mrbassman

I have a YS 120 AC that the previous owner ran 20/20, but the oil was all castor. I'm running 20/20, and the oil is all synthetic. Seems like this engine won't run the same way twice. Lately it's sagging at the top of a loop just as I get inverted. I need to throttle back and bail out, Also seems like I can't get this to idle reliably below 2100 - 2200 rpm. I'm wondering if I need reset the regulator and high end? Prop is an APC 15-10. I'm wondering if it's just worn out and needs a new piston/ring.


Also, Troy if you're reading this, I read all your posts about setting the regulator and high and low ends, but I can't seem to find a low end air screw. If it's on the barrel opposite the throttle arm, it's the strangest looking low end screw I've ever seen.

Thanks, Ken.

Troy Newman

First off the 120AC doesn't have the air bleed screw. That engine was before the air bleed screw on the throttle barrel was introduced.

As for the problems you have tuning the engine....It could be fuel related....I don't know what fuel you are trying to run now. Also I think you might have a tied engine and the 15-10 is a bit heavy of a load for it on only 20% nitro. I don't know....If the previous owner ran all Castor in the engine....then the exhaust ports, muffler, headers or what ever are likely all plugged up with baked on carbon. This is why I recommend not running Castor. This can also make the engine labor as it starts to close up the passages with the baked on gunk. This stuff is really hard to clean off.

When I ran the 120AC's I used 30% nitro on the 15-10.

But the engine could be time for e a bit of a rebuild. If its a worn out ring it can act like describe...Combine that with a heavy load...that a new one can haul but a worn out one will really labor.

I would recommend sending the engine to Richard at YS Parts and Service for some rebuild and cleaning work. I would bet this would solve your problems.

When the Castor starts to build up that baked on gunk its like choking the engine off...Thing of putting a banana in the tail pipe of a car. I have seen header pipes closed up all but 1/8" dia....The fuel is the culprit on that problem....

Plus being a well used 120AC....its likely time for a new ring. I usually put a new ring on the engines at about 250-300 flights....this is where it starts to fall off a little. You could probably go 400-500 flights before it really causes a major loss in performance....But when it starts to loose its seal and the gap in the ring gets way too big...the engine not only works on the top side compression and fire....but the downstroke of the piston works the super charger effect and runs the pressure system. So a worn ring here will affect the supercharger and the pressure system....This means not only low on power but the settings will wonder and become more difficult to setup.


Troy Newman
Team YS

mrbassman

No doubt the engine is tired, but I may have found the problem. I wanted to check everything over before I yanked the engine, and sure enough, the front fuel tank support broke and the front of the tank was sitting lower than the rear of the tank. Could explain why it never ran the same way twice. Let's hope !!!!

Troy Newman

Probably not.

The YS engines with their pressure system don't really care that much about tank orientation or location. Within reason that is...There is virtually no difference is the engine is upright inverted, on its side. Tank behind the firewall or back on the CG of the model.

Yes is the engine is a little too rich it will run better upright vs inverted....but as far as fuel supply and so it its not going to affect it.


NOW here is an idea. Could it be that the tank was moving around and the fuel lines were getting pinched off....then yes it could affect it like you are seeing. The tank full of fuel can be heavy and under G loads....its going to move if its loose. This movement can is the fuel lines are too short or restrained in some way pinch off the supply of fuel flow to the engine.


Troy Newman
Team YS

mrbassman

OK Troy......you're batting 1000. The tank made no difference, you thought it wouldn't. Here's the latest thing.....the fuel tank is not getting fully pressurized, not like it used to. I took my bicycle pump and gave the system a shot from the pump. The whole system pressurized and help pressure, so that tells me the check valve, tank and lines are all tight, the only thing left is the engine. (120AC). I sent an e-mail to YS parts and service, guess I'll be sending it in for an overhaul.

Thank you for your help. I'll let you know how I make out.

Ken

Troy Newman

The check valve pressure to the engine is important for two reasons. This exact pressure is what is on one side of the regulator. The other pressure is created by the spring tension you create with the regulator screw adjustment.

As the pressure builds in the engine past the pressure of the spring the little plunger opens, then as the internal pressure drops the spring pushes the plunger closed. I think you should replace the check valve that goes back to the tank...


Also I did NOT know this was a 120AC. The 120AC is a slightly different beast. It also has a check valve up on the air box. If you install that check valve backward it might act the way describe. The check valve on the 120AC up at the airbox, installs with pressure flowing into the airbox, and stops pressure from flowing out of it.

If this check valve is backward, then the pressure and the next charge will be "Light" in the air box, and a result is the engine could go lean.


To clean out the check valves pump raw fuel through them. This will clean out any gunk that has built up over the years. Your engine is 15yrs old or more...there is likely some gunk in those check valves. Of the tank could have a leak. Both of those options will make the engine go Violently LEAN.


Troy Newman
Team YS

mrbassman

Check valve on the air-box is good, the check valve going back to the tank is brand new. Both installed properly. I'm baffled. I just keep going back to that engine. The whole system held pressure when I took the engine out of the system, but today it wasn't building up pressure.

Troy Newman

Well its old and probably been run hard. Send it to Richard and He'll fix it up for you.

Troy