ASP 61AR (four stroke) petrol conversion by Razvan Rocas (M.A.M.S.) - September 2010

It's been a long time since I've started to play with the idea to convert a four stroke glow engine to petrol. I have already done a conversion (approx) 10 years ago while in Romania. The engine was a Russian Raduga 7, two stroke 7cc plain bearing. While the engine ran, it wasn't a good runner. The conversion involved a custom made cylinder head, flywheel and points. The spark plug and ignition coil were full sized ones. I feel the need to add that, this conversion has never been intended to fly or float. It was meant to prove to myself that it can be done. I believe that my brother still preserves my 'work of art'.

Back to the bright and lovely present. These days you can get at a reasonable price very good capacitive discharge ignitions (CDI) and some lovely tiny 1/4-32 spark plugs to directly replace the good old glow plugs. All nice and easy so far but, there is still a bit of a problem. The old points system has been replaced with a 'Hall' sensor and a very little magnet and, I dare to say that the biggest challenge the average modeler is faced to, is the sensor & trigger magnet installation. I have seen many ways of doing it (some of them way to crude for my liking). The photos I hope are self explanatory.

It is important to add that the aluminium ring to support the sensor has been fixed with 3 little grub screws to overcome the taper in the crankcase. The other ring has an interference fit onto the drive washer and is secured with a bit of Loctite. All the credits for the adapter rings shall go to Rex Handley. Thank you Rex once again and, who knows, I might ask you for some more rings one day (if you don't mind too much).

The ignition timing is actually easier to be set that it sounds. All I have done (as a starting point) was to draw a circle on a bit of paper with the sensor carrier ring diameter. Then I have drawn a 30 deg angle starting from the centre of the circle. The next step was to measure the length (on the circle) in between the angle lines and mark the same distance on the sensor carrier ring with the piston at top dead centre (TDC). It will be necessary to mark the TDC on the magnet carrier ring as well. When the marks are in line, the piston is exactly 30 deg before TDC. Ultimately, the sensor has been positioned in such a way that the spark will be generated when the marks are in line. Clear as mud.

By this time, some of you probably start to wonder 'what about the carby'. Well, I have left the original carburetor in place. I know it has been designed to mix a different ratio of fuel/air than needed for petrol but, following others experiences, I have decided to give it a try. After completely closing the low speed needle and opening the high speed one for about half of a turn, the engine is happy as Larry.

Another important aspect is the muffler pressure. It is so important that I have literary left it out. A small screw is now residing where not long ago, a nipple was so proud to be. To insure that a positive pressure is still kept inside the tank, I have connected the tank vent to a brass pipe facing the incoming airflow. It is important to add that the vent is positioned at the very top of tank and the brass pipe is well under the tank. We don't like to siphon-out the fuel don't we?

Finally, the question on everyone's lips: did it loose any power or not? The simple answer is: Yes. It did loose power. For all those speed daemons, 3D-ers and high speed 2-stroke fanatics it will be inadequate. But for everyone else it will be a pleasure to hear and fly. It still needs a bit of common sense and care in operation as with any other engine. Regarding the power: with the MAS 12x8 K-series prop it does 8400rpm. The fuel used is 5 month old petrol (91 octanes) from my motorcycle tank. It is mixed 20:1 (5%) with Castrol TTS (fully synthetic) motorcycle oil. While the power is more than enough to fly the Kyosho Tiger Moth 40 size (thanks Kelvin), I hope that some fresh petrol will add few extra rpm.

Most of you will know that the petrol engines generate more heat than their methanol counterparts. With this in mind, the modeler shall insure that proper air flow is maintained around the engine. So far, my converted ASP 61 has clocked approx 1 hour on the ground and a bit over 3 hours in the air. No dead sticks experienced. I have read over the internet that many others are flying converted engines some of them in excess of 300 hours. I do expect a very good life of mine too. Time will tell.

Did I mention how frugal the engine is on petrol? Or how clean the model is after the flight? No visible smoke is another side effect. Quieter exhaust note, and less engine vibration due to the variable ignition timing are also achieved.

Personal conclusions (at the end of approx 4.3 hours running time):

After the first hour of hard bench running I have removed the back plate, camshaft & rockers covers. While not much oil was visible around the cams, the big end and the rockers were very well lubricated. As a matter of fact, I was surprised to see the amount of oil accumulated into the crankcase. All these led me to the conclusion that the engine is ready for an airplane.

Probably all of the above sounds a bit too good so here is the not so good part. After the 3 hours air time, I have removed the back plate and the covers again to check for proper lubrication. This time I was surprised to see the lack of oil at the big end. However, the cams and the rockers were as oily as they used to be after the bench run. Good grief. What happened? Why is the big end so "dry?" It was time for a bit of an investigation. A quick check of the internals showed that while the big end was lacking oil, the piston skirt had plenty inside. Hmmm. My other ASP four stroke is powering the I.C.A.R. Universal and is installed inverted as well. In no time, off went the back plate of the little 30. Surprise-surprise. Not much oil on this big end either but, it was more than I could see on the 61. The cams and the rockers were showing good lubrication.

To cut the long story short, I have decided to stop flying the ASP61 inverted. A bit of virtual 'chat' on RC Universe made me to try 8% oil content. After a 15 minute trial ground run, I removed the back plate. Lots of oil at the big end this time. Back in business again. To be honest, I'm still puzzled by the difference in the oil content required by the methanol versus petrol. But, that is another can of worms I'm not prepared yet to tackle.

Parts used: complete ignition system made by Rcexl ( from DLEngines Australia) power supply is a 2-cell 800mAh LiPo with a 5V UBEC custom made aluminium adapter rings Tygon fuel tubing and (maybe) petrol resistant tank stopper. My original Chinese black stopper is OK with petrol so far. Also required is a certain dose of enthusiasm and interest in engines.

As a final note: personally I'm not a huge fan of the two stroke engines but, knowing what I know now, I consider that a ringed two stroke engine converted to petrol will be highly successful and desirable. As a final note: personally I'm not a huge fan of the two stroke engines but, knowing what I know now, I consider that a ringed two stroke engine converted to petrol will be highly successful and desirable. If you have a spare engine from 61-size upwards, they will give a good run for the money. This is it lads. You know as much as I know now. Keep building and flying those planes.