Troubleshooting Ignition issues 912UL

Do a full power run up for 20 or 30 seconds and then immediately retest. That always clears this up for me. From time to time I would also get RPM drops technically out of spec. On the advice of others here I stopped worrying about it and just go fly.

I agree with all of what Jeff B advises, with the exception of: "......ensuring that both throttle cables are lubricated ...."

To lubricate or not to lubricate????

You need to know what sort of cable is being used - many modern cables have Teflon (or similar) linings, which may be adversely affected by most lubricants ie lubrication may make them more prone to binding.

I also have reservations about this statement; "I sync at 2000 and 4000 RPM,   and if can’t get them both perfect, I split the difference."

Carb balancing is best done  at low engine rpm (I use high idle 2500 rpm) as this is where the inlet manifold vacuum is high and differences between the carbs will show up most.

Once balanced, at low rpm, the prudent pilot/maintainer will observe any changes to the balance as the engine speed is increased/decreased - 5000 rpm +.

It is quite common for small differences, in balance, to be observed as the engine speed rises/falls. Often the difference will be seen to resolve itself (catch up).  On repeat throttle movement, may not appear at the same rpm point. These differences are most likly due to throttle cable binding (frictional differences'). 

If the difference is small and or catches up  - leave as is, the carb balance tube will accommodate this.

If the difference is significant & repeatable at the same rpm point - might be best to replace the throttle cable.

Note; Cables should have large radius bends. Small bends will promote binding and accelerate wear. 😈

Hi Sean,

Quote:

"You need to know what sort of cable is being used - many modern cables have Teflon (or similar) linings, which may be adversely affected by most lubricants ie lubrication may make them more prone to binding".

This is why i use Inox. It doesn't hurt anything and is electric compatible. There are no harsh solvents or distillates. Many i know use it.

Thanks Roger -

I think you will find that the cables, with an inner low friction lining/sleeve, are not supposed to be lubricated.

Your tip on Inox not damaging such linings is good however any benefit may be short lived, if the lining is already worn. The lubricant may also attract dust, further exacerbating the problem.😈

Sean, 
It’s a good point, but my aircraft’s 100 maintenance and lubrication plan says to lubricate the throttle cables at the inlet (with motor oil).  Like Roger, I use Inox for this, and sparingly.  But like I said, your point is well taken and I make no attempt to lubricate further up into the cable sheath.  

Roger, 

I know you do a lot of balancing and I only work a few aircraft. I think I read a comment from you where you said you balance at 3500 RPM. I know that what sean says is true that the balance makes the biggest difference at low to idle RPM, but on the other hand you don’t spend much time at those RPM.  I find the most noticeable vibration on the carbureted engines is when you throttle back to 3500-4000 RPM so I pay attention to that range when balancing. What is your opinion?