rough mid-range and carb questions

Sean Griffin wrote:

"I have KN filter pods and maybe this is the reason stock settings don't work in my case"

Air filters can only effect fuel mixture by causing a restriction.

Restricted air flow, from too small a filter/partially blocked filter, will cause an over rich situation ie the air fuel ratio will have too little air: fuel. 

Changing your carburettor settings, to accommodate a restricted air flow, will only result in problems later on (usually of the too lean variety).

If you suspect a filter problem:

Replace filter

Clean filter & re dress using approved materials & system (if a serviceable filter eg K&N).

If filter to small (this is really filter medium) upgrade to larger filter.

it's the opposite.  Try running a CV carburettor WITHOUT any air filters and you will often find it does not work as the vacuum as the vacuum operated slide is not behaving as it did with an airbox. As opposed to normal slide carbs

An unrestricting KN filter pod is somewhere between airbox and no filter at all.  Regardless the AFR tells the truth and at least my vibration is related to lean misfire

Try running a CV carburettor WITHOUT any air filters and you will often find it does not work as the vacuum as the vacuum operated slide is not behaving as it did with an airbox. As opposed to normal slide carbs

An unrestricting KN filter pod is somewhere between airbox and no filter at all.  I have seen this on motorcycles with CV carbs too, that it may be hard to get jetting right when changing from airbox to filterpods.   Regardless the AFR tells the truth and at least my vibration is related to lean misfire.

My point is that it may not be given, under all conditions that the factory setting works best and it could be worth an attempt to raise the needles and see what happens.  If vibration is reduced then it could be related to mixture. Cheap check to do compared to sending in a gearbox 

Russel did not mention a change from air box to pods, he just mentioned pods.

Certainly any change to the air inlet system/geometry, may have an impact of carburettor performance/air fuel ratio.

Removing an air inlet/filter may also impact on the air fuel ratio, especially where the carburettor jetting has been fine tuned for a certain level of air inlet restriction.

Engines operating in dirty conditions, where there is a rapid contamination of the air filter (eg chain saws) are often tuned for restricted air flow, so will run lean if filter removed. This should not be the case for most aircraft engines.

No change in carbs, settings, or filters, but as I mentioned in the initial post, the big change was from 80HP cylinders to 100HP cylinders.  The mid-range roughness was always there, and I had been working to get rid of it.  After the cylinder upgrade, it was far worse, going from annoying to unacceptable.  As I mentioned, this is NOT a subtle vibration.  

The engine idles fine, and runs smooth and strong at high power.  It's everything in between that's a problem.  Since that's almost the definition of needle jet, I'm hopeful that a carb rebuild and inspection is going to fix this.  I should have the kits by the middle of next week.  

Today I replaced the coils (used set), wires and plug boots.  In a brief ground run, I don't see the same low temp on #3 with the A ignition that I had the past few flights.  I'll have to make a real flight to find out.  One thing's for sure, I have this coil and plug wire replacement down to pit stop speed.  Probably not something you want to be good at though.

Rusty

A complete misfire may give severe vibration. Try with the needle position.  If it changes then you are onto something.  Do you run with a proper airbox, or with separate filter pods as many do?    I see people talking about restriction in filter (dirty) and subtle changes to fuelling, but the issue is that in a CV carburettor with a diaphragm the slide is operated by the pressure difference between carb inlet and carb outside. It can be sensitive to any change in airbox much more so than a 'normal' carburettor where the slide is operated by a cable or linkage.  So when Rotax give 1 standard setting with 1 needle position and 1 needle jet size it may be off depending on application. I am not insisting that you don't have an airbox or suggesting you don't have air filters, but my observations from my set up is that too lean condition may cause vibration.   My engine runs 12.5 AFR at WOT climbing but leans out to an impossible 15.5 at cruise and vibrates.  It helps to pull the choke .  The eyeopener was when I installed the AFR meter. Without that I would thing propeller/gearbox/spark plugs/cables and plug caps/ignition modules/engine mounts.   In fact I changed most of those myself before locking in on fuelling.  Worth checking and over and out