[NSRCA-discussion] 120 AC issues UPDATE

Mon Jul 14 18:04:50 AKDT 2008

Surging after 5 minutes or so wound up being caused by a worn piston for me way back when.  Once the engine heats up, it allowed the piston to wobble causing a mid range surging that limited max power.  If you can get your hands on a "slug" piston, without all the weight saving machining done to the skirt, you would be better off, but a lighter piston will be OK, it just won't last as long.  Of course, when changing out the piston, use a new piston ring.  

Date: Mon, 14 Jul 2008 20:15:47 -0400From: schale at optonline.netTo: nsrca-discussion at lists.nsrca.orgSubject: Re: [NSRCA-discussion] 120 AC issues UPDATE
I want to thank everyone for their suggestions.  Too many possibilities :)First thing I did was to change the liner and the exhaust.  Replaced tank internals.  Decided to run it on the ground at home and save a trip to the field.After 5 minutes or so flying issue was reproduced.  Motor started surging and would not go to full throttle from idle.  Needle valve setting didn't seem to matter.  Switch to 30% heli fuel no difference.Changed both check valves no change.  Took the motor out again swapped the original liner back in and checked the regulator.  -Bingo the edges of the silicone diaphragm are frayed.  Replaced the entire regulator unit with one from an un-run 120AC (thanks Norm S.) figured that had to be the cause.No change still surging and no power after heating up.  The motor still turns free, no binding so although I will try a different piston and front bearing I do not believe that to be the issue.APC 14x12 used for all of today's tests.I think I will put a well used but last time ran fine 120 in it and try to diagnose this motor on the test stand.  I'll see if the same issue crops up with a different motor.Stuart C.William C. Harden wrote: 

I ran an APC 15 X 10 on my YS120 AC engine with 30% nitro (Cool Power).  

Bill

From: nsrca-discussion-bounces at lists.nsrca.org [mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Stuart ChaleSent: Sunday, July 13, 2008 9:52 PMTo: General pattern discussionSubject: Re: [NSRCA-discussion] 120 AC issues

Various props from 13.5x14 to 15x8I would think the 13.5x14 would be under propped so shouldn't have been an issue.StuartPaul LaChance wrote: 

Here is a stupid question, but what size prop are you running?  Is the engine fully broken in?  Just a thought.  Had similar issues with a 170 DZ.

Paul

----- Original Message ----- 

From: Ken Thompson 

To: General pattern discussion 

Sent: Sunday, July 13, 2008 7:55 PM

Subject: Re: [NSRCA-discussion] 120 AC issues

Stuart, 

I had the same problem with a 91AC a few years ago, like Ron, it turned out to be the front bearing.

----- Original Message ----- 

From: ronclark51 at comcast.net 

To: General pattern discussion 

Sent: Sunday, July 13, 2008 8:46 AM

Subject: Re: [NSRCA-discussion] 120 AC issues

Stuart,

I've had the same problem before. It was the front bearing. The bearing heats up the front of the motor, then the regulator, vaporizing the fuel. Better known as vapor lock.

Ron.....................

-------------- Original message -------------- From: Stuart Chale <schale at optonline.net> I wouldn't have thought about the pipe.  I should have another.Thanks,StuartRUDDERCABL at aol.com wrote: 

In a message dated 7/12/2008 11:33:24 PM Eastern Daylight Time, schale at optonline.net writes:

I'll have to pull it a part again and see if I see anything with the sleeve.Anyone else have any thoughts?Thanks,Stu

Sure do.

Try a different pipe. I know a flyer who had this same problem some years ago, not with an AC though. He tried everything and the problem kept hounding him. Turned out that the pipe was plugging up when it heated up and restricting the exhuast flow. 

Robert

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