I got another reminder about the use of excessive castor in AAC engines this weekend. A gentleman asked for assistance with his Jett76 that was "binding up" in the cool LA morning conditions we will euphemistically refer to as "fog" when attempting to burp it. I went over, and it was perfectly free through the bdc, and squeaking and "sticking" as it came up on compression. Usually, this is caused be excessive raw fuel washing out the oil, but it was happening far too low in the stroke and with only tiny amounts of prime. We got it to pop a few times but for the most part it would just stick coming up on compression, and was about impossible to back-bump because it would stop before it it got enough pressure to fire.
I had to go fly my flight, but then afterwards, in further discussion, he was taking 18% oil (50/50 fuel) and adding 7 ounces of castor, which makes the oil content roughly 24% and mostly castor. Since it was clear we couldn't prime it enough through the crankcase without binding it up from drag, I resorted to the old 40VF burp trick of removing the glow plug and priming it directly into the cylinder. While doing this, large amounts of varnish were found all over the engine and the glow plug was stained almost completely black from castor varnish. This was the tip-off, the engine had varnished up on the excessive castor and reduced the already very tight clearances, and become "sticky" in the cool morning air with cold fuel on it, and that's what was binding.
After a few tries with increasing amounts of prime we got it running on my fuel (15% Powermaster "Air" straight out of the can), and no problem, ran fine (and REALLY LOUD). We had him run it for a few minutes, and afterwards it was free as it could be and felt normal, hot and cold. He was sent off witha can of proper fuel and instructions to run 18% oil fuel with mostly synthetic, no manipulation, and expect the run to change for about the first gallon as the varnish is slowly reduced.
The point of all this is that you do not need and in some cases cannot tolerate excessive amounts of castor and the varnish it causes, in AAC engines. The varnish will build up to the point you are reducing the clearances to the extent that it will cause operational issues. Randy's fuel recommendations (pinned at the top) are very good - if *anyone actually read them before they asked the many questions that are asked* - but as long as you are reasonable confident you will not get too lean, you can probably and should probably be able to get away with even less castor than suggested in modern AAC engines. I expect the problem is not nearly as bad in ABC/ABN engines because the cold tolerances are larger. You do get the same oil drag issues when it is running but you can use that to manipulate the run characteristics to some degree.
Brett
p.s. the alternative, possibly superior, solution would be to take the engine apart and crock-pot it to clean it back to the original conditions. That was contraindicated in this case because it would have required someone with less-than-entirely-solid skill set to successfully disassemble and reassembly it properly, clean or not. That's no indictment of this particular gentleman, most of the people who *do* take their engines apart don't know how to do it either, and I know at least 2 cases of world-class modelers doing a routine "disassemble-clean-lube" process who destroyed the engines in the process (and dozens of people who are not world-class modelers the same). In this case at least there really was an issue, but cleaning it in operation seemed a lot safer. Based on discussions with Dub Jett in other cases, I am pretty sure that's what they would have done had it been sent back for repair.
Topic: Castor in AAC engines (Read 2013 times)
