Let me start by saying that there is no mechanical reason why a carbureted Twinkie engine should not idle just fine at 600 – 750 RPM. There is a lot of pseudoscience and other nonsense about oil pressure and the like on this forum concerning this sort of folklore and I don’t want this tread to devolve into that discussion.

My ’03 Low Rider is Carbureted. But when I lower the idle speed below about 800 RPM: First, The Tachometer drops to 0 RPM, although the engine is still running. Second, At about 700-750 I can get the traditional potato-potato idle that I used to love on my old shovelheads. Unfortunately it will only maintain this idle speed for about 15 seconds to a minute or so before the engine stalls. Third, Note: Oil pressure is just fine at 700 RPM and the engine oil scavenger pump is working just fine as well.


I’m guessing that the Tachometer signal drops off to 0 below 800 Engine RPM either due to the programming logic in the electronics of the Tachometer or because the Crankshaft Position Sensor signal amplitude is insufficient below 800 RPM to drive the Tachometer electronics. But in any case, the Crankshaft Position Sensor clearly generates a sufficiently strong signal to trigger the Ignition Circuit at 200-300 RPM or the Engine would never start. So at a 700 RPM idle speed the capabilities of the Crankshaft Position Sensor and the Ignition Module should be OK.

Questions:

1.) Is anyone aware of any ignition programming logic that distinguishes between an Engine that is at Cranking RPM versus one that is at a Running RPM? This is actually quite common on automotive systems, but the dividing line is usually about 200-300 RPM. Essentially, the question is; Is the engine stalling at ~700 RPM because the electronics of the ignition system reverts to a Cranking Engine mode? (Different amount of Spark Advance)

2.) Is there any reason why the original carburetor on this bike would have issues at low air flow rates, that the carburetor designs used on the older Shovelheads did not have? Venturi size would/might be the most likely thing but I don't know what the size of the bore was on either of the Shovelheads I owned many years ago.

 

Research your pressed together fly wheels and how easy they can be knocked out of true. Low rpm and lugging have been discussed in many tech articles. I noticed tach drop off also but never cared because I wasn't leaving the rpm there. Oil pump issue is a problem on Evo's with too low of rpm's. Crank isn't oiled properly by Evo's turning too low rpm. Like most things you can set your idle wherever you want. It is your bike not mine. You can also research that. Donny Peterson has wrote several good books covering Harley engines.

 

Let us know how it works for U..

 

My last bike wan an 03 E-glide carb model. I kept the idle @ 900-950 after it was warmed up good. It will be lower when cold. Any lower and you are asking for trouble.

 

I thought this was covered around here somewhere and went like this

Evo's should idle somewhere around 1000 rpm's to have enough oil pressure

twinkies had a different oiling setup which needed at least as much rpm's as an evo

& Shovels, and panheads would idle at any rpm, IF and when they decided to start.

 

twinkie?

 

I really don't get why some people want to go to so much trouble to make a bike idle slower. So it sounds cool sitting still? Do you feel the need to tinker with the idle settings on all your vehicles? Put it in gear and ride it.

 

If you ride a lot in cities with heavy traffic a lower idle can reduce fuel consumption, which, with gas at 1,75€ per litre, is quite a good thing. So, yes, I'm very interested in knowing how low my Twin Cam can safely idle, and why.

 

Your twinkie relies on more computer stuff than an ol' iron head or shovel ever did. No crank sensor was needed.
However I wonder if some of the old tech ideas are still at work here. like primary & secondary ignition.
Primary ignition would divert all available voltage to the coil to spank those spark plugs to life & light the gas. Than as the alternator kicked in & the voltage regulator began to stream power back to the system, the secondary ignition switched on & took over power management to the ignition.
If your bikes ignition system acts in the same way through the computer, It will allow the bike to run at a very low rpm because you are in start mode. Once switched to secondary or running mode, It will not allow prolonged running below a minimum rpm.
Just a thought.
Hatch.

 

Or, much simpler, the ECU reads an anomalous running condition and goes into protection mode switching off the engine to avoid damage. Many ECUs have this feature.