High idle when cold: my fix

Hi - have been lurking a long time on these forums but not had anything useful to contribute until now, wanted to share my solution to a common problem as I've not found this specific cause on a quick search.

TL;DR
Cold bike starts fine, idles at ~2k; put in gear and pull away, immediately starts revving to 4k and doesn't stop until bike reaches temp (~60 C), regardless of clutch engaged or disengaged. Solution was to replace partially functional clutch switch, not helped by alloy levers (Chinese) not fully actuating the plunger.

Longer
2008 6th gen with 47k miles: bought from an absolute yoghurt who had stripped the sump plug and glued it back in with silicon, replaced whole exhaust with catless headers and removed O2 sensors but not changed the fuelling, and generally balls'd up a PAIR/flapper mod. Fixed all the above (re-enabled PAIR/flapper, fitted PC5) during restoration work but hit with the infamous high idle when cold, as above. Was tempted to leave it as issue is always solved after 2 minutes of riding, but a colleague told me in no uncertain terms how much of a membrum virile I sounded leaving work.

Given the bike's history I was preparing to pull the throttle bodies, check for vacuum leaks, check fast idle wax, thermostat etc., maybe even get the clutch off. Was reading the shop manual on how the clutch fits together and noticed it mention that you should be able to start the bike in gear if the clutch lever is depressed, which I couldn't.

Pulled the clutch switch: it was only registering a closed circut when pushed in ~60-90% - I have no idea why it wasn't working when fully depressed as I could see the contacts touching, but regardless gave it a clean and re-installed. Then I noticed that my cheap alloy levers were only depressing the plunger to about 40%. Scrap yard OEM lever for £5, £5 for a fresh switch and the issue's solved.

Highly recommend checking the clutch switch before even thinking about lifting the tank as it takes all of 30 seconds!

 

I must be missing something here. Are you suggesting that you fixed a high idle by changing a clutch switch? If so I can't imagine how that affects anything but I am prepared to be enlightened.

My understanding is that idle speed is set by airflow into the engine, the airflow is controlled by the starter valves with a closed throttle, and the starter valves are pulled open by the wax unit and then relax back to the hot position set by the throttle stop screw.

The clutch switch will certainly allow starting in gear (provided the sidestand switch is also closed/up). I do know that it also affects the function of the flapper solenoid in the airbox as the switch opens the flapper in gear at low revs (but don't ask me why). Where in all of that does it also affect the idle speed?

 

Think what the OP is referring to is Honda's Launch Assist. I'm sure I remember my 6gens having it and most certainly the 8gen does.

When in gear and on clutch release with a warm engine, as soon as the clutch switch opens rpm is increased by about 250 rpm. Its a neat feature and almost unnoticeable. However, when the engine is cold and moving off the rpm increase on clutch release can be quite higher than 250rpm.

The OEM lever is nicely set up such that deactivation of the clutch switch is around about the friction zone, just where you want the rpm increase. Some non oem levers don't consider this function for when the clutch switch should deactivate.

 

Well I am prepared to be amazed at Honda's cleverness but how does the Launch Assist actually achieve that? I can understand how a RBW throttle system would be able to do that but a cable operated throttle would seem to be difficult to manipulate in this way.

 

Hi Terry.
Check out this link below on VFRD.
I haven't found any info within service manual or owners book. Sometime back I heard or read something that mentioned launch assit. Its very subtle and most riders wouldn't realise its there. Suzuki also have it on some their bikes.
As to how it's done I guess the ECM can do that easily and given the small change in rpm air control may not be a factor - only guessing!

https://www.vfrdiscussion.com/index.php?/forums/topic/86207-am-i-the-only-one-who-didnt-know/

 

Can't argue with that Grum; wonder how that works?

 

Yeah assumed it would be some sort of launch control: my old S1000XR did this too, but it was comparatively subtle and presumably just controlled by a stepper motor in newer bikes.

Worth noting that the VFR still does rev to 4k when cold but only when in gear and with the clutch out, enough to roll it about 20 mph with no throttle. Does seem a bit high, maybe starter valves need balancing? Will be doing this on next service so will update thread if this changes anything.