OS.40 FP ABC Running away

[Juan Valentin]

   
              My OS.40 FP ABC is running away and I can`t find out why. This engine is an OS.40FP with a Tower .40 ABC cylinder and piston reworked by Randy Smith. I had this engine for a while and it always ran beautifully. I used it in a Brodak Oriental before placing it on my scratchbuilt P-51 Mustang. It was working like I said beautifully but suddenly it started to run away when I tried a new fuel using 5/20 in place of the 5/22 that I was using. I had never had any problem with the fuel mixed by one of our club members. I have been using a uniflow tank no pressure,Randy`s muffler Xoar 11 X 6 prop.
            I tried using fuel from a few of my friends that was 5/22 and replaced the glow plug twice. I took the engine apart cleaned and there is no sign of trouble,The cylinder has no deep scratches and the piston is in excellent shape. The only wear in the piston is in the top of the skirt but again no deep scratches just normal wear. Engine has excellent compresion and The glow plugs used are Thunderbolt long reach with idle bar.I have also replaced the uniflow gas tank, Any suggestion?  Your help will be appreciated.
                                                                                                              Juan

[phil c]

What is the oil composition?  Most ABC motors need 50%or more synthetic oil to keep castor oil varnish from building up.  A couple of hard runs can build up a significant amount of varnish that will keep the motor running hot until it is removed.

I'd suggest soaking the motor in kerosene and scrubbing off any varnish, particularly on the piston top, the inside of the head, and on the piston and liner with a no-abrasive pad- copper mesh, or a kitchen pad with no abrasive in it.

Other possibilities are crud in the fuel, or specs of congealed oil/wax in the fuel.  They may not be easily visible even on a good filter.

[Tim Wescott]

Did the run change with the change in fuel, or the change in plane?

If it's not what Phil said, and if your P-51 is cowled, it's possible that the engine's not cooling properly.

[MarcusCordeiro]

Is your P51 a profile?? I remember seeing the pics, but don't remember the fuselage...
Anyway, if it's a profile, maybe you have foam forming inside the tank 'cause of vibration.

Marcus

[ray copeland]

As Marcus say, fuel foaming would be my "guess" .  Try to duplicate the runaway on the ground and then hold the fuel tank firmly with your hand and see if the rpm's lower.

[Steve Helmick]

I'd say too much prop load. What sort of prop did Randy recommend? I'd start with my favorite TT Cyclone 11 x 4.5 and I'd try uniflow/muffler pressure, tho I know Randy doesn't like muffler pressure. You do need to keep your system tight, so if you have trouble with things coming loose or falling off, it's probably not for you.  Steve

[Juan Valentin]

  Thanks for the reply`s. The engine had been working perfectly on my profile mustang. I had no problem with the prop loading the engine correctly and I was getting thru the pattern well. This just happened suddenly as I started with a new batch of fuel but didn`t go away as I tried other fuel. Foaming is out of the question since we use Armor All as an antifoam agent. The fuel I was using was 5%nitro 11% castor 11% synthetic the fuel that I started to use when I had the problem was 5% nitro 10% castor and 10% synthetic. I have checked and cleaned the filter at the engine many times. I will be trying a new gallon of 5% 11% castor 11% synthetic Next sunday in hopes that the problem goes away. My engine installation is tight as I use threadlock on all screws. I will be bench running the engine to see how it does but the real test is in the air.Here is a pic of the plane.Thanks again to all for replying.
                                                                              juan                                           

[Dennis Moritz]

Nice plane. Pinhole somewhere. Check fuel lines. Tank plumbing, solder joint letting go. Crud in tank. Pressure test tank make sure there is no air leak, even a minute leakage can cause runaway. Check needlevalve. Is the hole pointing down? Is it tight against the case? Is there any play in the needle. Possible to leak air through the threads. (More a Fox issue, but I have seen at least one FP need a new stock needle.) Tighten mounting screws. Plywood collapses after a time. Loosening mount will cause extra vibration. I've had models that needed tightening periodically.  Check muffler mounting. Is it tight. Has crud accumulated in muffler. Check and clean. If using muffler pressure, is pressure outlet clear. Different brands of fuel have different qualities, actually use different lubes, different qualities of castor possible, synthetic lubes differ. Try the fuel that worked for you in the past. FPs wear out the front bushing eventually. This can also cause run aways. Is there a lot of time on the case? Do you use a starter? I do. That will wear a case eventually. I like Phil's idea of cleaning varnish from piston and liner. Deposits will happen. Even when using a synthetic lube mix. Weird profile bad vibes sometimes happen after there is time on the plane. Fues/wing joint can loosen a bit. Has the vibration level gone up far as you can tell? Usually this is noticeable when holding the plane by the leading edge of the wing as engine is running ready for launch. Onward. Switch out the engine. Switch out the tank. If the airframe is vibration prone, it will show up then. If all else fails there is the top sekret inverse tetra tank.

[Dennis Moritz]

Check prop balance. Prop can get nicked or even cracked. Run a different prop. Your aging airframe may have developed a troublesome harmonic. Changing prop, going to wood for instance, or changing diameter or pitch a bit, can move the engine to a less sympathetic vibration zone. Onward.

[MarcusCordeiro]

I'd replace the glow plug too... Just in case...

Marcus

[Juan Valentin]

Hello Guys
                       I was not able to fly today due to rain,will be flying it next year if the weather allows me. Our flying field was soaked this morning but a valiant effort was made by our friend Rafael Usaf Irizarry to mow the field for us. We got there early with hopes of having a good day since previous days had been sunny with  little rain but no luck. I had checked everything on my engine and decided to replace the aluminum head gasket since it seemed compressed Other than that I checked all lines,pressure tested the tank,made sure filter was clean,bolted securely and installed a new Xoar 11 x 6 since the engine had been running great with it also have a few Zinger and Bolly props to try. Will keep trying to make it right. Thanks all for your suggestions.
                                                                                                                           Juan

[Juan Valentin]

Hello Guys
                      I went flying this weekend since the rain stopped and we were able to mow the field. My engine was still running away with the set up I had then we went to a Bolly 11 X 6 and the engine ran like it should. I guess all Xoar 11 X 6 are not equal Since it had been running good previously with the one it had. I tried with a fresh batch of fuel and my first two flights were runaways then I switched props and the next two were great but the wind came up and couldn`t put anymore flights.Next time I have a runaway I will remember to try different props and will keep more props in my flight box. I hope my experience will help others and thanks for all the advice I received.
                                                                                                                       Juan