Spark plug hole finger test — do this first

Start by removing the plug for your version:

🔩 Version A — Engine Kit Remove the glow plug from the cylinder head.

⚡ Version B — Generator Set Remove the spark plug from the cylinder head.

Press your fingertip firmly over the plug hole to seal it completely. With your other hand, slowly rotate the flywheel. As the piston rises on the compression stroke, you should feel a noticeable puff of air trying to push your finger away — even on a new engine this should be clearly felt.

Interpreting your result:

• ✅ Clear, definite puff felt — compression is sufficient. Tuning or starting procedure issue, not mechanical. Go to Section 4.
• ⚠️ Very faint pressure, barely noticeable — some compression present but low. Proceed to Test 2. Go to Section 3 → Path B.
• ❌ Nothing felt at all — genuine compression loss. Proceed to Test 2 immediately. Go to Section 3 → Path C.

Soap bubble test — only if Test 1 showed weak or no compression

This test shows you where the air is escaping from.

• Remove the rocker arms completely so the valves close under spring pressure alone. Leaving them in can hold a valve slightly open and give a false result.
• Mix a small amount of dish soap with water
• Brush the solution around the intake manifold inlet and the exhaust port outlet
• Reinstall the spark plug / glow plug, then gently blow into the intake port

Interpreting your result:

• ✅ No bubbles, or one slow bubble after 5+ seconds — minor seepage, normal for a new engine. Go to Section 5.
• ⚠️ Small but steady stream of bubbles — moderate leakage. Valve lapping will likely fix this. Go to Section 6.
• ❌ Continuous strong stream of bubbles immediately — severe sealing failure. Valve lapping required. Go to Section 6.

Use a slightly richer fuel mixture for the first two tanks

A richer mixture carries more oil through the engine, lubricating rings and valve stems while they seat in.

🔩 Version A — Engine Kit Set the main needle to 2.5 turns out from fully closed — half a turn richer than the standard 2.0.

⚡ Version B — Generator Set Mix at 20:1 for the first two tanks (50 ml of 2T oil per litre of 92-octane gasoline). Return to 25:1 from tank three onward.

Keep throttle below 50% for the entire first tank

High RPM before the rings have seated can glaze the cylinder bore and permanently reduce compression. Vary throttle between idle and 50% — no full throttle.

Allow the engine to cool fully between tanks

After each tank, shut down and allow to cool to room temperature — typically 15–20 minutes. Heat cycling helps metal parts gradually conform to each other.

Gradually increase throttle on tank two

Extend the upper range to around 70%. Brief bursts at higher throttle help seat the rings, but avoid sustained full throttle until break-in is complete.

Re-test compression after two tanks

Repeat the finger test and soap test from Section 2. In most cases the puff of air will be noticeably stronger and soap test will show little or no bubbling.

If compression has not improved at all after two full break-in tanks, move to Section 6 — valve lapping.

Step 1 — Take off the valve cover

The valve cover is the flat panel on top of the engine, held down by 4 small screws. Loosen them a little at a time, working in a cross pattern (loosen one corner, then the opposite corner, then the other two) — this stops the cover from warping. Lift it straight off. You will now see the rocker arms and springs inside.

Step 2 — Remove the rocker arms

The rocker arms are the small lever-shaped pieces that push the valves open. Remove the screws holding the rocker shaft in place and slide the shaft out — the rocker arms will come free with it. Put a small piece of tape on each one and write "IN" or "EX" (intake or exhaust) so you know which is which when you put them back. They look similar but are not interchangeable.

Step 3 — Remove the cylinder head

Loosen the head screws a little at a time in a cross pattern — same as the cover. Once all screws are loose, lift the head straight up. Underneath you will find a thin paper or metal gasket. Set it aside carefully and check it — if it is torn or crushed in any spot, you will need a replacement before putting things back together. A damaged gasket will cause an air leak even after the valves are perfectly fixed. If it looks intact, just set it to one side.

Step 4 — Remove the valves (the fiddliest part — go slow)

Each valve is held in place by a spring and two tiny metal clips called keeper clips. This is the trickiest part of the whole process — not because it is difficult, just because the clips are very small.

Do this over your parts tray. Use tweezers to compress the spring slightly and slide the two keeper clips out sideways. The moment they come free the spring will release, so keep a finger over the area. Once the clips are out, the spring and valve lift straight out.

Keep each valve's parts together in a separate section of your tray. Do not mix intake and exhaust valves — they go back into the same holes they came from.

