why kickback afflicts cold engines?

"soft start cdi same 4° btdc"

The regular modules were 4 BTDC, but the soft start modules are 3 ATDC with the newer fly wheel. Kick back is usually easy to fix. It is usually only the few things I mentioned in the other post. Like I said don't keep starting and getting kick back or it WILL ruin the sprag clutch. Depending on what your idle rpm is set at when warm it may just be your starting technique or even a slightly weak battery or the carbs need some attenttion. It could be one thing or a couple of small things causing the issue. Things like plug gaps way too wide and needing a carb sync or idle set really low and then it would be better to crack the throttle during start. 

Resurrecting this thread to ask a question. Would rotating the propeller to position a piston (which one?) at TDC (or some other position) prior to start help minimize kickback? 
Just thinking out loud.

Hi Jim

First let's correct some issues with the old thread. 

If the engine is from before 2010 production year it did not have soft start.  The soft start changes the timing on the start circuit from 4 degrees BTDC to 3 degrees ATDC.  In addition the modules were changed to have a timing delay of between 3 to 5 seconds before advance to 26 degrees BTDC.  (912 80 and 100 HP engines)  

Kickback can still occur regardless if one does not do preventative maintenance on the gearbox and have it correctly shimmed internally.  Additionally weak RPM on cranking will not produce a spark.  This is normally due to old battery, undersize cables or bad connections.  The tendency of many seeking STOL performance from engines and using extra mass propellers is a main cause of gearbox wear.  The gearbox damping gets damaged and this effects the sprag on cranking when it allows kickback.  

The crankshaft position relative to the firing point is not really the issue and I have no idea how one would calculate a best position from initial power to the starter.  Start power from the battery is perhaps the most important.  Generally one test to know if you have a sprag issue is to do a test start with a boost on the system.  If this overcomes the issue it tells you to look at battery, wires and the like.  

In my opinion the best is to use the new e-modules with the start delay.  if you also install the new flywheel then you will get the best chance to start without kickback, as long as you have also addressed the other issues.  The long list of things to check in in the old SB from 2004 on kickback. 

Cheers

Thanks RW. My five year old, 200 hr. 912UL (no gearbox clutch) in my Zenith 701 started exhibiting what I assumed to be mild kickback symptoms last Winter. Only happens on first start of the day, Winter or Summer. One or two loud noises and a slight shake of the airframe and that’s it. Never happens when the engine is warm. I synch the carbs, including confirming the cold start system linkage, and check prop (3 blade Whirlwind, MMI within Rotax limits) dynamic balance annually. I start with throttle at idle stop, electric boost pump on, cold start system full on for first start of the day, closed for subsequent starts. Idle rpm is 1,800 cold, 2,000 when fully warmed up. When I release the key as the engine starts with the choke on, rpm will surge to 2,500 unless I quickly push the choke in partway. Then I slowly close the choke and she idles at 1800. That rpm “choke surge” didn't happen the first four years of cold starts. All warm engine starts are smooth and drama-free. Battery is an EarthX 680, five years old, typically reads 13.2V at startup, 13.8v at shutdown after a flight. Not sure if my start technique needs to change but the first fours years there was nothing but smooth starts, cold or warm. 

Hi again Jim

on a gearbox with no clutch there is still a dog and ramp torsional damping of the starting pulses.  The pressure that holds the parts together can still get worn and allow more reactive forces, more bounce, within the gearbox.  As long as you do not hear that "bounce" in the gearbox, it runs smooth at idle, you should not have to reshim it.  If you get that metallic clatter at idle and it goes away with a slight increase in idle speed that suggests it is time to reshim it.  

Don't worry about 2500 while you warm it up, you need some heat to warm it up and that requires some load in the prop. The choke can also be operated mid point, it is a enrichment system and functions at the engines low speed like a tiny carburetor when below 3500 RPM.  I always like to run the boost pump for a short while to let fuel fill the bowls and remove any air that might have accumulated in the lines before cranking.  

I would think the only change you have had it the time you had it would be some wear on the Belleville washers in the gearbox. 

Cheers