Most of this has been posted in various threads on different forums over the last year or so but in putting it together for Bob Z's four stroke group thought it might not be a bad idea to also post it on the forums. It's possible that some have not seen or read what a year of trying to get my Saito's to run consistently resulted in.....
First off, I only use the carb for break-in to control the RPM per Saito's recommendations. If the carb is the only choice you have, try it but I was never able to get what I was looking for using the RC carb. UHP
http://www.egpworld.com/ultrahobby/ makes a Control Line venturi for most of the 4 stroke engines we might use. My 56 has a Tom Dixon venturi but understand they are no longer being made so UHP, home made or buying one of the new CL engines are about the only choices.
After a year of trying everyone else's recommendations, discovering why so many that tried 4 strokes went back to two strokes (which I almost did myself the day I flew my Score in a 15 mph breeze and it was doing about 90 at the bottom of the vertical eight ). Started talking (listening) to Brad Walker out of Dallas. The end result is what I believe to be the holy grail of making a 4 stroke run (at least Saito's). I am talking about rock solid stunt runs with no slowing down, speeding up or sagging overhead. I really believe my 56 powered (67 ounce) Score can be launched vertically and I could go straight up, fly an overhead eight and not loose line tension. When I get it's replacement built and trimmed I am going to try it.
When I first installed the 40 on my TwistMaster I was running an 11-4 with a uniflow hard tank on Powermaster 10-22 50/50 with the RC carb wired wide open. Even brought home some hardware with this set-up, most notably placing in Intermediate P-40 at Brodaks. Ya it ran OK but it had to get through the reverse wingover before it would start running right and was very critical on the needle. Every once in a while it would simply quit for no apparent reason. It would sag over the top and flying the square 8 was a challenge in power management.
Now with both my 40 and 56, the best way I can describe it is one word "confidence", when I pull the nose up for that first wingover I have no doubts.. That little Saito looks at me, says oh you want me to pull this overweight Twister (or Score) up over the top and maintain the same line tension I give you in level flight.. ok.. no problem..... When I get to the square eight the airplane is flying the exact same speed in the last corner it was in the first. Oh and when the wind kicks up and I am setting up for that first loop of the clover, I no longer have to whip or otherwise worry about if it will do it.. I know it will... I would bet the confidence factor alone improved my scores by 25 points or better.
I am sure this isn't the only combination that will work but it was the only one I found that met my expectations and provided the consistency I was looking for. The formula I use is basically, load it down with a high pitch prop and adjust the choke area to get the desired RPM (lap times). It's very important you completely forget everything you perceive and all your 2 stroke experience. None of it applies...
If you can find them, Rev-Up Four Stroke props are excellent. Baring that, my next choice would be a Zinger Pro, the Zinger needs a little thinning work to make it as good as the Rev-Up but it should work stock well enough to get you in the ball park. I believe whoever designed the Rev-Up four stroke props knew what he was doing. They are close to the actual pitch marked on the blade and have a slight undercamber. I run a Rev-Up 11-7 on my 40a and a Rev-Up 13-7.5 on my 56. No these are not typos.. Funny how many people looked at the prop on my Score this past season and walked off shaking their heads.
Ok we have a prop now we need to make the fuel delivery system four stroke friendly. This is easy, use an RC clunk tank plumbed more or less like the instructions that come with the tank. Non uniflow, vent in top inside corner and run it on muffler pressure. Funny both my 40a and 56 use the same amount of fuel, 4 ounces. Use PowerMaster YS -20-20 fuel if you can find it, I have not found anything that works as well even some home brew 20% nitro 20% all synthetic I made, almost but not quite... Don't forget the after-run oil, a necessary evil with synthetic lube.
Next you either need to provide some way to adjust the stock RC carb or some means to adjust the choke area. The stock carb is the easiest but if the barrel tolerances are a bit off you will have a variable air leak that will give you trouble with inconsistent runs. They say a picture is worth many words so I am attaching a photo of what I did with the 56 and the 40. The 56 is a Tom Dixon manifold but the same can be done with the UHP. The 40 is a UHP but I made another insert with a thicker wall thinking I needed it for the screw. Have since heard from a few that did the same thing with the stock insert and it worked fine. The picture of the 56 was before I finalized the adjuster screw setup, I added another hard wood block closer to the venturi and epoxied both to the firewall. If you look closely at the reflection in the inspection mirror (Saito 40) you can see how much of the venturi I ended up choking off with the nylon screw.
If the airframe will allow it or you are designing a ship for a four stroke mount the engine on it's side, like a profile. They just work better that way.. The needle is less critical and the oiling system works better. Please don't ask me to explain why I just know it works.
First time out decide what length lines you feel you will need. I choose lines for the airframe not the engine displacement. I fly my Score on 63 foot eye to eye, the TwistMaster on 58, would probably go to 65 for something like a Strega. Start the engine and while on the ground adjust the carb or air intake for about 8100 RPM.. You will need to adjust both the carb and needle, I start with the carb wide open and start closing it down, drop 3 or 4 hundred and re-adjust the needle, drop another couple hundred and re-adj the needle till you get running at about 8100 with the needle a click or two on the rich side of peak rpm.
Interesting side note.. As I closed the venturi down on the 40 it started actually using more fuel than when it was wide open. Remember what I said above about none of your preconceived notions applying.. 4 strokes simply do not draw fuel as well as two strokes and we need to do everything we can to make it easier for the engine to get the fuel it needs. Reducing the venturi size improves fuel draw, using low viscosity synthetic lube helps fuel draw, muffler pressure helps fuel draw, non uniflow helps fuel draw. More fuel = more power and better lubrication, how can you argue with that.
The needle is critical and needs to be set on the rich side of peak RPM, I have found I can adjust it better by ear than I can with a tach. I turn it towards the lean side till I hear the RPM drop than go on the rich side till I hear the rpm drop, then try to come back to the spot half way. This should be close to peak RPM, then I go to the rich side and try to find the place half way between max and where it starts to drop RPM. This sounds complicated but once you play with it it isn't as bad as it sounds.. Also once you have the carb and needle set you can pretty much forget about it, I haven't changed the needle on my 56 since I came back from Brodaks, 3 contests and 3 states ago.
Once you have it running at 8100 with the needle set right, fly it and see how close you are to your desired lap times and needle settings. It may be fast or slow but shouldn't change when you do a wing over. Now you can play with the carb and needle to get the laptimes you and the airframe are happy with. If it seems to go lean over head richen it up a click but don't go too far sometimes it's hard to tell if it's too rich or too lean.
That's about it, if you have any questions drop me an email and I will try to help.. I recently purchased two of the new Saito 62's and am planning on applying the same ideas when I get around to trying them, probably start with a 14-7 Rev-Up. BTW: the UHP 56 manifold also fits the 62... My Saito line up has grown to three 40's, two 56's, two 62's and one 30. I think that should cover about anything I would enjoy flying.
My next project is going to be a Dee Rice Ringmaster 576 (576 sq in wing) with a Saito 40a Golden Knight. This airplane was designed for the OS LA 46 but I have no doubts the little Saito will handle it. Also have wanted to build a SIG SomethinExtra into a CL stunt ship for several years. My experience with the Score has been so positive the SomethinExtra will use Score numbers and a Saito 56 or 62. Really looking forward to getting these two in the air.
My thanks to Bob Z, Brad Walker, the Moon brothers and everyone I forgot who helped get me here.
Bob Reeves
Topic: Summery of my Saito experience (Read 858 times)