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Can't Reach High RPMs


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I bought a used ParaJet Volution 2 Macro last year (it's a 2012 model).  An injury kept me from flying much but it seems like it has never run well.  I’m not able to get over 6500 RPM with it and even that seems to be inconsistent.  

That manual says to shoot for 7600 RPMs.  I can run it up, get 6500, let it back to idle, then get 5500 when I run it up again.  Here’s what I’ve tried so far.

  1. Replaced the fuel tank cap and fuel filter.
  2. Replaced the fuel line and put in fresh fuel.
  3. Rebuilt the carb with the OEM kit (Walbro 37c).
  4. Adjusted the throttle cable to ensure there was no slack or kinks.
  5. Replaced the spark plug twice.
  6. Had a friend check the compression.  I don’t recall what it was but it remained the same each time which led him to believe there were no air leaks.
  7. Adjusted the high and low jets as well as throttle after each change.
  8. Checked the packing on the exhaust silencer.  It's relatively clean and nothing is loose.
  9. Checked the resistance of the CDI and spark plug wire

I’m at the point where I may have to replace the carb and/or replace the reed valve.  I also read that loose packing in the exhaust system could cause problems.  I’m getting very frustrated and don’t want to just start replacing things.  We fired up my buddy’s Volution 2 and it ran right up to 8000 RPM.

Since the high-end RPMs have been inconsistent (even when manually holding down the throttle lever), it's been hard to diagnose.  Has anyone seen a similar problem and, if so, how was it fixed?


Edited by TonyE
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hmm,  on each flight do you check the color of the spark plug?. just to see how the engine is running? e.g. too hot? and spark plug number. as mentioned the reed valves should be checked too.   this is my advice.. i have had problems with the running of my paramotor.. done what you have tried. and its abit like sherlock holmes quote" when you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.' 'It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth.' "'    back to me. renew the carb = brand new 37c. and the old carb keep for parts...

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Responding in order...

I removed the reed valve and it looks fine. I'm going to replace it but am waiting on a part quote.

The prop is a ParaJet 3 blade.  My motor is a ParaJet Volution 2 with an XT172 motor.

No, I haven't checked the plug on each flight.  It's been six months since I was able to fly and it didn't perform well on the first flight, so it's been grounded since then while I try to troubleshoot this.  During the flight, it maxxed out at 6500 RPMs.  I let off on the throttle to loase some altitude, then when I maxxed the throttle again, I only got 5500 RPMs.  The top end RPMs have been very inconsistent.

I have changed the spark plug three times now to be sure that wasn't the problem.

In addition to the reed valve, I'm also going to replace the carb.  I rebuilt the one I have but I'm running out of ideas.

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i rebuilt my carb and still ran the same.. = crap.  but when i renewed the carb 37c it ran fine.. no problems just check the small tube that connects to the carb and the other end to the engine block.near to reed valves casing or block  . and the tube has to be the correct length .. and make sure no air escapes   and even when you run with brand new carb.. check each flight the color of spark plug...

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Hi, there has been some brilliant work by people on this forum describing "pop off pressure". It is the pressure at which the small spring behind the carb diaphragm allows it to lift. 

Search this forum on "carb pop"

Here is one discussion....



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    "YouTube walbro pop off pressure "                    all to do with the 37c carb? and this is what i say to my son quote "you either learn the easy way or the hard way? "and ive already said the easy way? no comments from me concerning this topic. what do i know? rebuilt mx engine raced and won. now flying paramotors same basic engine.but not race? i have years of experience.what have you ?

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