.
Just need to make brackets for it and a mid pipe....

--
Mikko

 

I'm just using this thread as my personal log of my jetting, easier to find this than some hand written notes on back of notebook...

With my previous jetting the bike ran REALLY well except for slight lean area at very small throttle, I'd say between 1/8 and 1/4.
Even that was only noticeable on the A/F meter and probably gave me really good cruising fuel mileage.
My guess was that at that point the jetting is mainly set by the constant diameter section of the needle and ID of the needle jet.

I was running 9EBBY-50 needle with P-4 needle jet.

Straight forward option would have been to go to P-6 NJ but I don't have one at hand. But I did have 9EBBY-51 needle which is one step leaner than the -50.

So I decided to try the 9EBBY-51 with P-8 needle jet. -> Excellent results!

It is clearly richer than -50 with P-4 at the constant diameter area so I went down to 15 Pilot Jet.
Now I have idle A/F ~ 13.5 to 14
Small throttle A/F under load ~12.5
On very small, or no throttle, pretty much on decel the A/F goes up to 11.
WOT at full RPM is also around 12.5

For my own record:
PJ = 15, MJ = 115, 9EBBY-51 clip @ top grove, NJ P-8.

For TM36-68 jetting reference, Id say the two needle combos that work just super well on this bike are:
9EBBY-51 with P-8 needle jet and 9EBBY-50 with P-6 needle jet.

All jetting testing is done riding the bike with LM-2 A/F meter hooked up.

--
Mikko

 

This is great work.