Hi there,

i just bought my F4i Sport (2001, 24k KM. Recent oil change + filter and air filter) about 4 weeks ago, it ran great since I bought, only thing I noted was that it sometimes had difficulties starting when cold (about 10 seconds cranking). But not the biggest deal.

I decided to do some maintenance and mods this week and did the following:
- Coolant flush (OEM Honda coolant)
- Spark plugs (OEM Iridiums)
- APE Manual CCT (I was having some rattling at 5k rpm before)
- New tires
- Chain maintenance
- Changed all lights to LEDs

Note: I stripped 2 of the bolts holding in the velocity stacks so I had to drill them out and havent replaced them.

Now onto the issue, beforehand, my bike would start and cold idle at around 4k rpm and once fully warm would settle to about 1.4-1.5k (I set it like that using the idle screw).

Now, after assembling everything back and I rode around, I noted that bike idles at 2.5-3k even when warm (I rode for about 30 minutes), and the idle screw does nothing. And once I got home from my ride I also noticed a now more pronounced rattling sound (coming from the engine?), which is pretty loud, like if I step away from the bike I can hear it pretty well even with my loud exhaust.
https://youtube.com/shorts/sd3qJMXQD6E?is=YouG2Q5JrC5rhwgs

Any ideas?

 

Per Grok,

High Warm Idle (2.5-3k, idle screw does nothing)

This is not normal F4i behavior. Stock warm idle is ~1.3-1.5k once the wax unit fully retracts. The two biggest culprits after intake/airbox work (exactly what you did) are:

1. Vacuum leak / unmetered air (very high probability here)
   • Missing or improperly sealed velocity-stack bolts is a known leak point. The stacks (air funnels) bolt directly to the throttle-body tops inside the airbox. Stripped/drilled bolts (even if the stacks "sit" in place) commonly allow extra air to sneak in post-throttle plates. This leans the mixture, raises idle, and makes the main idle screw ineffective because the leak bypasses the idle circuit entirely.
   • Forum consensus: Vacuum leaks anywhere in the intake tract (stacks, boots, hoses, sync caps) are the #1 cause of sudden high warm idle after disassembly. Spraying carb cleaner/start fluid around the stacks, boots, and joints is the standard test—if rpm jumps, you found it.
   • Coolant flush can also introduce tiny air pockets that affect related systems, but the stripped bolts are the smoking gun here.
2. Wax unit (fast-idle plunger) disturbed or stuck
   • The wax unit is coolant-heated and controls the big cold idle drop (your old 4k → 1.4k behavior). Coolant work + reassembly frequently leaves air in the lines or gums it up slightly, so it doesn’t fully retract when warm. Result: idle stays artificially high and the screw can’t override it. Many owners report this exact “idle screw does nothing” symptom after coolant changes or TB work; some fix it by bleeding air or doing a quick external WD-40 clean/retract test.

Throttle cables binding or plates not closing fully are less common but also mentioned after airbox removal. Throttle-body sync is frequently recommended after any intake work because it can amplify these issues.

Bottom line from research: Your velocity-stack bolt situation + coolant flush timing perfectly explains the sudden change. It’s almost never the ECU, sensors, or fuel pressure in these exact scenarios.

Louder Rattling (now audible from a distance)

This is textbook under-adjusted manual CCT.

• APE (and other manual) tensioners do not self-adjust like the stock hydraulic one. After install (especially the first heat cycle), they almost always need ½–1+ turns of extra tightening. Too loose = exactly the chain-slap rattle you’re hearing (worse than the old 5k rpm one because the old tensioner is gone).
• Owners report the same “new, louder rattle right after APE install” dozens of times. The fix is almost universal: warm the bike fully, loosen the locknut, and slowly turn the center bolt in (clockwise) in tiny increments (⅛–¼ turn) while listening until the rattle just disappears—then lock it. A little residual rattle is safer than too tight (which causes whine, power loss, or accelerated wear). Many do this test-ride-adjust cycle 2–3 times.
• Adjustment is best done hot and running (or at least test-revving through 3–6k rpm where the old rattle lived). Do not guess cold.

No evidence of timing-chain jump or valvetrain damage in typical APE installs unless the engine was rotated with the old tensioner removed (you didn’t mention that).

Quick Confirmation Steps (prioritized by what you changed)

Do these in order—90 % of identical cases resolve here:

1. Fix the velocity stacks first (critical). Replace the stripped bolts properly (helicoil inserts or longer bolts that grab fresh threads are the common fixes). Even temporary sealant + new bolts can test it. Then do the carb-cleaner spray test while idling.
2. Adjust the APE CCT (warm engine, fairing off for access). Loosen locknut → turn center bolt in slowly until rattle stops → tighten locknut. Test ride and re-check. This is the #1 fix for post-manual-CCT rattle.
3. Check/bleed wax unit & coolant. Verify coolant is at the correct level and fully bled (air pockets love the wax circuit). Some owners loosen the wax-unit nut slightly or clean externally to let it retract fully.
4. Quick extras.
   • Confirm all vacuum hoses (especially the two small ones at the back of the airbox) are reconnected.
   • Feel the throttle linkage—plates should snap closed.
   • After fixing the above, a throttle-body sync is cheap insurance.

These bikes are extremely well-documented, and your symptoms line up 1:1 with the most common post-maintenance issues. The stripped velocity-stack bolts + new manual CCT are the two direct changes that explain both problems simultaneously.

Try the CCT adjustment and stack sealing first (easiest and most likely).

If anything works keep us posted I have the same problem!