That good old hanging rpm topic again

[georgesgiralt]

Funny, I just bought "Classic Bike", the Britain magazine and they explain how to cure a bad return to tick over for Amal carbs. On the adjacent page there is a picture of the Colortune plug !
as per the factory base settings, I can't tell because I'm 1200 km away from home and have no access to the factory manual. Sorry. But one full turn seem OK to start the bike.

[Barry]

Me being shown as an ex member was only a system glitch which has been sorted but thanks for the kind words at the beginning of this thread which made me blush.


Carb related idle speed hang up can be due to the butterflys being too far open at idle to the extent that the transfer ports are brought into play and continue to flow a mixture of air and fuel. 

The two tiny transfer or transition ports are intended to flow mixture just off idle to smooth the transition onto the needle jet. They should only function when sufficient vacuum is created as the edge of the butterfly passes over them.

In order to make sure the transition ports are not brought into play at idle it's important to achieve the desired idle speed with the minimum possible throttle opening which is effectively the same as saying that the mixture screws are set for maximum idle speed. If the mixture is wrong and the throttle stops are used to compensate by raising the idle speed then this problem can occur.  If the advance mechanism has been eliminated as the source of the problem, a simple test is to temporarily lower the idle speed below normal using the throttle stops. If the problem goes away you know it's carb related and the idle setup needs retuning.

[Justin B.]

On a hanging high idle a test that's always worked for me is hold the brake, let out clutch to bog the engine down, and then pull clutch back in.  Most often, if it's a sticking advance weight, this will cause the idle do drop back down to where it was originally set.  This is predicated, however, on the idle being properly set in the first place.

[tunnelrider]

In order to make sure the transition ports are not brought into play at idle it's important to achieve the desired idle speed with the minimum possible throttle opening which is effectively the same as saying that the mixture screws are set for maximum idle speed. If the mixture is wrong and the throttle stops are used to compensate by raising the idle speed then this problem can occur.  If the advance mechanism has been eliminated as the source of the problem, a simple test is to temporarily lower the idle speed below normal using the throttle stops. If the problem goes away you know it's carb related and the idle setup needs retuning.

Aw Barry, I'm glad you haven't escaped!  As they say, once institutionalised, they can't survive on the outside. He he.  And.. once institutionalised, they learn a lot more..  Good to have some theory behind the symptoms, thanks.  Good news is I bought a couple of wire spokes today to set up the plug shorting technique for adjusting idle mix to get max rpm on each cylinder.  This'll be fun! Won't do it tho until the weather gets ride-able (atrocious at the mo, tho not as bad as Texas, hopoe you guys are OK.. Monte, are you out there too), borrow a great cooling fan from work to set up in front of the bike in the garage and go for a long ride first.  I'm still not 100% the ATU has been discounted but will be able to get a better handle on that by doing this too.

On a hanging high idle a test that's always worked for me is hold the brake, let out clutch to bog the engine down, and then pull clutch back in.  Most often, if it's a sticking advance weight, this will cause the idle do drop back down to where it was originally set.  This is predicated, however, on the idle being properly set in the first place.

Yes Justin I've found dragging the clutch in gear has always reigned in the hanging idle and it doesn't go back up, which is why I'm still open to the idea the ATU may be the problem.  But as you say, set up the idle correctly first.

[tunnelrider]

a simple test is to temporarily lower the idle speed below normal using the throttle stops. If the problem goes away you know it's carb related and the idle setup needs retuning.

Yeah I have kinda done that before and the hanging idle problem hasn't exactly disappeared but is less noticeable.  But it makes it a pig of a thing to ride cold around town stopping at lights etc.  So it will be interesting to see what happens with the improved carb syncing method.

[tunnelrider]

(Photos below...)  Is this a suitable plug shorting technique?  I haven't tried it out in practice yet.  Is simply laying the screwdriver over the spoke enough to short the plug well?  Or does it have to wedged/held pretty tightly down?  The jumper lead in the last photo has a much better result on the ohmmeter as far as resistance goes, (laying screwdriver over spoke and in between cylinder fins gave a variable reading and quite high resistance) but I don't know how successfully I'll be able to attach the jumper wire with the bike running, it has insulated alligator clips but the voltage pulses may be able to come through.  I could always turn the bike off in between doing each side I suppose.
P.S. Keen observers will see a couple of broken cooling fins on one of the heads - that wasn't me! 

