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Hop Up Kit - GX340 - GX390, 420 Predators, and 13/15hp OHV, Stage 4
Hop Up Kit - GX340 - GX390, 420 Predators, and 13/15hp OHV, Stage 4

Our Price: $859.99

Stock Status:In Stock
Part #: STAGE4KIT390

Head Casting (Application)

13 HP Aftermarket (Chinese) Casting - Large Port
Genuine Honda GX390 UT2, Small Port (High Torque) [+$13]
Genuine Honda GX390 UT1, Large Port (Max RPM) [+$22]
420 Hemi Style Head [+$120]
Select Valve Package

Valve Package*:
Single Springs with Aluminum Retainers
Dual Springs with Titanium Retainers [+$30]
Dual Springs with Aluminum Retainers [+$6]
Select Carburator

23.5 mm (Bored) [+$58]
Carburetor, 24mm Bored 3-Circuit Carb - Gas
GX390 Bored - Methanol [+$89]
34mm Round Slide Mikuni - Gas [+$131]
34mm Flat Slide Mikuni - Gas [+$155]
33 Mikuni Pumper - Gas [+$250]
33 Mikuni Pumper - Methanol [+$326]
Header & Exhaust Options

No Exhaust

Add Header, Kart & Mini-Bike [+$35] .......WARNING: Pipe should be supported with a bracket/brace to either the frame or the engine otherwise pipe may fracture or break.

Add Muffler and Clamp [+$41]
Select Rocker Arm Type

Billet 1.2 Roller Tip Rockers (included in Kit)
1.2 Roller Rockers (includes Spacer) [+$133]
Select Camshaft

280 RPM (Mild cam for larger tracks)
280 TQ (Pulling cam)
306 RPM (Med to large tracks) BEST
306 TQ (Small Tracks)
307 RPM (33mm Carbs or Bigger Carbs)
320 0807 (not recommended)
Select Flywheel

No Flywheel
Add Aluminum Flywheel - Recoil Start [+$166]
Add Aluminum Flywheel - Electric Start [+$202]
Additional Options

Add UT2 Coil w/o Rev Limiter, for UT2 Hondas Only [+$67]

Add Genuine Honda UT1 Coil [+$32]

Description -

The Stage 4 kit starts with package similar to that of the stage 3 but adds a Ported and Milled Head with stainless, stainless valves, stud kit, & HD valve train components. This kist allows up to 35+ HP out of a GX390 and 40+ out of 420/440cc engines.


  • Race/Open Air Filter & Adapter,
  • Choice of Carbs
  • Choice of Cams
  • Ported Head, Milled .100" (Chinese OHV)
  • Stainless valves
  • Aluminum Retainers & Split Keepers
  • Chrome Moly Pushrods
  • Single or Dual Springs
  • 8 degree Timing Key,
  • Billet Rod
  • Carb, Head, & Case gaskets
  • Stud Kit
  • Roller Tip Rocker Arms
    • Optional Aluminum Roller Rockers (NA on Hemi)
  • Optional Header/Exhaust
  • Optional Aluminum Flywheel

Picture for reference only and may show optional parts. See above description for actual content.
Header not included unless specified.

Due to head work required, allow 1 week to ship.

Note: Use of the Choke Bracket requires use of a stock Honda (or Chinese OHV) "Air Filter to Carb Gasket". This is not included in the kit. You can use the one that came with the motor, or can purchase one at
Filter should be sprayed with K&N type oil, especially in dirt applications. Oil not included click here

These kits are intended for experienced engine builders. Some clearancing, fitment, or other engine building "tricks" may be needed.

Billet Aluminum flywheel recommended. Stock flywheels, esp "Clone" are prone to failure and very DANGEROUS

Note: UT2s series Hondas (2012 & up) or later have a built in Rev Limiter in the coil and will require a new coil to exceed 4000 rpm. The UT2 coils (4 wire) can only be used with stock flywheels. If using a Billet Flywheel, you must run a UT1 style coil (single wire).

13/15 Chinese Engines (414, 420, 460) usually have UT1 style coils already. However some Lifans and 420s have a digital ignition and will need a Honda UT1 coil to run t he billet flywheel

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