rough mid-range and carb questions

it is not odd that jet size is smaller on the larger capacity engine.   The larger capacity engine sucks better vacuum and velocity through the venturi is higher so jetting must be reduced to avoid overfuelling.   It is similar to going to larger carbs; you need to increase jetting because velocity in the venturi is reduced

You say your EGTs at cruise are fine.   If EGTs are good then combustion is good too and I would look elsewhere to explain vibration issue. .  Apologize for the noisy posts...

The jet size explanation makes perfect sense.  Thanks.

EGTs in cruise are fine, but there's also no vibration issue at cruise power.  The vibration is maybe 2300-4800.  It's probably worst right in the middle of that range around 3500-3700. 

My prop is pitched for full throttle level RPM of about 5600-5700, so at 5300 in cruise, the throttle isn't far from being fully open.  I'm thinking the needle has very little effect at that point.  The needle should be the primary path for fuel control at 3500 +-1000 RPM though.  Am I correct about that?

My plan now is to raise the needle one notch.  I'd love to find it at the 2nd, so it would end up at the 3rd where it's recommended to be.  If it's in the 3rd, I'll move it to the 4th for a test.  Unless raising the needle magically cures that issue, I'll be rebuilding the carbs later this week anyway.  

Thanks,

Rusty

Rusty,

Yes you are correct, with a slight clarification, it’s not specifically rpm, but throttle arm travel, although the two should correlate. If your carbs have not been tampered with from factory settings, and there is a bit of overlap, but from about 25% to about 60% of throttle opening, the jet needle and needle jet are metering fuel to the engine. Below 25% it’s all idle jet, above 60% it’s all main jet.

Thanks Jim. 

For those who are playing along, I found the needles in the 3rd slot, so now they're in the 4th.  All the  advice I read about customizing a screwdriver to fit the screws, and heating (beyond belief) to release the locktite was necessary.  Those screw slots were pretty well abused already, but no worse now.  

Everything I can see in the carbs looks like new, and I'm going to resist installing those rebuild kits if at all possible.  I'll be able to at least run this tomorrow, and maybe fly depending on time and wind.  At this point I'm  hoping for some obvious change, even if it's worse.   

Rusty

The screw that holds the needle in position is called the "fixation" screw. It absolutely needs heat to loosen it because if you try too much muscle you'll snap the head off down inside.  You don't want to go there. 

I wrap a rag around the piston (slide) so I don't cook my hand. Then I use a small butane flame to apply heat only where I want it. A heat gun works, but it cooks everything including your fingers. LOL

Once it's hot the screw comes right out without applying muscle.