Flathead Kart Engine Training Manual

Source: Experienced Builders wealth of knowledge

1. Engine Identification & Core Components

Block Identification

• Look inside the valve spring box: a "13" marking indicates a 5 hp; a "10" indicates a 4 hp.
• Blower housing side usually has stamped numbers (5 hp blocks often start with 13xxxxx).
• Steel sleeve vs. aluminum bore and single vs. dual bearing are common variations. Dual-bearing blocks need careful endplay management.
• Steel sleeve blocks can usually take more abuse and are preferred by many builders for modified engines.
• Older blocks (pre-R4) may require welding on the cam boss or lifter bores when using high-lift cams.
• Dual-bearing blocks require the side cover to be shimmed carefully so the crank does not bind when the cover is torqued.
• Always check the lifter bore condition before installing a high-lift cam. Worn or egg-shaped bores need welding and re-boring.

Crank & Timing Gear

• Older cranks use a 3/16" wide key; newer use 1/8" woodruff key.
• Press-fit crank gears (Horstman or ARC billet) are preferred for limited/modified engines to eliminate play.
• Pin-punch the key in the crank to remove play if using stock gears.
• For stroker cranks, gear tooth count remains the same as stock block flathead (26 or 29 tooth depending on application).
• Crank endplay is set with side cover gaskets (.012–.014" ideal; up to three gaskets allowed in some rules). Shims are sometimes used when not under strict rules.

Governor Gear

• Remove the governor gear for racing; the plastic gear will slip off the shaft on the side cover. Remove the tab arm and replace the hole with a 1/4" bolt.

Flywheel

• 3 hp flywheels spin up faster than stock 5 hp wheels and are noticeably different on tracks with big RPM drops.
• Stock cast iron/steel 5 hp flywheels are preferred over aluminum for durability in racing.
• Always perform the "ring test" and inspect for puller marks or damage.

2. Fuel Systems & Carburetion

Carburetor Setup (Tillotson & Stock)

• Stock-appearing carbs are limited to .695" bore (checked on engine side).
• Short stem ID and height affect fuel volume and signal. Opening the short stem too much reduces vacuum on small-plate engines.
• Remove the short stem entirely for some builds; chamfer the bottom of the hole.
• Two gaskets between carb and tank are common and legal in many classes; helps prevent long tube bottoming out.
• FelPro thicker gasket (.068") is used by some but requires a perfectly flat tank surface.
• Welch plugs: seat evenly with a socket, then lightly tap edges with a ball-peen hammer in two spots. No silicone needed if press fit is good.
• Fuel tank inserts limit sloshing in tight corners and help maintain fuel level in the short stem cup.
• Fuel tank inserts also reduce the chance of the pickup tube sucking air in left-hand corners.
• The long tube screen should be checked every race weekend. A partially clogged screen leans the engine at high rpm.
• Needle and seat should be replaced as a pair. A worn needle will not seal even with a new seat.

Fuel Types & Changes

• Methanol engines can be converted to gas by changing to a smaller jet (example given: .038" starting point) and retarding timing.
• Engines built for methanol run rich on gas without jet change.
• Pump gas (87 octane, non-ethanol preferred) works in low-compression flatheads. Higher octane does not equal more power and may reduce performance.
• Ethanol in fuel requires thorough purging with gas or WD-40 after racing to protect aluminum.
• Methanol must be stored airtight; it absorbs water. Old methanol can be used for starting fires once diluted.
• Happy Cap allows fuller tank without venting issues in corners.
• Overfilled tanks cause pause in turns.
• Fuel vapor/reversion at high RPM is common on rich Jr. drag setups.
• When switching from methanol to gas, run several tanks of gas through the system and change the oil before the first race.

Jetting & Tuning

• Jet size is chosen to keep cylinder head temperature in the 400–425° range (some builders target ~375°).
• Muffler pipe generally requires slightly richer jetting than loop pipe.
• Needle tuning is more sensitive on gas than methanol.
• Air density changes require jet adjustments; larger jet for higher density.
• Higher air density (cooler, drier, higher pressure) requires a larger jet. Lower air density requires a smaller jet.
• Do not automatically richen just because the day is warmer. Density is the real variable.
• CHT is an insurance number; EGT is considered more accurate by some.
• Tape the blower housing to reduce air pumping losses.

