2018-2024
PWK 28mm bored- Main Jet
- 145
- Pilot Jet
- 45
- Needle
- NRKC OEM
- Clip
- Clip 3
- Air Screw
- 1.5-2 turns out
- Baseline
- 1,000 ft / 75-80 deg F
Loretta Lynn Championship Proven
KTM 85 Carburetor Setup Guide
HP Race Development baseline specs, RAD adjustment tables, and Keihin jet math for KTM 85 bored carb setups.
HP bored carb baseline
Starting Specs
Use these as starting points for HP Race Development modified KTM 85 carburetors. Final jetting still needs to be verified at race-day conditions.
2025+
PWK 28mm bored- Main Jet
- 145
- Pilot Jet
- Contact HP Race Development
- Needle
- NRKC OEM
- Clip
- Clip 3
- Air Screw
- 1.5-2 turns out
- Baseline
- 1,000 ft / 75-80 deg F
2025+ pilot jet note
The 2025+ platform has different pilot needs. Contact HP Race Development before treating older pilot specs as a direct swap.
Fuel warning
Pump gas and standard race fuels follow these starting points. High-oxygenated fuels may require richer settings. Always plug chop.
Tuning order
Work From Idle To WOT
A two-stroke carb has overlapping circuits, so the order matters. Get the lower circuits honest before chasing the main jet.
1
Set air screw
Start at 1.5 turns out. Adjust at idle for the crispest response. If it wants more than 2.5 turns, go up one pilot size. If it wants less than 1 turn, go down one pilot size.
2
Dial pilot jet
Clean up throttle pick-up from idle and through slow technical sections. Hesitation off corners points lean; loading up and blubbering points rich.
3
Set needle clip
Tune the 1/4-3/4 throttle roll-on feel. Higher clip number is richer. Lower clip number is leaner.
4
Calculate main jet
Once the lower circuits are right, use RAD math for the main jet and verify at full throttle with a plug chop.
Air density
RAD Calculator
Relative Air Density estimates how much oxygen is available compared with sea-level standard conditions. Higher RAD wants more fuel; lower RAD wants less.
Race Conditions
Baseline is about 93% RAD.
Calculated RAD
92.4%
18-24 main jet145
Calculated size144.5
NeedleClip 3
DirectionLeaner than HP baseline
Carb circuits
Throttle Opening Influence
These circuits overlap. A change in one area can show up somewhere else, especially needle straight diameter at low throttle.
Idle1/41/23/4WOT
Air screw
Broad in-day trim
Pilot / slow jet
Idle to 1/2 throttle overlap
Slide cutaway
Initial opening transition
Needle straight
Low throttle fuel leak / control
Needle taper
Midrange fuel curve
Clip position
Needle height
Main jet
Top-end / WOT
RAD tables
Adjustment Charts
Main jet changes follow Keihin diameter math. On a close call, choose the richer jet and verify under load.
2018-2024 KTM 85
| Conditions | RAD | Main | Pilot | Needle | Air Screw |
|---|---|---|---|---|---|
| Sea level, 60 deg F, dry | 100% | 152 | 48 | Clip 4 | 1.5 out |
| Sea level, 75-80 deg F | 96-97% | 148 | 48 | Clip 3 | 1.5-2 out |
| HP baseline - 1,000 ft, 75-80 deg F | ~93% | 145 | 45 | Clip 3 | 1.5-2 out |
| 2,000-3,000 ft, 70-80 deg F | 87-90% | 142 | 45 | Clip 2 | 2 out |
| Hot / humid low altitude | ~88-90% | 138 | 40-42 | Clip 2 | 2 out |
| 3,000-4,000 ft, 70-80 deg F | 84-87% | 138-140 | 42 | Clip 2 | 2 out |
| 5,000+ ft, 70-80 deg F | 80-84% | 135 | 42 | Clip 1, consider leaner needle | 2-2.5 out |
2025+ KTM 85
| Conditions | RAD | Main | Pilot | Needle | Air Screw |
|---|---|---|---|---|---|
| Sea level, 60 deg F, dry | 100% | 152 | 55 | Clip 4 | 1.5 out |
| Sea level, 75-80 deg F | 96-97% | 148 | 55 | Clip 3 | 1.5-2 out |
| HP baseline - 1,000 ft, 75-80 deg F | ~93% | 145 | 52 | Clip 3 | 1.5-2 out |
| 2,000-3,000 ft, 70-80 deg F | 87-90% | 142 | 52 | Clip 2 | 2 out |
| Hot / humid low altitude | ~88-90% | 138 | 48-50 | Clip 2 | 2 out |
| 3,000-4,000 ft, 70-80 deg F | 84-87% | 138-140 | 50 | Clip 2 | 2 out |
| 5,000+ ft, 70-80 deg F | 80-84% | 135 | 48 | Clip 1, consider leaner needle | 2-2.5 out |
Keihin math
Why The Formula Works
Keihin jet numbers are based on orifice diameter in hundredths of a millimeter, so the fuel-flow change follows area, not straight diameter.
New Jet = sqrt(Current Jet^2 x (New RAD / Old RAD))
Example: if a 148 main is correct at 100% RAD and race-day RAD drops to 94%, the math lands near 143.5. Round to the richer available Keihin size, so you would start with a 145.
Needle clip limits
Baseline is clip 3. Each clip change is roughly a 6% RAD correction. Once you run out of clip range, switch needle taper instead of forcing the fix with jets alone.
Running Rich
- Black, sooty plug
- Bluish-black smoke
- Blubbering or loading up
- Sluggish top-end
- Hard warm starts
Running Lean
- White or light gray plug
- Overheating
- Popping on decel
- Harsh or flat power
- Power falls off up top
Plug chop reminder
For a main jet read, run full throttle under load, cut throttle and kill the engine together, coast in, and inspect the plug immediately.
Need setup help?
Talk To HP Race Development
These specs are starting points from dyno and track testing. Conditions, fuel, engine health, and rider load still matter.