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       OS Two Stroke 1.20 AX carb Improvement

The OS AX series engines using the 70D carb could use some help with topend tuning. The problem is, the lowend needle does not fully retract when the carb is at full throttle. This basically blocks the full throttle fuel flow through the spraybar. No matter how much you open the highend needle the flow stays pretty much the same because of the lowend needle blocking it. So, your options are to have your lowend set very rich, or shorten the lowend needle. I tried the lowend rich method and it works, but the engine runs wet an sloppy at idle & mid throttle. I then decided to shorten the lowend needle. That fixed the problem completely. I now have a perfect lowend, midrange, and killer topend!   Reducing the lowend needle length didn't affect the idle at all, but it gave the topend the kind of fuel flow it needed. Here's how to do it:

There needs to be approximately a 1/16" gap between the needle and spraybar at full throttle. Remove the carb, and remove the screw on the backside, this will release the rotary drum. Using a needle file, file down the tip of the lowend needle until you get the 1/16" spacing between the spraybar and lowend needle. This simple modification will greatly improve the topend tuning.  

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NOTE: The lowend needle is tapered to provide a varaible fuel flow at different throttle settings. The largest diameter is where the lowend resides, and the mid-taper is midrange fuel. The topend should be wide-open at the tip of the needle. This is where the filing takes place.

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Filing the tip of the needle has absolutely no affect on lowend or midrange adjustment. As long as you don't get too carried away with the filing. This modification gives you lots of highend adjustment that you don't currently have!
 
 
It's important to understand that on AX carbs (70D) the SPRAYBAR actually adjusts the lowend. Therefore, the gap will change according to your lowend setting.

I flight tested the AX and it was perfect. This engine never ran so good. Originally, my highend needle was about 4 turns out to get a half decent tune. Now its less than 2 turns out to get a perfect tune.

 

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mark.fuess@comcast.net