Cleaning & Maintenance

• Fuel tank: use Black Diamond blasting sand or nuts + gas, then rinse thoroughly with methanol/acetone if switching fuels.
• Never use gas to rinse a methanol tank if fuel tech is present.
• Carb cleaning: check passages, diaphragm, screen on long tube, and needle/seat.
• After methanol, remove all residue from carb (gas, WD-40, or Yamaha fuel mix).
• After methanol use, pull the carb and spray the passages with gas or WD-40 before storage.

3. Clutch Systems

Clutch Types

• 2-disc clutches work fine for flathead classes; 3-disc for higher HP.
• Disc clutches (Bully, Revolution) are preferred by many over shoe clutches.
• Disc clutches generally handle heat better than shoe clutches on long green-flag runs.
• Shoe clutches (Noram GE series, Hilliard with Bully conversion) are easier for low-maintenance drivers.
• Heavy shoes (200 g) with white springs or light shoes (150 g) with red springs are common for Noram.

Engagement RPM

• Set clutch to slip at peak torque (from dyno sheet if available).
• Common starting points mentioned: 3800–4000 rpm for adult stock heavy; 3200 rpm for purple plate peewee.
• Use tach replay to observe the first RPM drop for true engagement point.
• Chatter after initial engagement is normal but excessive means rebuild needed.
• If the clutch chatters on engagement, the air gap is usually too large or the discs are glazed.
• Noram recommended numbers are chatter engagement and too low.
• Bathroom scale against wall method can locate peak torque.

Installation & Removal

• Clutch bolt removal: use strap wrench or oil filter wrench to hold clutch; long wrench, not impact.
• Greased Lightning clutch removal: longer 5/16" fine thread bolt + pry bars + hammer. Thread bolt in, rap with hammer while prying.
• PB Blaster on crank/keyway helps. Keep pry bar against driver to avoid damaging basket.
• Spacer or washer may be needed on new Blockzilla blocks with longer shaft.
• On new Blockzilla-style blocks, measure crank length before installing the clutch. A spacer is often required.
• Always safety-wire or Loctite the clutch bolt after final torque. A loose bolt destroys the crank keyway.

Settings

• Orange Crush clutch: .035" air gap ideal; 1/8" is worn out. Backing screws out lowers stall; turning in raises it.
• Purple plate example: lightest springs gave 2200–2300 rpm; target peak torque.

4. Camshaft, Valve Train & Timing

Cam Selection

• 96-3 and 96-3 OM are frequently recommended for plate engines.
• 04-3 is finicky; may require careful spring setup.
• 07-3 is used in some WKA torque plate setups.
• 7X and 458 cams appear in limited/modified builds.
• Adjustable cam gears allow timing changes.
• High-lift cams almost always require the cam boss and lifter bores to be welded and re-machined on older blocks.

Valve Springs & Coil Bind

• Coil bind numbers around .370" intake / .350" exhaust are referenced for some cams.
• Coil bind should be checked with the spring at full lift. Less than .030" remaining travel is risky.
• 1.400 HD and 1.485 Dyno springs are common.
• Installed height and pressure must be measured with a spring tester (drill press + scale method works).
• Exhaust springs are stronger than intake. Do not mix them up.
• Springs drop pressure after first run; check after every weekend.
• Float valves intentionally on high-RPM cams.

Valve & Guide Work

• Stock valves: intake ~1.115", exhaust ~.990".
• Valve-to-guide clearance: run as tight as possible without seizing.
• Lapping valves ensures seal. Lapping valves after every refresh restores the seal better than just replacing the springs.
• Replace guides if excessive wiggle or valve slides sideways on seat.
• Seat width: narrow high seat on intake for flow; wider on exhaust for heat dissipation.

Timing

• 04-3 gold plate: 29–29.5° starting point.
• Lower timing on tight tracks.
• Degree wheel or .280" reference used for crank gear timing.

Welding & Repairs

• Weld cam boss/lifter bores for high-lift cams.
• TIG welding requires very clean, oil-free aluminum; pre-heat to 350° and use low heat.
• Holes in blocks can be TIG welded or patched with epoxy.
• Welch plug holes and other repairs follow same clean/pre-heat process.

