Home > Engine Performance > Small Engine Performance Parts > Honda GX390 / Predator 420cc > Honda GX390 Big Block Hop Up Kits
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Main Jet Performance Upgrade Kit For GX390 and Predator 420cc Main Jet Performance Upgrade Kit For GX390 and Predator 420cc

Our main jet kit is a must for the Honda GX390, Predator 420cc engines that have had the performance K&N style air filter setup installed on the engine. Maximizes the engine for best performance and air fuel ratio.

Our Price: $8.14
Header Kit, GX390 Mud Motor Header w/Muffler Straight Header Kit, GX390 Mud Motor Header w/Muffler Straight

Mud Boat header kit that fits the big block Honda GX240, GX270, GX340, GX390, Predator 420cc and 460cc engines. Our header kit features 1 1/4" O.D. header tube and is TIG welded for best durability. Our header kit includes the following components:

  • Straight header tube to fit mud motor specifically
  • Our custom TIG welded header flange
  • Screw on muffler

Our Price: $36.00
Header Kit, GX390 Mud Motor Header w/Muffler Angled Header Kit, GX390 Mud Motor Header w/Muffler Angled

Mud Boat header kit that fits the big block Honda GX240, GX270, GX340, GX390, Predator 420cc and 460cc engines. Our header kit features 1 1/4" O.D. header tube and is TIG welded for best durability. Our header kit includes the following components:

  • Header
  • Our custom TIG welded header flange
  • Screw on muffler

Our Price: $38.00
Bullfrog Pro Velocity High Performance Air Filter Kit for Honda GX390 Bullfrog Pro Velocity High Performance Air Filter Kit for Honda GX390

Our Pro Velocity high performance air filter kit fits Honda GX340, GX390, Predator 420cc and 460cc big block clone engines. Upgrade your go kart, mini bike or drift trike with our high performance air filter kit. Our kit includes the following items:


  • High flow racing style air filter 4" filter length
  • Foam Pre-Filter
  • Radiused air filter adapter
  • Choke hold bracket

Our Price: $44.95
Mud motor Predator 420cc engine kit Mud Motor Stage 1 Kit for Honda GX390, Predator 420cc, 460cc Big Block

Our Mud Motor Stage 1 Kit is designed for the Honda GX390, Predator 420cc and 460cc engines.  Our Stage 1 kit includes our performance air filter, radiused air filter adapter, air filter adapter gasket, foam air filter and choke hold down.

Our Price: $45.99
High Performance Air Filter Kit for Honda GX390 High Performance Air Filter Kit for Honda GX390

High performance air filter kit that fits Honda GX340, GX390, Predator 420cc and 460cc big block clone engines. Upgrade your go kart, mini bike or drift trike with our high performance air filter kit. Our kit includes the following items:


  • High flow racing style air filter 6" filter length
  • Foam Pre-Filter
  • Radiused air filter adapter
  • Choke hold bracket

Our Price: $54.95
High Performance Air Filter Kit for Honda GX390 w/Outerwears High Performance Air Filter Kit for Honda GX390 w/Outerwears

High performance air filter kit that fits Honda GX340, GX390, Predator 420cc and 460cc big block clone engines. Upgrade your go kart, mini bike or drift trike with our high performance air filter kit. Our kit includes the following items:


  • High flow racing style air filter 4" filter length
  • Outerwears Pre-Filter
  • Radiused air filter adapter
  • Choke hold bracket

Our Price: $58.95
Mud motor Predator 420cc engine kit Mud Motor Stage 1 Kit for Predator 420cc (Outerwears Pre-Filter)

Our Mud Motor Stage 1 Kit is designed for the Predator 420cc engine.  Our Stage 1 kit includes our performance air filter, radiused air filter adapter, air filter adapter gasket, outerwears air pre-filter and choke hold down.

Our Price: $59.95
High Performance Air Filter Kit for Honda GX390 w/Outerwears High Performance Air Filter Kit for Honda GX390 w/Outerwears

High performance air filter kit that fits Honda GX340, GX390, Predator 420cc and 460cc big block clone engines. Upgrade your go kart, mini bike or drift trike with our high performance air filter kit. Our kit includes the following items:


  • High flow racing style air filter 6" filter length
  • Outerwears Pre-Filter
  • Radiused air filter adapter
  • Choke hold bracket

Our Price: $68.90
500cc Big Block Racing Engine, Longblock, No Cam 500cc Big Block Racing Engine, Longblock, No Cam

Bullfrog Performance Products is the first in the industry to offer a 500cc big block racing engine package. We are always innovating and bringing new engine packages to the small engine performance enthusiast. Our 500cc longblock is custom built and comes with the following high quality components:

  • Bullfrog Performance Products custom Big Bore tall deck cylinder block for stroker crank applications
  • Bullfrog Performance Products custom forged piston and low drag ring kit
  • Head Dowels
  • Side Cover 10-Bolt
  • ARP side cover bolt kit
  • Solid side cover dowel pins
  • Dipstick and oil fill cap
  • Cranksaft oil seals made from Viton material for higher operating temperatures
  • Roller main bearings
  • Custom stroker crankshaft
  • Bullfrog Performance Products billet connecting rod kit
  • ARP rod bolts
  • Babbit Rod Bearings
  • Victor Reinz big bore fire ring style head gasket
  • Bullfrog Performance Products custom head package
    • GX390 aftermarket cylinder head
    • Big valve package
    • Custom multi-angle valve job
    • Ported, polished and flow tested head
    • Dual valve spring package
    • Billet valve spring retainer kit
    • Chrome moly pushrod kit
    • Roller rocker arm package
    • ARP head bolt kit
This engine package does not include a camshaft or lifter set. We have many cam grinds available so just email us on what you are looking for or put a note at checkout on what cam you are looking for. We will then send a separate invoice for the cam and lifter kit.

Our Price: $2,495.00
Sale Price: $1,900.00
Savings: $595.00