After a rebuild or new parts, the engine can be stiff and hard to start the first time. All the parts are new and rough. Some engines also seem to have no compression for a short time, and then start by themselves.
Is it safe to ride?
Do not ride until it starts and runs well. Break in the engine carefully afterwards: Break in a new or rebuilt engine.
Quick checks first
- Does the plug spark? Test for spark.
- Is the fuel on and reaching the carburettor? Is the choke or enrichener set properly? Start a cold carburettor engine.
- Were the push rods, valves and rocker parts fitted in their original places? Parts marked at strip-down avoid this: Keep parts in order.
- Are the valve clearances right? Set valve clearance.
Diagnose it step by step
1Does the engine have spark and fuel?
- No: Engine turns over but will not start.
- Yes: go to step 2.
2Does a very small squirt of petrol into the plug hole help it start?
- Yes: use only a few drops, with a fire extinguisher near. Refit the plug and start. A new engine often starts this way. Starting fluid works too but use very little. If it does not start, find the cause. Do not keep adding fuel or fluid. A stray spark can blow the exhaust off or start a fire.
- No: go to step 3.
3Does the engine seem to have no compression right after assembly?
- Some engines have hydraulic tappets (valve lifters filled with oil). They are pumped up during assembly and may take some time to bleed down to the right clearance. Wait for a while, then try again. If it starts after about an hour, this was the cause.
- No hydraulic tappets: Test engine compression and check the valve timing.
4Does a two-stroke start only with starting fluid and run for the rest of the day, but need it again on a cold morning?
- This usually means worn crankcase seals: Two-stroke is hard to start, runs lean or smokes.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| New parts not yet wet with fuel and oil | Starts with a few drops of fuel in the plug hole | Prime lightly and start |
| Fuel or spark fault | No spark or no fuel at the carburettor | Cranks but will not start |
| Hydraulic tappets still bleeding down | Compression returns after waiting | Wait, then retry |
| Valve timing or clearance wrong | Marks do not line up | Re-time the engine |
| Worn crankcase seals (two-stroke) | Starts only with starting fluid | Replace seals |