STOP THE STUTTER STARTER PACK for FCR 37 37mm FLAT SLIDE KEIHIN CARBURETOR

Stop Tuning in Circles: The Case for Genuine Keihin

We’ve all been there. You’re staring at a bike that won’t idle, or a quad that stumbles the second you crack the throttle. You’ve cleaned the carb three times, but the "bargain" jets you bought online just aren't doing the job.

Here’s the cold, hard truth: In the world of fueling, a "close enough" fit is just a fancy way of saying "broken."

When you choose OEM Keihin Jets, you aren't just buying brass; you’re buying the exact calibration the engineers intended.

Why Genuine Keihin Matters

• Micro-Precision Flow: A Keihin #105 isn't just a label; it’s a guarantee. Knock-off jets are notorious for inconsistent sizing—sometimes varying by as much as two or three sizes from what’s stamped on the side.

• Proprietary Metallurgy: Keihin uses high-grade, debris-resistant brass that resists fuel gumming and corrosion better than generic alloys.

• Perfect Atomization: The internal taper and finish of a genuine jet are polished to a mirror shine. This ensures the fuel breaks down into the finest mist possible for a crisp, clean burn.

• Zero Guesswork: Why spend six hours "plug chopping" and chasing a ghost in the machine? With Keihin, you install it, sync it, and go.

The "Cheap Jet" Tax

Buying a $10 kit of 20 jets might feel like a win, until you realize you’re paying for it in lost shop hours, frustrated customers, and potential engine damage from a lean condition you didn't see coming.

The Mechanic’s Rule: If you don't have time to do it right, when will you have time to do it over?

Comparison at a Glance

The Bottom Line

Your engine is an air pump that depends entirely on a precise ratio of fuel to stay alive. Don't let a $5 savings on a jet ruin a $5,000 engine.

Give your machine the precision it deserves. Stick with Keihin.

Determining the right jet size is a mix of science, intuition, and a little bit of "mechanical voodoo." Since carburetors are analog devices, they don't have sensors to adjust for you—you have to be the computer.

Here is the professional approach to dialing in your jetting.

1. Understand the Variables

Before you swap a single brass piece, know that these three things change your air-fuel requirements:

• Altitude: Higher altitude = thinner air. You need smaller jets (leaner).
• Temperature: Colder air is denser. You need larger jets (richer).
• Modifications: Adding a high-flow exhaust or air filter usually means you need more fuel (larger jets).

2. Diagnose by Throttle Position

Don't just change the main jet and hope for the best. You need to identify where the bike is struggling:

3. Identify the Symptoms

Listen and feel for how the engine reacts. This is your best guide for whether you are "Lean" (too much air) or "Rich" (too much fuel).

Signs You Are LEAN (Need Bigger Jets)

• The engine "pops" or backfires on deceleration.
• The bike hangs at a high RPM after you let go of the throttle.
• The engine feels very hot and "pings" under load.
• The spark plug is white or ashy.

Signs You Are RICH (Need Smaller Jets)

• The engine sounds "blubbery" or stutters (like a rev-limiter) at high RPM.
• The exhaust produces black smoke or a heavy smell of raw gas.
• The bike feels "lazy" and won't crisp up when you crack the throttle.
• The spark plug is black, sooty, or wet.

4. The "Plug Chop" Test

This is the gold standard for testing your Main Jet:

1. Put in a brand-new spark plug.
2. Warm the engine up, then find a safe straightaway.
3. Hold the throttle wide open through several gears for about 10–15 seconds.
4. While still at full throttle, kill the engine and pull the clutch (don't let it idle!).
5. Remove the plug and look at the porcelain:
• Tan/Light Brown: Perfection.
• White: Dangerously lean (Go up a jet size).
• Black: Rich (Go down a jet size).

Pro Tips for Success

• Change one thing at a time: If you change the pilot and the main jet at once, you won't know which one fixed (or broke) the performance.
• Start Rich and go Lean: It is much safer to have an engine that stutters from too much gas than an engine that melts a piston from too much air.
• Clean the Carb first: Half of all "jetting issues" are actually just a piece of dirt stuck in a passage. Ensure the carb is spotless before tuning.

Would you like me to look up the factory stock jetting specs for your specific ATV or dirt bike model?