RE: FL 70 4stroke
I have one. It is a factory replacement for the 1st one I bought when they first came out. Actually, it's the last one I bought too. Needless to say, I had some trouble with it. OK, I had trouble with both of them.
In the course of trying to use the 1st one, I heard a lot from the 4stroke clan about how effortless they were to use. That was what I heard until I started having trouble and did what every bright modeler does when he has trouble, ask the experienced users. Turns out they were the same bunch who'd said how trouble free 4strokes were...... ok, I wont turn this into a rant.............
It was suggested that they really need higher nitro fuel than otherwise. I use 5% with all my 2strokes so borrowed some. Later on, it seemed to help.
I was trying to use it inverted. It flooded in a heartbeat, and then it's heart wouldn't beat if I blocked the flooding. I started using a hemostat to clamp off the fuel delivery line while fueling. That led to trouble getting any prime into it. It was fully cowled and the intake is on the backside of the engine. The intake's design can lead to flooding problems when the sucker is inverted. There never was a real simple, elegant solution to that pair of problems. That was very much a problem of the plumbing combined with the intake's orientation, but it was still a problem. I built a priming tube into the front end and by controlling the amount and timing of a quick shot of prime into the sucker, it became fairly dependable to start.
After a short war with the sucker it gave a session that seemed to suggest that all was good. Except that it really wasn't broken in and didn't seem interested in doing so. A couple of sessions and it still was running ragged at some throttle positions. I finally swapped it out of the airplane. Put an OS46AX in, pulled out the unnecessary priming plumbing and that engine ran like a clock from the first flip. So there really wasn't anything special about the airplane's plumbing with that engine at least.
A buddy showed up with a FL70 about that time. He had it on the front of a stick. It was upright and totally exposed. It didn't give him a bit of trouble. I had mine out so decided to see how it ran upright on the bench. It appeared to have been beaten up internally by the previous running. I was fairly certain it had some kind of problems with it's timing. So I sent it back to OS/Tower in hopes of discovering what parts were replaced (as a way to find out what I'd been fighting). Turns out they sent me a new one with no explanation. I ran it on the bench and it was good to go. Upright and not cowled. I ran it awhile on a trainer. Upright and out in the breeze. It was ok. It didn't seem to need 15% but didn't seem to like 5%. So I ran 15% and 10% in it as long as I used it upright. It didn't really show much preference to those two but didn't seem to like the 5%.
It was my first 4stroke. Good thing it did for me was get the 4stroke fanboys to drop their salesman personna BS, and since then they've all been up front from the gitgo on what special tricks they use on each of their planes. It appears only one of 'em has actually used an FL70 however. And I'm not expecting many more of them to try one because OS/Tower has discontinued the engine. hhhmmmmmmmm
My impression is that the thing might be damage prone because of it's layout and internal parts. But that's a guess based on exposure to only two of them. If installed upright the intake points down. Lots of 4strokes have the same design, but mine seemed to suffer badly from it when installed inverted. My advice would be to break the sucker in upright if you plan to install it inverted. And then try to run it on the bench inverted to figure out a fueling/starting sequence it likes before you hide that venturi away and out of reach.
Short version of my advice? Run it upright and out in the breeze. All the other stuff, like extra hot plugs and high nitro...... doesn't seem to be needed.