RPM hang at idle throttle.

[Lobo]

Engine tuning is a black art to me...

So, my neighbours K1; which I set the valves, timing strobe @ 4000 rpm / full advance marks), one spark plug gap ( was ok / others too tight for me to free) and carb vacuum-balancing checked (no adjustments necessary)....
My ears are not up to individual air-screw adjustments - and so the 4 carbs set at 1 turn out, ie the book figure. (Note - they were 2 turns out)

The bike is tired & unloved, but overall appears to be totally original and unmolested.

Queries.
 (1) by winding out the air screws the revs increased... no ‘ peak’ was achieved at the 3 turns out point, the revs just kept (slightly) increasing. I gave up here and simply returned the ‘screws back to 1 turn out.
(2) the bike starts easily on the kick-start (duff batt) and needs only a minute before the choke can be fully backed off (OAT about 20 degs) My K2 needs significantly longer..
(3) a slight hesitation on pulling away, but once rolling throttle response is good through all rpms & gears. She pulls smoothly and with no hesitations.
(4) on closing the throttle, the RPM will hang at 3000rpm for 3-4 seconds before dropping back to idle - even if rolling the throttle against the back-stop (2 cable type)

So... my guess is the idle circuit set-up.... help please?

Ta,
Simon

[K2-K6]

The idle settings are curious certainly,  at three turns out it would ordinarily  be going toward lean and possibly starting to miss a little.

It suggests that the idle jets are larger than need be as you can't lean it off enough,  but would explain why it's set on arrival at 2 turn out which would probably be better for it.

Too rich idle would give you that "off choke early" necessity,  which seems to tally.  Looks like you'd need to pull a float bowl and use a mirror to see if you can identify the idle jet size.

But it would usually "quench" the revs quickly,  for which I'd suspect something else. You've checked the timing,  but did you assess if the advance/ retard is also dropping back fully when you close the throttle?
If it's holding onto advance that could give you the hanging revs temporarily until it closed the mech down.

Also a remote possibility that the intake rubbers are leaking and someone has responded incorrectly by putting in larger idle jets,  giving you a mix of two problems.  Can you assess for air leaks?

[Lobo]

Great info K2-K6; never considered the Adv/Ret unit and will check that out...

During my ‘service’, as standard I check all hose clamp jubilees, and found all a bit slack. Ie the clamps are all tight... thought that doesn’t necessarily infer no air leaks, and so will investigate that too.

In Brunei I had a twin engine 2-stroke jet boat, and had exactly this prob of rpm hang on closing the throttles (it had electronic ignition) ... but I can’t remember how I fixed it.... there was little you could do on the Rotaxs other than fiddle with the carb settings / jettings.

Many thanks - will get back in time,
Simon

[Andrew-S]

Also a remote possibility that the intake rubbers are leaking and someone has responded incorrectly by putting in larger idle jets,  giving you a mix of two problems. Can you assess for air leaks?

Simon,

What might seem airtight may not be, you could try spraying carb cleaner at the individual intake rubbers e.g. rev to say 4,000 rpm and spray on the overrun (and at idle) and if the revs rise you have a leak on that particular intake rubber and possibly others? 

Cheers, Andrew

[K2-K6]

"In Brunei I had a twin engine 2-stroke jet boat, and had exactly this prob of rpm hang on closing the throttles (it had electronic ignition) ... but I can’t remember how I fixed it.... there was little you could do on the Rotaxs other than fiddle with the carb settings / jettings."

Sounds interesting  Rotax are good engineers too.

I'd suspect the fuelling adjustment was for low speed mixture and perhaps a long taper valve to modulate fuel rather than airflow.

The bike is odd though as your work so far suggests rich at low speed but the elevated idle contradicts that,  hence the reasoning to look at the other elements.

Something along those lines I often find on a car engine type I'm familiar with. There's a significant number that think they can set their timing "dynamically " by swinging the distributor and listening for smoothest running,  yeah that's a bit organic  often they settle on an advanced position and then proceed to completely screw up the tickover monitoring  by the ECU by trying to adjust things that are concisely set.  I know that's not the case here but its surprising just how much the tickover rises with just a few degrees extra advance, so if the advance retard mech is slow to close,  I could envisage it causing that slow decay in idle speed.

[Johnny4428]

Hi for what it’s worth I thought that characteristic was something that I was just going to have to live with on my K3. Until I removed carbs for the third time and pulled pilot jets cleaned system thoroughly. It now idles as it should runs smoothly and no longer hangs on to revs. The reason I pulled the carbs was because of the poor starting and rough idle.

[philward]

The idle settings are curious certainly,  at three turns out it would ordinarily  be going toward lean and possibly starting to miss a little.

It suggests that the idle jets are larger than need be as you can't lean it off enough,  but would explain why it's set on arrival at 2 turn out which would probably be better for it.

Too rich idle would give you that "off choke early" necessity,  which seems to tally.  Looks like you'd need to pull a float bowl and use a mirror to see if you can identify the idle jet size.

But it would usually "quench" the revs quickly,  for which I'd suspect something else. You've checked the timing,  but did you assess if the advance/ retard is also dropping back fully when you close the throttle?
If it's holding onto advance that could give you the hanging revs temporarily until it closed the mech down.

Also a remote possibility that the intake rubbers are leaking and someone has responded incorrectly by putting in larger idle jets,  giving you a mix of two problems.  Can you assess for air leaks?

K2-K6, for my info, when you say 'idle jet size' is that the slow running jet (normally 40)?

[K2-K6]

Yes philward,  I usually call them idle but some refer to pilots or mixture. But on a 750 normally about 40 in size.