This is a step by step how-to for modifying a standard Saito 56/62/72 RC carb for use on a Control Line Stunt Ship. Over the last couple years I have tried just about every intake configuration imaginable on every Saito I use 30,40,56 and 62 and in my opinion a spigot with choke adjustment screw provides the most consistent runs of any other intake configuration. This How-To also applies to the Saito CL intake.
Have to give proper credit as the basic idea for this mod wasn't mine. It resulted from seeing what Allan Perret did with a Saito 40 and tips freely shared by Brad Walker and the Moon brothers. All I can say is everyone that has seen my Saitos run say they are among the best running 4 strokes they have witnessed. Now lets get started...
Modifying Saito 56/62 Carb for Control Line
By Bob Reeves
After proper break in remove the carb and completely disassemble as in the photo above. While you are at it remove the throttle leaver and idle screw from the barrel. The barrel will be used later during reassembly.
Next step is to press out the spray bar so the tip can be cut down. This is necessary to allow clearance for the choke adjustment screw. This vice has aluminum inserts in the jaws and will not damage the end of the spray bar. The spray bar is a press fit but comes out fairly easily. NOTE: Have since ran into a couple of these that were in so tight that simply using a vice as above would mess up the end of the spray bar. I now screw the needle valve in part way and use the needle valve to press out the spray bar.
I chuck the spray bar in my lathe and cut .06 inches off the end. The photo above shows a comparison between a stock Saito part and a modified part. .
On the 30 and 40 the idle stop screw is 90 degrees to the carb barrel and easily replaced with a set screw to lock the replacement barrel in place. This isn't the case on the 56/62 carb so I drill and tap the cast in boss on the carb body for a 4-40 set screw. The scrap aluminum block shown already had a 45 degree angled surface, I simply drilled and tapped it to facilitate mounting the carb body in my mini mill. The angle isn't perfectly perpendicular to the boss but close enough to work just fine.
Now comes the fun part, making a new barrel. I start with a piece of round aluminum stock first turning the OD for a snug fit inside the carb body. This ends up being close to .424 inches.
Once the OD is finished it's back to the mill to drill the venturi hole. I use a letter D drill bit which is .246". First center drill to be sure the placement of the hole is centered in the stock and 0.315" from the end. I use a centering jig from LittleMachineShop.com to make sure the bit is centered when drilling round stock.
Then it's back to the lathe to drill and machine the end to fit over and clear the steps on the spray bar. First center drill then drill all the way through with a #46 drill bit. This provides clearance for the spigot. Next drill almost all the way through (at least 0.15" deep) with a # 28 bit to provide clearance for the first step.
To provide clearance for the spray bar flange I first drill the end of the new barrel using a letter J bit and just go far enough to make the hole the full drill size. Stop when the hole just gets to the largest diameter of the bit. Then use a small boring bar to square up the inside edge. The flange on the spray bar is only 0.02" thick and all you need is enough to clear it.
Last steps to finish up the new barrel is cutting it off at the proper length 0.775" then chuck it back in the lathe to drill and tap the other end for a 1/4 - 20 nylon bolt. The above rough drawing shows the steps needed for the spray bar, test fit, the spray bar flange should be flush with the end of the barrel.
The spray bar can now be reinstalled into the carb body. Use the stock barrel turned around and a 1/4 inch drive socket to back up the carb body. Line up the feed hole in the spray bar with the threaded hole for the fuel nipple and press the spray bar back in using caution not to damage the spray bar or carb body.
The needle retainer, spray bar nut and fuel nipple can now be reinstalled. I use a dab of lock tight on the threads of the spray bar next to the carb body and a dab on the threads of the fuel nipple. This is more to seal everything up rather than a thread lock. The above photo shows what it should look like when reassembled.
The new barrel can now be installed in the carb body. I use a dab of JB weld around the outside of the barrel just to be sure I don't have any air leaks between the barrel and carb body. Insert a 4-40 set screw to lock the barrel in place then the nylon choke adjust screw. When the JB weld has cured it's it's ready to install on the engine.
