120 decreasing rpm
 

I am trying to get a older 120 running for a buddy of mine. I just replaced the regulator parts today and it is idling well. I am assuming the pressure port coming out of the nose of the engine should be pressurized with fuel coming out of it. This engine is only getting pressurized air coming out of it. When I rev it up to full throttle it will go wide open fairly well but will slowly loose rpm and eventually almost die. He bought this engine in a ultimate knowing it had this problem. I also put in a new os #f plug and replaced the fuel lines and tank stopper. When I open the vent line it is pressurized. Any advise would be appreciated.
thanks
Jeff

RE: 120 decreasing rpm
 

I just realized that the pressure port cant put out fuel or when I would vent the fuel system fuel would come out instead of air like it is. I also failed to mention that I have the regulator screw backed out about 1/2 turn. I also meant to add that it will run great with the high end needle turned in all the way.
thanks
Jeff

RE: 120 decreasing rpm
 

What do you mean by backed out 1/2 a turn? 1/2 a turn from flush with the regulator body or 1/2 a turn from fully closed? if its 1/2 a turn from fully closed the regulator probably needs some new components, plunger, spring and diaphragm.

RE: 120 decreasing rpm
 

I just replaced all the regulator parts minus the body itself. I just adjusted the low end so that the screw was flush with the housing and got it running. It will rev up but will start to loose rpm pretty much right away and if I hold it at full throttle it will drop all the way down till it dies. It did this before I replaced the regulator parts. I currently have the high end at 1 1/2 turns out. Not sure where to try next?
thanks
Jeff

RE: 120 decreasing rpm
 

Is this with the cowl on? My first thought is overheating.

RE: 120 decreasing rpm
 

completely cold motor with cowl off. Will do it the first start before the engine gets a chance to warm up.
Jeff

RE: 120 decreasing rpm
 

Jeff,

I had a similar problem years ago, I fixed my engine by replacing the piston and ring, maybe the engine you are working on is just worn. By the way, my engine started to loose power in flight one day, and after three flights with the same problem I had to change the piston as I mentioned, it was something we will think it happens gradually, but no. And which 120?


Regards

RE: 120 decreasing rpm
 

Is there any indication by looking at the piston? I know to look for the obvious scoring and such. I believe it to be the sf or nc model, not sure which.
thanks
Jeff

RE: 120 decreasing rpm
 

Not really, mine looked like new when I discarded it, they wear very evenly and smoothly, I really doubted if I needed to change it, but based on the behavior I did and fixed,sorry I can not have better news on that. I'll get a picture of the discarded piston tomorrow, I still have it.

The reason why I changed the piston is because there was a new version of that piston which produced more power, but try changing the ring at least, and make sure the piston grove is very clean.

Regards

RE: 120 decreasing rpm
 

I took apart the engine today. The rear bearing is alittle rough. Everything looks pretty good. I would like to know how easily the back plate disc valve should turn. It turns smoothly but does have some resistance while turning it. Just wondering if that is normal?
thanks for the advise
Jeff

RE: 120 decreasing rpm
 

A little resistance is normal. if you are that far into the engine that you have the bearing out change it and the ring at minimum.

RE: 120 decreasing rpm
 

I finally got all of my parts in today and put it back together. We put in the plane and got it running. Same issue. It will not run with the high end needle set at 1 1/2 turns out. It will start and run but while holding full throttle it will still slow down until it would eventually die. If I adjust the needle to 1/4 turn out it runs good and has quite a bit of smoke. With a 16/8 prop we got 7400-7500 rpm If I go alittle bit leaner to 1/8 turn out I can get 7800-7900 rpm. still gets good smoke at the 1/8th.
So far I have replaced the ring, both bearings, and the regulator diaphragm, spring, pin. Im not too sure where to go from here. Not sure if we should just run it at the 1/4 turn point as long as i keep good smoke?
Is there anything maybe in the carb I should be looking at? I havent taken it apart yet.
thanks
Jeff

RE: 120 decreasing rpm
 

Take a look at the actual needle itself. I have seen and had engines that will wear the taper on the needle into a step due to a long needle extension being used. It will exhibit going from too rich to lean or perfect in a click or two after a long time of no response to turning the needle.

If the needle has a nice smooth taper then you might consider sending to Richard far a check up.

RE: 120 decreasing rpm
 

Petec you are the man!! I just now tried the needle valve out of my 120 in my buddies that I have been working on and it runs great at the 1 1/2 turn out setting, even chokes and starts to die when I start to lean it out more then that. When I compare the bad one to mine it is about 1/16- 1/8 in shorter and doesnt have as sharp of a point as mine. I have had to change quite a few parts to find the problem but at least the engine is in like new condition. I guess another order from central hobbies is in order.
thanks for all the help.
Jeff

RE: 120 decreasing rpm
 

I was able to pull the actual needle part out enough to match my needle. It looks as if the previous owner must have bottomed out the needle and kept turning the screw forcing the needle up into the thumb part of the valve. I havent tried it but I am pretty sure that it will work good.
thanks
Jeff

RE: 120 decreasing rpm
 

Thats good news. I am glad to hear the engine is running as it should.