912 ULS starting but stalls

Jeff Blakeslee wrote:

Jeffrey Schinkel wrote:

Jeff Blakeslee wrote:

I believe that the Lucus Octane Booster effective ingredient is MMT (Methylcyclopentadienyl manganese tricarbonyl), about 5%, suspended in naphtha. It is marginally effective at doing what it says, but can cause fuel system and engine deposits. I would not use it in my aircraft engine. 

I read that too and stopped using it after the second 200hr carb service.  

Can you reason that that greasy, slightly gritty, gray goop on the idle mixture screws is deposits caused by Lucas?

I am really sorry I did not save it to be tested instead of cleaning and moving on.  It never recurred after that first finding before the ~25hr inspection/cleaning I did. 

But thread drift and complaining aside.  I will get to the engine after I finish with the project of moving my EFIS to another position.  Something to finish before trying to start it again after "one change at a time". 

Jeffrey

No I can’t make a direct connection between the gunk in your carburetors and either of your fuel additives.  In fact, most of the concern about MMT is combustion deposits that are typically a red brown color.  The grey sludge you describe is typically just from old stale fuel varnishing.  Sitting unused is one of the hardest things on an aircraft, and if that is going to be the case, you need a program to cycle out old fuel and drain the carb bowls before extended periods of non operation.

I do not believe fuel has been left in my aircraft tank any longer than 35 days since new. Sugeries, hospitalizations and such, you know....😕

Yea, I just don't get it.

But I have my list to go through so will start with idle speed, then idle mixture screw setting.

At least with the old ignition modules, starting circuit failure was pretty common ... both of mine failed. If you always start with both mags switched on, the failure of one may not be noticed until the other one fails too. However, it may run extra crappy until the ignition timing advance to the run value. Since my mags are switched individually and I have a separate starter button, it's easy for me to verify that both circuits are working. With a fully warm running engine, shut down normally. Then engage only one mag and hit the button. It should start instantly and, after the normal delay, jump to it's normal, but just a little slower, idle. Shut down, then start only on the other mag. If you have only the usual key start, you'll have to get behind the panel find the p-lead for one mag, ground it and start on both. Rinse & repeat. Or perhaps it would just work to start normally, but before the retarding delay ends, quickly switch to a single mag. If the engine dies, the start circuit of that mag is toast.

Anyway, I do this test with a warm engine because a cold start with a single mag can cause it to run crappy, even if the start circuit is okay.

Flipping the tale, bad starts could be due to one of your two ignition module start circuits acting up.

Jeffry Stetson wrote:

At least with the old ignition modules, starting circuit failure was pretty common ... both of mine failed. If you always start with both mags switched on, the failure of one may not be noticed until the other one fails too. However, it may run extra crappy until the ignition timing advance to the run value. Since my mags are switched individually and I have a separate starter button, it's easy for me to verify that both circuits are working. With a fully warm running engine, shut down normally. Then engage only one mag and hit the button. It should start instantly and, after the normal delay, jump to it's normal, but just a little slower, idle. Shut down, then start only on the other mag. If you have only the usual key start, you'll have to get behind the panel find the p-lead for one mag, ground it and start on both. Rinse & repeat. Or perhaps it would just work to start normally, but before the retarding delay ends, quickly switch to a single mag. If the engine dies, the start circuit of that mag is toast.

Anyway, I do this test with a warm engine because a cold start with a single mag can cause it to run crappy, even if the start circuit is okay.

Flipping the tale, bad starts could be due to one of your two ignition module start circuits acting up.

Very interesting.

Now I want to ask about how the separate mags work.  I thought that was what a "mag check" before takeoff was for. They both drop rpm and both about the same amount = all is well.  The Rotax has a different circuit for the mags on start up vs after starting?

Also confusing to me is,  "It should start instantly and, after the normal delay, jump to it's normal, but just a little slower, idle."  My engine always has started at a lower idle and after 3-4 seconds jumps to a higher idle.

😕

Hi Jeffrey

To be clear the soft start is a circuit within the modules that is excited when you first power your starter, normally from the starter solenoid.  What this activates is a timer in the circuit that delays the advance from start timing to run timing.  This is the 3 to 5 second delay you hear.  It is not directly related to idle but you will feel the advance and cant go to high throttle until this moves to the 26 BTDC degree run timing.  Engines from before 2010 did not have this feature and had different timing for the starting of the engine.  (4 BTDC as compared to 3 ATDC on start) 

I disagree that starting on a single mag (single module actually) will give you a rough start.  This suggests something wrong in your system.  On one module you still have one spark plug active in each cylinder.  Cold starts run badly normally because you are too lean on fuel while the engine is trying to warm up.  

Cheers

I always start mine on one ignition so I can detect dormant failures. I alternate between starting on left and then right on the following flight. That way I can identify one that will not start an engine. Normally they are fine at engine running speeds, just not at cranking rpm. I've successfully detected several faulty SMD modules over the years.