# 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.