Expected rpm on 120ac

[Ed Miller]

Yes, sometimes the engine will ingest a bit of debris and score the aluminum back plate causing a slight decrease in power and a wetter than normal intake area. A piston/ring/sleeve fit nearing the end of it's life will also cause an rpm drop and spitting but is usually accompanied with richness at midrange and a difficult to adjust needle. Does the engine still have the original "spool" piston ?? If so, in the future if not now, replace it with the 0600 "bucket" piston from the standard YS120.  Durability will increase tenfold. There are 2 locking set screws on the pin for the back plate rotary valve as well as the screw visible on the outside. I have found the aluminum back plate not to be flat in most cases, I surface it with 400 then 600 grit paper wet on a piece of glass and shoot for .002" clearance between the disc ( rotary ) valve and back plate on reassembly. Another problem area on the AC is the air chamber "O" rings, they tend to wear from the weight of the airbox rocking side to side in a soft mount. No matter how tight the screws are on the little strap from the valve cover to the air box, the airbox can and does move a bit on the 4 "O" rings.
Check the gasket for the air box also, most times the mating surfaces of the 2 half's of the air box are not true and the gasket will not seal correctly. No 2 tachs are exactly the same and rpm figures are only meaningful if done with the same tach during the same atmospheric conditions. If you are used to seeing 8300 with your tach and it wasn't a very hot and humid day, it wouldn't hurt to take a look at the engine before your next flying session. 
Ed.M.
From: John Ferrell 
  To: discussion at nsrca.org 
  Sent: Wednesday, July 02, 2003 9:19 PM
  Subject: Re: Expected rpm on 120ac


  Spitting excess fuel is sometimes excessive rotary valve clearance. Don't forget the locking setscrew if you go there!
  In my experience, AC's don't always look impressive on the ground runup.

  John Ferrell 
  6241 Phillippi Rd
  Julian NC 27283
  Phone: (336)685-9606  
  johnferrell at earthlink.net
  Dixie Competition Products
  NSRCA 479 AMA 4190  W8CCW
  "My Competition is Not My Enemy"

    ----- Original Message ----- 
    From: Del Rykert 
    To: discussion at nsrca.org 
    Sent: Wednesday, July 02, 2003 10:35 PM
    Subject: Expected rpm on 120ac


    Would appreciate opinions.

    I have a YS 120ac that has been a strong runner in the past. Would turn a 15x12 apc 8300-8400 on 25% powermaster. It was rebuilt last season but never run till this season. It was giving me fits at Hamburg contest and taching is only showing 7800 to 8000. Needle 1 1/4 out. Seems to spit more fuel out of air inlet than I used to see. 

    My question being am I being unrealistic expecting it to continue to give me 8300-8400 on that prop & fuel. Valves check at 002 piston TDC cold. Good seal in valves. Don't feel any drag from bearings but I replaced bearings last time and was surprised on ratty they were after dis-assembly. Might my bearings be the culprit and next item to explore?

    TIA for all who reply.

      If you choose to reply off list...   drykert at rochester.rr.com 

         Del K. Rykert
         AMA - 8928 
         NSRCA - 473
         Kb2joi - General 



-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.f3a.us/pipermail/nsrca-discussion/attachments/20030702/bf223f5d/attachment.html