[Barry]

As long as the connections to the spark plug are secure it's fine. Using a screw driver to short to the cylinder head will allow you to move from one side to the other with the engine running. That's pretty much how I do it.

[georgesgiralt]

Are you sure it is safe to use the shorting method on post 1981 bikes (with electronic ignition) ?
I seem to remember it could kill the ICU  ?
But I'm not so sure...

[Barry]

It's safe as long as the HT never goes open circuit which would allow the coils to generate excessively high voltage and possibly break down the insulation.  That why it's important that any temporary HT connections made to facilitate shorting are properly secured.  The temporary act of shorting the HT to earth with a screw driver is not harmful. 

The objective of the shorting method is to stop the spark on one cylinder. The risky way of stopping the spark that really will damage electronic ignition is the old fashioned method of pulling off the HT lead.

Edit:

If we think bout it the HT is strictly speaking not been shorted directly to earth.  It's being connected to earth via the 5K resistor in the plug cap just as it is in normal operation which limits the current.  I wouldn't advise any method of shorting where the plug cap and resistor are taken out of circuit or where a resistor spark plug and non resistor plug cap have been adopted.

   

[Barry]

Going completely off on a tangent, nearly 50 years ago I learnt something of shorting HT literally first hand.

At the factory where I worked it was the custom at Christmas time for all Engineering Apprentices to tour the various trade workshops singing carols in exchange for a money.  At the electricians shop we were also put to the infamous megger test.  Basically you were asked to grab the test leads and receive a shock.  If you timidly held the test leads lightly the shock was quite severe but if you grabbed them firmly like a man the shock was insignificant.  The Electricians knew what they were doing.  A megger is not dissimilar to an ignition coil in that it generates a very high voltage but with very little current which is another way of saying it is a high impedance source. Just as with an ignition coil if you operate a megger nearly open circuit a very high voltage is generated which can give a nasty shock where as if you operate it nearer to short circuit the voltage collapses due to the relative mismatch in impedances of source and load. 

It was a subtle lesson in ohms law never forgotten. I doubt health and safety would permit such a hands on method of learning these days.

[tunnelrider]

As long as the connections to the spark plug are secure it's fine. Using a screw driver to short to the cylinder head will allow you to move from one side to the other with the engine running. That's pretty much how I do it.

Thanks for the affirmation Barry, I pushed a crimp eye terminal over the the spoke and soldered it to the spoke for the spark plug connection, it's solid enough I think. And thanks misterpepper for describing the set up, in another post I'll prob have some time to go for a ride and use it later in the week.
Cheers.

[skippyc]

Shorting one lead is the same as a single outlet coil which has one end earthed.

[Motu]

Electric fence is the same, grab it hard and it's not too bad, be a bit timid and a spark  jumps to your hand with a crack...and it hurts. Old engines in equipment used to have a blade bolted to the head, which you pressed against the live sparkplug terminal to shut it down...it didn't pay to think about what you were doing.

Engines have been missfiring for a 100 years, either by the spark shorting to earth or open circuiting - it's not total death of a coil just by going open circuit a few times.  I've seen car engines with the same ignition system as Airheads with a cyl miss firing open circuit for days at a time...and the coils are just fine.

[Justin B.]

Old engines in equipment used to have a blade bolted to the head, which you pressed against the live sparkplug terminal to shut it down

Man, I haven't seen one of those in years!

[tunnelrider]

My mower still has a spark- ground kill switch, broken off the bracket so it's manual push together!

Anyway, doing the plug shorting for syncing the carbs is an easy way of getting the idle mix right, but I was already in the right ball park.  So it hasn't improved the slow drop to idle.  Basically, no matter how I tune the bike, the only way I can set the idle speed at 1000+ rpm is by using the throttle stop screws and I will get the slow return to idle from 3000+ rpm, approx 3-4 secs from 3000rpm.

If I set the idle speed below 1000 rpm, say 600-700 rpm, the idle speed will return far more quickly from 3000+rpm. Obviously this is no good for everyday riding though..

What should I expect as a time for revs to return to idle from 3000 rpm, in neutral, stopped with a hot engine?  Am I just expecting too much for it to return to idle quickly?

If not, I think I may have to do a carb overhaul as well (eh BPT), before pulling the ATU apart.