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Rough @ 4,000 to 5000 RPM


Runtoeat

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All of a sudden, I've got a very rough running with strong vibration in the 4,000 to 5,000 rpm range.  If I go full throttle in this range and run thru it, no roughness.  If I pull throttle at 5,500 rpm and coast down thru the 5,000 to 4,000 range, no roughness.  No problems at any throttle settings at other rpms, including idle.  Just have a problem when I'm at "low to moderate throttle" that starts just past 4,000 and ends just short of 5,000.

 

Any guesses?  Carb balance?  Maybe the carb vent tube is out of position (which I haven checked yet)?  I'm wondering if there is a mid throttle circuit with it's own jet in the Bing or do we just have two circuits. I,E., the idle circuit and then the main jet/needle mid/hi speed circuit?

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Another quick check you can do on the SW is remove the air intake hoses, reach in the carb and lift up on the venture slide. Make sure one is not gummed up and sticking. If they are sticky it will cause the engine to run rough at times and smooth at others. I have seen this on 2 different airplanes. It is a little harder to check on the LS with the Rotax intake system.

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Hi Tip and Tom.  Thanks for the replies.  Tip, I think you mean the vent tube that runs thru the airbox?  As we've discussed before, I really don't like the design of this and it's always suspect when I run into a rough running concern.  Maybe I'll talk my mechanic into making me a new vent tube that has the same cutouts as the original but have these spaced closer to one another and well inside of the two walls of the airbox.  What also irks me is the vent tube has, over time, opened up  oversize holes in the airbox similar to what the intake bypass door pivot does, and these enlarged holes are not far from the vent tube cutouts. This has been down on my  squawk list for a long time and I need to give it my attention.  Tom, I just had both carbs removed and inspected and new floats installed for my condition inspection but goop on the slides might go under the radar and could be a causal factor.  I'll put this on the list to check.  My zip tubes are a PITA to get back on.  I'm wondering if I can just remove the tops of the carbs and check the slides this way?

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Doug, good thought....but......the vibration is pretty bodacious if I hold in the vibration period due to the high amplitude forces shaking the engine.  Not sure if I can hold it long enough to look for differences in EGT?  Might give this a try though.  Icey, I've got to take a look at the tube.  The tube could very well be cut as you indicate.  Even if not blocked, this would open up a hole in the tube that is exposed to ambient air outside of the airbox.  Thanks guys.

 

As a reminder, this only occurs when I'm at low to mid throttle from 4,000 to 5000 rpm.  No problems when power is completely pulled and coasting down thru this rpm range and no problems when at full throttle going up thru it.

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, this only occurs when I'm at low to mid throttle from 4,000 to 5000 rpm  when the needle circuit controls the mixture No problems when power is completely pulled and coasting down thru this rpm range idle cuircuit takes over and no problems when at full throttle going up thru it main jet controls.

 

Dick, you are focused on RPM. Maybe things change due to throttle setting?  Sounds as though closed and wide open are working but when leaning should be taking place it gets rough.  What about at run-up?  Does it get rough on the ground too?

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CT, things definitely change relative to throttle setting.  Mid throttle produces heavy rough running only in 4,000 to 5,000.  All throttle settings at other rpms do not produce rough running.  I have not checked for rough running during static runnup but this would be good to try.  I think you probably hit on the key when you said "when leaning takes place".  I'm hoping a carb re-balance fixes this or that I might find my my vent tube needs repositioning.  Thanks.

 

Icey, I recently had my gearbox reshimmed due to vibrations as you indicate.  This did resolve other vibration concerns I was having when it was combined with tightening of all areas of my stabilator.  After the reshimming and stabilator tightening, my CT was humming along and running exceptionally good.  Since this engine roughness just popped up and it needs a certain throttle setting in a discreet rpm range, I believe my concern is carb related.  FWIW, I burn Mogas exclusively.  Thanks.

 

Note:  Resolution of this might take a few days.  The temps are forecast to be single digits with winds of 20 to 30mph for a while.

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CT, things definitely change relative to throttle setting.  Mid throttle produces heavy rough running only in 4,000 to 5,000.  All throttle settings at other rpms do not produce rough running.  I have not checked for rough running during static runnup but this would be good to try.  I think you probably hit on the key when you said "when leaning takes place".  I'm hoping a carb re-balance fixes this or that I might find my my vent tube needs repositioning.  Thanks.

