How to Jet Keihin Carburetors

While this post will be specific to the last few Keihin Carburetors we’ve jetted, a lot of the concepts can also apply to other brands.

Update October 30, 2023: Both bikes got clogged pilot jets from a bad batch of gas(with ethanol). Kept the 38 Pilot in the KTM, but bumped up to a 40 Pilot in the Xtrainer. Still hot here in central Texas(80s) and it might be a tad rich, but that will work well when it cools down. We’ve not located a station to get Ethanol-free gas and will only use that going forward.

Update October 28, 2022: From February 2022 through Most of October 2022 we rode at altitudes ranging from 3000ft in Oregon to 10,000ft in Utah. In my Beta Xtrainer i was running the Suzuki NECJ needle, but during a training session with Rich Larsen(IRC Tire Guy) he suggested I try the slightly richer NECW. I did and the bike ran great! My settings at altitude were: NECW Clip #2 – 162 Main – 38 Pilot whichFMF Fatty & Airbox restrictor removed. We just recently got back to Texas and the only change I made was moving the clip down(richer) to Position #3. Recently tested it a local MX track (DDMX Ranch) and the bike ran great! Motocross run: https://youtu.be/UZjFmLrDWf4

On our 2016 KTM 250 XC we were running: 172 Main – 38 Pilot – Suzuki NEDH Clip #1 at altitude and just changed that to 175 Main and NEDH on Clip #2. That bike also ran great at DDMX Motocross ranch. https://youtu.be/7dYfr0PRiQE

Why Change the Jets on My Carburetor?

Great question and there are several reasons why you’d want to or need to jet your carburetor.

1. Engine Mods and Upgrades

Your dirt bike comes from the factory with general jetting settings. Once you start doing mods such as exhaust or air boxes, you’ve changed the air/fuel requirement for the engine and likely need to change the jetting to accommodate these changes. For example, my Beta Xtrainer comes with a restrictive plastic airbox (compared to the other Beta 2 strokes), and removing it allows the engine to intake more air. You need to increase the jets to balance out this increase in air with more fuel. If you put a big bore kit on your dirt bike engine, you will also need bigger jets vs stock.

2. Altitude and Temperature

If you ride at different altitudes or extreme temperature changes you would also want to jet to compensate. Colder air equals more oxygen which can use richer jetting. Higher altitude equals less oxygen and that requires leaner jetting.

Below you see a copy of Beta’s Xtrainer jetting chart. The shaded area is the stock setting. You can see as altitude increases, jetting gets leaner but for the same altitude, as the temperature gets colder, jetting gets richer. A NOZJ needle is richer than a NOZK needle.

SLM Altitude Above sea level
AVA Air screw opening (from all closed)
Gm Pilot Jet (Slow Jet)
GM Main Jet
SPL Needle
POS Needle position (from top)
VLV Valve

You can see Beta jetted for 1001ft to 2500ft with a temp range of 61f to 78f. Does that mean if I ride at 6000ft my bike will blow up? Look at the chart again and you can see the jetting only changes slightly depending on altitude/temps. If you look at the chart again below, I’ve highlighted several jetting settings that are identical except for the Pilot Jet.

If you’re planning a ride that will go from sea-level to 10,000ft, you should jet the bike for where you will be riding most of the time and always err on the rich side. Usually, as you go higher in altitude, the air also gets colder sometimes balancing out the need to jet leaner.

The charts above reference a Pilot Jet, Needle, and Main Jet. How do these come into play with throttle openings?

The Pilot Jet is from idle to 1/8 throttle, the Needle is between 1/8 and 3/4 the main is from 3/4 to wide open. Think of them as 3 separate circuits that overlap where they blend together at 1/8 and 3/4 throttle positions. The air screw affects idle to 3/4 and is more of a general fine tuner which can also tell you if your Pilot Jet is too large or too small.

What if I don’t want to figure all of this out or make it easier?

JD Jetting sells kits for your specific model dirt bike with an easy-to-read table of jetting settings based on mods and altitude along with the jets you need for your bike. These kits cost about $79 and take the guesswork out of jetting. We have used them on both our Honda CRF150RB and our Beta Xtrainer and I can tell you both bikes ran great. We highly recommend JD Jetting!

But I Like to Tinker!

Now if you spend any time on Thumper Talk or on Dirt Bike Facebook groups, you may find that several others have fine-tuned their jetting beyond the factory or JD recommendations. If you like to tinker, a very popular jetting modification is using Suzuki needles such as the NECJ for 300cc 2 strokes and the NEDW for 250c 2 strokes. Here’s a thread from Thumper Talk: Beta 300RR Jetting Thread. Search on ThumperTalk and you’ll likely find a thread for your make/model.

Carb Cleaning

For our recent 2016 KTM 250 XC, we took a different route. We bought this bike with only 1 hour on it. The seller was honest and told us we’d probably need to go through the carb. When we got the bike home and were test riding around when it started pissing fuel out of the carb overflow tube. I knew this meant the float was probably stuck, I tried to trick of tapping the float with the handle of a screwdriver. This didn’t work so we pulled the carb out and took it apart. Look what we found in the float bowl: gunk and some suspect KTM Orange bits!

That definitely needs to be cleaned, but it’s not causing the fuel to overflow so let’s look deeper.

Yup, that is some (KTM) orange piece of plastic in the float needle seat! This kept the float from moving overflowing fuel. We cleaned that out and out the carb back in. It started up(yay!) and we took it for a test ride…no more fuel pissing out the overflow! But, too early to celebrate as the bike would not idle. Idle is handled by the pilot jet so we took out the stock #38 and out in a #40 that the bike came with. Voila! The bike now idles and runs pretty well. The other recent change we made was installing a Suziki NEDW needle as I had read a few others like the way this needle runs on their KTM 250 2-stroke.

Our current settings for the KTM 250 XC are: 172 Main – 40 Pilot – Suziki NEDH Clip 2 – AS 1.75 Out

Now that we’re at higher altitudes (3000ft and higher) we’re going to try one position leaner on the needle and go back to a 38 Pilot Jet. I feel like it is running slightly rich based on the spooge on the exhaust. That said we’re trying to test the mileage range on the KTM and so far have gone 80 miles without hitting the reserve(2.6 gallon tank using 50:1 pre-mix with Motul 710). Would like to have a range of 100 miles.

We’ll take some new pics of the needle access and pilot jet access next week when we do those updates in Durango, Colorado!