Not legal2 parts
Only atmospheric engines are eligible, so the class admits no turbocharger or supercharger for this part to serve. 502.01.01.01
- Aftermarket intercooler · Forced induction
- Boost relief / overboost control valve · Forced induction
UIM · runabout
Runabout Juniors GP4
UIM 2026, 302.01. The rulebook does not state a governing principle for this class.
Read out of the published UIM rulebook on 2026-09-08, so confirm anything that decides a purchase. The full note is at the foot of the page.
Running the class
- Rider requirements
- Ages 13-15, reckoned at 31 December; Minor Release Form.
- Race duration
- 12 minutes + 1 lap.
- Riders on the grid
- 15.
- Entry fee
- Free.
- Championship status
- World Championship and Continental Championship class.
Read from the same class section as the part answers. The rider side of the sport, tiers, ages, licences and what gets inspected, is set out across the bodies at Racing.
Places where the rulebook contradicts itself, or where a figure could not be settled. Nothing in this list is decided, and it sits here rather than in a footnote because it is what you would want before spending money.
- Whether any Junior-specific restrictor or power limit is applied in practice - none is published.
What the rules say (57 parts)
7 permitted outright, 16 permitted with stated conditions, 11 refused by a rule we can quote, 5 held to the original part by a rule we can quote and 18 refused by our reading of the class rather than by a quoted rule. Grouped where the same rule gives the same answer, so two "Not legal" groups can sit side by side under different rules.
What each answer below means
- Not legal
- This comes off. The rules refuse it in this class.
- OEM part required
- This comes off. Only the part the manufacturer fitted is allowed here.
- Not permitted, not listed in the rules
- Treat it as coming off. The class requires the craft to stay stock except where the rules allow otherwise, and this part is not on the list, so this is our reading rather than a rule we can quote at you.
- Legal, with conditions
- Keep it, but measure it: the rules permit this part only inside stated limits.
- Legal
- Keep it. The rules permit this part in this class.
Read the refusals as a list of work. Anything below marked Not legal, OEM part required, or Not permitted has to come off the machine, or go back to the standard part, before it races in this class.
Not legal1 part
ECU must be original. Aftermarket ECU prohibited. 502.01.05-02
- Aftermarket standalone ECU · Ignition
Not legal1 part
No internal engine modifications of any kind; ports may not be cleaned with abrasive material. 502.01.03-01
Not legal1 part
Only atmospheric 4-stroke 950 cc is listed.
- Aftermarket turbocharger or supercharger · Forced induction
Not legal1 part
ANY internal modification - grinding, surfacing, polishing, machining, shot blasting - of any transmission component, including the stator and exit cone, is prohibited. 502.01.06-02
Not legal1 part
The driveline article opens only the impeller, wear rings and pump seal, bars any modification of transmission parts, the exit cone included, and lets the trim go manual only without modifying the nozzles. 502.01.06.01, 502.01.06.02, 502.01.06.04
- Aftermarket pump nozzle and directional nozzle · Driveline
Not legal1 part
Nothing in the driveline article opens the couplers, bearing housing or driveshaft, and it bars any modification of transmission parts, so they stay stock under the closed list. 502.01.06.02, 502.01-02
Not legal1 part
The driveshaft, impeller housing and stator are not opened to replacement and transmission parts may not be modified, so a titanium one cannot be fitted under the closed list. 502.01.06.02, 502.01-02
Not legal1 part
Homologated production craft only - 50 specimens, marketed, conformity certificate. 500.02.01
- Aftermarket or prototype hull · Hull and handling
Not legal1 part
NO MODIFICATIONS TO THE HOOD are allowed beyond those listed. 502.01.02-15
- Aftermarket hood · Hull and handling
OEM part required1 part
Crankshaft must remain stock; replacement bearings or bearing shells only, of original type and dimensions. 502.01.03-02
- Aftermarket four-stroke crankshaft · Engine
OEM part required1 part
Camshaft(s) must remain stock; replacement bearings of original type and dimensions only. 502.01.03-03
- Aftermarket camshafts · Engine
OEM part required1 part
THE HEAD GASKET MUST BE ORIGINAL - stated expressly and uniquely for Runabout GP4. 502.01.03-01
- Aftermarket head gasket · Engine
OEM part required1 part
THE WHOLE ORIGINAL EQUIPMENT SEAT for that model and type must be used. 502.01.02-12
- Aftermarket seat assembly or seat base · Hull and handling
OEM part required1 part
Repairs may move the hull no more than 5 mm from its original configuration, and sanding, grinding and polishing are allowed only where the original shape does not change at all. 502.01.02-02, 502.01.02-03
- Modification of the OEM hull · Hull and handling
Not permitted, not listed in the rules14 parts
Not listed in this class. Illegal under the class's own stock clause.
- Through-hull exhaust outlet flap removal · Exhaust
- Aftermarket intake manifold · Fuel
- Aftermarket electric fuel pump · Fuel
- Aftermarket fuel filler cap · Fuel
- Long-distance fuel filling system (stopper and tank connection) · Fuel
- Aftermarket or modified throttle body · Fuel
- Aftermarket charging system / stator · Ignition
- Aftermarket valve springs, washers and retainers · Engine
- Aftermarket valves · Engine
- Replacement engine mounts / engine bed · Engine
- Replacement starter motor and bendix · Electrical
- Ballast weight kits · Hull and handling
- Hull extension · Hull and handling
- Supercharger internals, clutch and pulleys · Forced induction
Not permitted, not listed in the rules1 part
The class is four-stroke only and prints no exhaust article, so the original water box stays fitted under its own stock clause. 502.01-02
- Aftermarket waterbox · Exhaust
Not permitted, not listed in the rules1 part
The class is four-stroke only and prints no exhaust article, so the original manifold and head pipe stay fitted under its own stock clause. 502.01-02
- Aftermarket exhaust manifold / head pipe · Exhaust
Not permitted, not listed in the rules1 part
No fuel-system hardware article; treat as stock under the closed list. 502.01-02
Not permitted, not listed in the rules1 part
Only the manual trim system substitution.
