If you'll build the 4-speed freewheel that'll deal with some of your shifting issues. The freewheel is on a 2-stage 6-speedcore. The cogs are (34,28,22,16). As long as both cores have lock rings you should be able to build this using the 6 speed 34-14 mega-range and your old 7 speed 28-14.
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Check your axle and see if it's bent. Chances are you'll have to replace it with a cro-molly axle. Get a Sram X4 rear derailleur; it has a very strong tensioner spring. The Sram X3 shifter is for a 7-speed or lower freewheel. The Sidewinder came stock with a Shimano Tourney rear derailleur. It exploded on me.
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Mark20 suggested using a Sram X5 rear derailleur. In doing my research I found the difference between the X5 and X4 was weight. Well, most here will sacrifice weight any day for strength.
Here's a picture of my triple chainring shifter. Before I built the Sidewinder; I had a LandRider which shifted 7 gears automatically. On the Sidewinder, I put a 7-speed custom-built 34-13 freewheel. One would think 21 gears that are way too many and too complicated. Not so, I developed a color-coded shifting system. All shifting is done with the left hand. For a total of 9 sequential non-redundant ratios.
Red gears
1(1-3)
Steep hill climbing
Yellow gears
2(3-5)
Around town general use
Green gears
3(5-7)
Open road use
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