Step 5 — Clean the valve and its seat

Take the valve and look at the angled ring around its base — that is the seating surface that needs to seal. Now look inside the cylinder head at the matching angled ring where the valve sits — that is the valve seat.

Wipe both surfaces with alcohol on a lint-free cloth until they are completely clean and dry. Any oil or grease left on these surfaces will stop the lapping compound from working properly — so take an extra minute here to make sure both are spotless.

Step 6 — Lap the valve (the actual fix — easier than it looks)

Put a very thin smear of lapping compound around the angled face of the valve — about as much as you would put toothpaste on a toothbrush. Less is better. A thick blob just squeezes out to the sides and does nothing useful.

Drop the valve back into its seat in the cylinder head. Hold the stem between your thumb and index finger. Now just rotate it back and forth — half a turn one way, half a turn back — while pressing it gently but firmly down into the seat. It is a bit like grinding pepper: steady, gentle, back and forth.

Every 30 seconds or so, lift the valve out and look at the angled face. You are looking for a dull grey ring that runs continuously all the way around — no breaks, no shiny patches, just a consistent matte band about 0.5–1 mm wide. If the ring is uneven or has a gap somewhere, just put the valve back and keep going, rotating it a quarter turn before you restart so you work the whole surface.

Most valves are done in 2–5 minutes. You will be able to see and feel when the surface is becoming more even.

Step 7 — Clean off all the lapping compound (most important step)

This step matters more than any other. Lapping compound is an abrasive — it is designed to grind metal. If any of it is left on the valve, in the seat area, or anywhere nearby when you start the engine, it will grind away the piston rings and cylinder wall within minutes. That kind of damage cannot be repaired.

Wipe the valve face and seat with fresh alcohol-soaked cloths. Then wipe again. Use cotton swabs to clean the valve stem, the guide bore, and every corner around the seat. Keep going until you wipe across the surface with a clean white cloth and nothing grey comes off. That is when you know it is clean.

Step 8 — Check your work with the soap test

Before putting anything back together, do a quick check. Place the valve back in its seat by hand — just drop it in, no spring needed. Brush a little soap solution around the port opening and gently blow air in. If no bubbles appear, the valve is sealing — you are done with this one. Repeat for every valve before moving on to reassembly. This only takes a minute per valve and saves you from having to take everything apart again later.

Step 9 — Put the valves back in

Put a small amount of high-temperature grease on the valve stem — this is just to protect it on the first start before any fuel reaches it. Slide the valve back into its guide, fit the spring over the stem, compress the spring, and slide the two keeper clips back into their groove in the stem. Give the valve a gentle tug upward — if the keepers are seated properly, the valve will not pull out. If it does, compress the spring and reseat the clips.

Step 10 — Refit the cylinder head

Place the head gasket back in position — make sure it lines up with all the holes — then lower the cylinder head on top. Start every screw by hand before tightening any of them. Then tighten a little at a time in a cross pattern, the same way you loosened them. Firm is enough — do not go hard. Over-tightening strips the threads or crushes the gasket, which creates a new leak right where you just fixed one. Reference torque is 1.38 N·m for M3 screws if you have a torque screwdriver.

Step 11 — Refit the rocker arms and grease the camshaft

Before fitting the rocker arms, apply a good smear of high-temperature grease to the camshaft lobes (the oval-shaped bumps on the shaft) and to the curved pads on the rocker arms where they contact the cam. This is important: the ST-NF2 has no oil pump, so there is no automatic lubrication to these parts. The grease you apply right now is all they have for the first run. A generous smear here will protect the cam lobes for many hours of running. Slide the rocker shaft back in and tighten the retaining screws.

Step 12 — Check the valve clearance

With the rocker arms back in place and the engine completely cold, check the small gap between the tip of each rocker arm and the top of each valve stem. The target gap is 0.05–0.08 mm. If you have a feeler gauge, slide the 0.05 mm blade into the gap — it should pass through with just a little resistance.

If the gap is too large you will hear a ticking noise when running. If it is too small the valve cannot fully close — which means you would be right back to the leaking problem you just spent time fixing. If adjustment is needed, loosen the locknut on the rocker arm adjuster, turn the adjuster screw until the gap is right, then retighten the locknut.

Step 13 — Close everything up and test

Place the valve cover back on and tighten the screws in a cross pattern. Reinstall the spark plug or glow plug.

Now do the finger compression test one more time: plug the hole with your fingertip and slowly turn the flywheel. You should feel a clear, definite puff of air pushing against your finger. If you do — that is it. You fixed it. The engine now has the compression it needs to start and run.