- Aftermarket hull trim tabs · Hull and handling
Legal, with conditions1 part
Equivalent aftermarket air filters/flame arresters available on the market; the flame arrestor/air restrictor DOWNSTREAM of the throttle body may be removed. 502.01.04
Legal, with conditions1 part
Excess oil or fuel leaving it must be caught and must not spill into the engine compartment. 502.01.04-03
Legal, with conditions1 part
Must fit the OEM battery box and be securely fastened. GP2 additionally allows relocation of the battery, box or housing limited to the strict minimum needed to fit an aftermarket exhaust.
- Replacement battery · Electrical
Legal, with conditions1 part
Original diameter maintained. 502.01.06-01
- Aftermarket impeller · Driveline
Legal, with conditions1 part
Permitted, subject to the limits UIM applies to every class, namely the sound ceiling, the hull repair tolerance, the fuel specification, and the intake grate, sponson and pump dimensions.
- Replacement wear ring · Driveline
Legal, with conditions1 part
The original trim system may be replaced by a MANUAL system, without nozzle modifications. 502.01.06-04
- Aftermarket nozzle trim system (pump-mounted) · Driveline
Legal, with conditions1 part
Max 177.80 mm. 502.01.02-20
- Aftermarket pump cover plate / extension · Driveline
Legal, with conditions1 part
Full-length type with at least one bar parallel to the drive shaft, all leading edges radiused, extending no more than 12.00 mm below the flat plane of the pump intake area. GP1 adds that the projecting member must be filed so as not to create a hazard.
- Aftermarket intake grate · Driveline
Legal, with conditions1 part
TWO REAR SPONSONS required. Max 92 cm each side, 100 mm protrusion, 63.5 mm channel, 63.5 mm below the hull intersection, minimum 6 mm thick.
- Aftermarket sponsons · Hull and handling
Legal, with conditions1 part
The cover may be changed, including covering seat and backrest as a single piece, provided the shape and dimensions are unaltered. The glossary caps aftermarket seat covers at 10 mm thickness and forbids altering the original shape.
- Aftermarket seat cover · Hull and handling
Legal, with conditions1 part
Aftermarket shaft, holder, handlebar holder and cables ARE allowed, but QUICK-TURN STEERING RATIO MODIFICATION IS NOT GRANTED IN GP4 - it starts at GP3. 502.01.02-11
- Aftermarket steering shaft, holder, handlebar holder, cables · Hull and handling
Legal, with conditions1 part
Padding required. Kill switch may be relocated; start/stop switches may be replaced or aftermarket. 502.01.02-11/16/17
- Aftermarket handlebars, throttle, cable, grips · Hull and handling
Legal, with conditions1 part
Start/stop switches may be replaced or aftermarket; the kill switch may be moved from its original position. 502.01.02-16/17
- Aftermarket switches and switch housings · Electrical
Legal, with conditions1 part
Replacement bumpers allowed. A soft flexible water-spray deflector may attach to the hull sides or bond flange, not exceeding the perimeter of the original bumpers or the external hull edge measured with a plumb line. SAFETY BUMPERS ARE COMPULSORY in both GP1 classes.
- Replacement bumpers and water-spray deflectors · Hull and handling
Legal, with conditions1 part
Water-exit derivations modified or replaced, directed downward and/or rearward. No new water exit on head, cylinder or casing. 502.01.03
Legal, with conditions1 part
All cooling system valves must be of FIXED OR AUTOMATIC type - thermostats, pressure regulators, solenoids. Manually controlled devices that alter cooling water flow while under way are prohibited in every class, as are electronic water injection systems unless of origin.
Legal1 part
The ducting between the flame arrester and the throttle body or intake manifold inlet may be modified or aftermarket. 502.01.04-02
Legal1 part
ECU must be original; SOFTWARE may be reprogrammed. 502.01.05-02
- ECU reflash / software reprogramming · Ignition
Legal1 part
Aftermarket spark plugs of a different heat rating are allowed in every class that addresses them.
- Aftermarket spark plugs of a different heat rating · Electrical
Legal1 part
The reverse gate must be REMOVED. 502.01.06
- Reverse gate removal · Driveline
Legal1 part
Padding and mat kits may be added and custom painting is allowed. Metal components above the bond flange may be polished, shot peened or painted.
- Padding, mat kits and custom paint · Hull and handling
Legal1 part
The original bilge pump may be modified or disconnected, and aftermarket bilge draining systems are allowed if no hazard is created, but removing the pump is not listed. 502.01.02.14
- Aftermarket bilge draining system · Hull and handling
Legal1 part
Cooling system flush kits are allowed in every class that addresses them.
- Engine flush / rinse kit · Cooling
UIM 2026, 302.01, read from the rulebook on 2026-09-08. Read the rule yourself, Runabout Juniors GP4. Every UIM class · where the rulebooks differ · how this site reads a rulebook.
This is one class. Every part below also has a page of its own showing where it stands across every class this body runs, and legality by part is the index of them.
Cite this page
“Runabout Juniors GP4, UIM class rules”, BaddJet archive, https://baddjetarchive.com/legality/uim/runabout-juniors-gp4/
Add the date you read it.
57 parts read · 7 permitted, 16 with conditions · 2026 rulebook