 

 

Maybe one of the needles is not functioning properly.

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In CV Carbs:

 

High RPM problems are a result of jet issues.

Midrange RPM problems are a result of needle, needle clip position, or plunger problems. This is where movement of the carb plunger is occurring.

Low RPM problems are a result of float level issues (serious float issues can affect all ranges).

Idle RPM problems are a result of mixture screws needing tuning.

 

Also, repair of problems and carb tuning occur in that order as well. Anything done at the top of the list will affect things at the bottom.

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Corey, in my case, low to mid throttle produces engine roughness at 4,000 to 5,000. Idle and full throttle in this rpm range OK and no roughness. Based on your cause/effect above, what would you be looking at? It looks like jets could be ruled out. I deduce carb slides and needles would be suspect from your post.

 

Question: When we talk about jets for the Bing, are there only the idle circuit and main jet or are there other jets?

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Main jet and needle jet (not to be confused with the jet needle) are in the central discharge assembly where the jet needle lowers into. During full throttle, the main jet is the fuel limiter, during part throttle, the needle jet manages it.

 

Anyways, these carbs are still freaking magic sometimes. Suggest disassembly and cleaning of the jets and checking for needle/clip wear and free movement of the slide.

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OK.  Got it.  Needle is tapered and as the slide, with needle attached, moves, this moves the needle thru the main jet to meter fuel while the slide controls air flow.  Same as my Bing and Mikuni carbs on my old dirt bikes.  I've got to start a step by step to figure out why I'm running rough.  It'll take some time and I'm not in a hurry to go out to my hangar in this winter blast we're having right now.  I'll report back.  Thanks to all for the inputs.

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OK. Got it. Needle is tapered and as the slide, with needle attached, moves, this moves the needle thru the main jet to meter fuel while the slide controls air flow.

That was my (apparently mistaken) take.

 

But it appears the jet the tapered needle moves in Is called the "Needle Jet", while the "Main Jet" is below that.

 

Tried to post an image, but it gave me problems.

 

Let me try again.

 

bmw-om-carb-cv.jpg

 

If I understand right, #10 is the Main Jet and #4 is the Needle Jet.

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Eddie, a picture tells a thousand words. I was in your camp too before seeing the diagram. Thanks. Now, going back and reading CT's and Corey's remarks and your last remark makes sense.  If I have a mid-throttle engine roughness this should be related to the needle jet, needle or slide.  Funny that this only occurs in the 4,000 to 5,000 rpm range though.  I've got some detective work to do. 

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That was my (apparently mistaken) take.

 

But it appears the jet the tapered needle moves in Is called the "Needle Jet", while the "Main Jet" is below that.

 

Tried to post an image, but it gave me problems.

 

Let me try again.

 

bmw-om-carb-cv.jpg

 

If I understand right, #10 is the Main Jet and #4 is the Needle Jet.

 

Here's something even better :D. It's almost accurate to our bing carbs too!

 

The exception is where the throttle vacuum passage is shown. In the bing 64, it's actually in the plunger, and is affected by the amount of air rushing by the plunger. The vacuum is indirectly affected by throttle valve position, unlike this video at part throttle.

 

The full throttle operation as shown in this video is more similar to how the bing 64 works, and it varies based on how much air is rushing by.

 

In addition, our bing 64 carbs have a tiny hole drilled into the top center chamber, which acts to dampen the movement of the plunger. It is shown in FastEddieB's photo connecting chamber 23 to the center vertical chamber via the tiny pilot hole drilled up and to the left of where 23 is pointing.

 

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That is a VERY cool little video. THANKS!!!

 

I need to watch it a few more times to burn it into my brain. Not anything I didn't basically grasp, but a video is worth a million words!

 

One thing - I think it shows pretty clearly that the starting circuit is designed to work with the butterfly closed. That's consistent with BMW motorcycles that warn you when using the "choke" to leave the throttle closed. The more you roll the throttle on when starting, the more problematical starting becomes.

 

It therefore surprised me that my checklist calls for throttle open 1/4". I know from experience that the BMW technique of closed throttle works best for cold engine starts in my Sky Arrow with the "choke" on.

 

Anyway, thanks again!

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