5. Exhaust & Pipes

Header Choice

• Loop header (non-muffled) helps bottom end.
• A loop header improves bottom-end response on short tracks.
• Muffler pipe generally needs richer jetting.
• A muffler pipe is often required by rules and needs a richer jet.
• Leave first 2–3" of header unwrapped.
• Header wrap on the first few inches can raise under-hood temperature. Leave the first 2–3 inches bare.
• Header wrap does not cause excessive heat issues when applied correctly.
• Pipe length affects both power and heat. A longer pipe generally cools the engine slightly and moves the power curve down.

Temperature Management

• 420° head temp is not excessive.
• Lean mixture, jetting, ignition timing, spark plug range, and pipe length all affect heat.
• Warmer days do not always require richer jetting.

6. Maintenance, Storage & Refresh

Storage

• Drain fuel and oil.
• Spray carb and internals with fogging oil or WD-40.
• Tape all openings.
• Rotate engine occasionally to compression stroke (valves closed).
• Rotate the engine to TDC on the compression stroke before long-term storage so both valves are closed.
• Store on wood in cool, dry place.
• Check for rust on valves and rings before re-use.

Refresh / Rebuild

• Rings and valve seal are primary wear items.
• Lightly hone cylinder, lap valves, replace springs if pressure low.
• Replace rod bearings every rebuild; rod bolts every 2–3 rebuilds.
• Rod bolts should be replaced every 2–3 rebuilds even if they look fine. Stretch is hard to see.
• Piston replacement when skirt scuffs or ring lands wear.
• Piston skirt scuffing is the most common reason to replace a piston. Ring land wear is the second.
• Cam lasts indefinitely if oiled properly.

Oil

• 12–14 oz typical in flat-mount 3–5 hp engines.
• Conventional or high-zinc oils are commonly used.
• Change oil after practice, heats, and features.
• Milky oil after methanol use indicates fuel contamination (check rings, jetting, timing).

Puke Tank

• 1.5 oz after 2 hours running is noted; considered normal blow-by or crankcase ventilation.

7. Racing Classes & Setup Tips

Classes

• WKA stock flathead rules are stable and used for vintage/exhibition classes.
• Limited modified allows porting, bigger bore (2.643" max), billet rod, etc.
• Weight breaks exist when mixing gas and methanol (example: 360 lb gas vs. 400 lb methanol).
• Weight breaks between gas and methanol are common. Always confirm the current class minimum before adding or removing weight.
• Purple plate, blue plate, gold plate, turquoise plate are common restrictor classes.
• Plate engines generally want lower clutch engagement than unrestricted engines.
• Purple plate engines usually want the lightest clutch springs and the lowest practical engagement rpm.
• Gold plate engines can often run a slightly higher stall than purple plate engines.
• Jetting and timing are more sensitive on plate engines because airflow is restricted.
• Always confirm the exact plate and allowed modifications for the class.

Gearing & RPM

• Match gearing to track length and corner types.
• Match gearing to track length and corner speed so the engine stays near its preferred rpm range.
• Same tires + same gearing should produce similar peak RPM regardless of cam (within reason).
• Same gear and same tires should produce similar peak rpm. Large differences usually mean a clutch, jetting, or engine health issue.
• Use tach replay to confirm peak and drop rpm before changing gears.
• 50 green-flag laps on ¼-mile track is unrealistic; 35 green + 15 yellow is more typical.

Clutch & Engagement

• Always set at peak torque RPM for best starts and corner exits.

General Dos & Don'ts

• Do: Keep fuel tank clean, change oil frequently, check valve springs often, lap valves for seal.
• Do: Use two gaskets on tank if needed, index spark plugs properly.
• Don't: Run high-octane fuel expecting more power; run ethanol without purging; overfill tank; use impact on clutch bolt.
• Don't: Use gas to clean methanol tanks if tech is present.
• Don't: Leave a loose clutch bolt. It destroys the crank keyway.

8. Spark Plugs

• Heat range is chosen to match compression, fuel, and track conditions.
• Colder plugs for higher compression, methanol, or sustained high rpm.
• Hotter plugs for low-compression plate engines or short tracks.
• Index the ground electrode so it faces away from the intake valve when possible.
• Always check gap and condition after a session. Fouled or blistered plugs indicate jetting or ignition issues.

9. Common Failure Modes

• Loose clutch bolt or worn keyway causes crank damage.
• Ethanol left in the engine corrodes aluminum.
• Overfilled fuel tank causes lean pauses in corners.
• Excessive valve spring pressure loss after a weekend leads to float and valve damage.
• Running high-octane fuel in a low-compression flathead can reduce power.