Another view of the completed intake, this has Saito's velocity stack installed. Most Saitos do not come with the velocity stack although it is available from Horizon as an option. The jury is still out on if it's worth adding or not. As soon as I get my test airplane back in the air I'm going to put up test flights with and without to see if it helps or not.
Setup and Flight Performance
Maybe I should revisit a little on how I got here. When I first started messing with 4 strokes back in 2003 I was having the typical issues with trying to run a low pitch high RPM setup with the RC carb wired open. I was running an 11-4 on a 30 and it just wasn't working, level laps were fine but it just didn't have it when I pulled the nose up.
Switched to a Saito 40 with a UHP intake thinking the 40 would give me moor oomph and ended up with the same run issues I had with the 30. Almost completely gave up on 4 strokes then started talking with Brad from Dallas, he advised me to go to a higher pitch prop which I did. First time out the airplane was doing 4 1/2 second laps but it ran great. Back to Brad who told me to start putting panty hose over the intake to slow it down, this I did but when I got to 7 or 8 layers it got a little difficult to manage. In-step the nylon screw idea which allowed me to choke it down enough to get reasonable lap times.
From these less than great beginnings I saw enough good in 4 strokes to continue on and see if I couldn't make a larger engine perform as well as the little 40. The results of all my experiments and trial flights have proved to me that a few things seem to always work with Saito 4 strokes.
1. Plastic Clunk tank plumbed conventionally on muffler pressure.
2. Never use a prop with less effective pitch than 6 1/2.
3. If possible mount the engine on it's side.
4. Run PowerMaster YS 20-20 or equivalent fuel.
5. Do not use the needle to set speed, this is what the choke screw is for.
6. If you are running over 7 minutes on 4 ounces (3 1/2 on the 40) something isn't right.
Having experimented with several props on all my Saitos the following is what I have settled on. One would think the 5.5 pitch 3 blade conflicts with the above but this particular prop bought for a Stalker 61 from Modusa is effectively at least a 6.5 even though the pitch gauge measures 5.5.
Saito 40.. Rev-Up 11 X 7, Thunder Tiger 11.3 X 6.5, Evolution 11 X 7
Saito 56/62.. Rev-Up 13 X 7, Rev-Up 13 X 7.5, 13 X 5.5 Three blade CF.
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.. The needle should always be set within a couple clicks of this point, do not try to set speed with the needle it doesn't work.
Adjusting the choke screw:
The combination of a high pitch prop and restricted intake allows the engine to run at an RPM that is down in the torque curve for speed control. It isn't allowed to breath enough to wind up or run away and because we are running in the torque curve it doesn't slow down. We want to select a prop that will fly the airplane with the engine running 8000 - 8500 RPM and use the choke screw to set the speed.
For example, most of my engines will turn a 6.5 - 7 pitch prop at over 9000 wide open, to get it running at a reasonable speed, start screwing in the choke screw. Drop a couple hundred and readjust the needle, keep this up until you have the engine running at around 8200 with the needle set as described above. Fly the airplane and make the final adjustments for your desired lap time.
Something I have noticed with a conventional tank is you will get a slight speed up during the flight as the fuel is used up. When everything is right this typically is only about 0.2 seconds. Personally I like the slight increase in speed as it gives me a little more juice for the hard stuff especially if it's windy. If you are seeing a large difference in lap times you are probably running too far on the rich side of peak. This is a good indicator of when you have the needle set just right.
That's it folks, if you have any questions drop me an email.. Email Bob
NOTE: I have received several requests to modify carbs for others and will be happy to do it. I charge $40.00 payable through PayPal or you can send a check with the carb. The price is the same for either the RC carb or the CL intake and includes return shipping. The above applies to Saito 30 through 72 engines only, If interested just drop me an email and I will send you my address.