GSX750 Chopper #6 Choke and other bits...

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Prof
Founder, Choppers Australia
Posts: 5625
Joined: Sat Oct 22, 2005 3:54 pm
Location: Willunga, South Australia
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GSX750 Chopper #6 Choke and other bits...

Post by Prof » Sun Jul 23, 2017 10:42 pm

James GSX is almost ready to dismantle, final weld mould and paint. Seat and backrest are done and will be written up in a separate post.

Original choke is a long cable (red arrow) running up to a bulky pull button that moves the choke bar outwards 15mm (blue arrow)...
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The essence of a traditional chopper is simplicity. This junk needs to go. The following plan shows the simplest choke set up we can have. Unfortunately the choke bar connecting the four carbies is heavily sprung, so we can't just have a pull push set up but need to hold the choke out against this spring pressure when cold starting. We will pull the bar out with a piece of 6mm stainless round bar with a simple knob on the end. A couple of roll pins will give us full and half choke. They will be retained by a slotted nut attached to the outer carby top...
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A piece of 20x3 angle is the starting point. You can see the texta marks showing the centre of the screw hole...
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A piece of 3mm is welded to the open end for attaching the nut. Sorry about the blur, but I wanted to illustrate a method of welding on small pieces... don't cut the whole piece to shape, but leave some length to hold onto while tacking...
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...then trim it to shape...
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6mm stainless round bar and an alloy knob off a computer case. Fne male thread is trimmed off in the lathe and a 6mm hole drilled down the centre while still in the lathe...
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Knob is drilled on the side at 4mm and an M5 thread tapped in. Thread is started with a taper tap and finished with a bottoming tap. You can also see a small flat on the round bar that the grub screw will bed into preventing the knob ever coming off...
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End of round bar is threaded with a thread die set up on the lathe to keep it square to the shaft. Blue arrow shows thread. The die holder is retained on the lathe tool holder and the chuck is turned until we have a couple of threads. Then removed and threading finished in the vice in the normal manner...
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Required clevis gap is measured...
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Clevis cut in some 12mm square stainless with a 1mm blade...
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Clevis is drilled for an M5 Socket Head Cap Screw. First drilled with a 4mm bit and then one wide with 5mm bit. Thread tap enters through the 5mm hole to help keep it square...
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Something to practice is cutting off small pieces without them dropping; cut until the piece starts to push away...
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Our new lever almost complete. The thread on the end allows the shaft to pivot (only half a turn needed)...
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An M6 nut is drilled out to 6.5mm and the nut welded to the carby bracket...
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A bit of measuring earlier demonstrated we needed to set up the bracket on the outside of the carby top not the inside..
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Shaft and bracket installed and works. The shaft is blacked with Texta and marked for the roll pins...
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... and then removed and a flat ground on the shaft so we can easily drill the two holes. Roll pins are nominally 3mm, but actually 3.2mm in diameter so they will hold in a 3mm hole...
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Nut is now slotted 4mm with a 1mm disc...
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Renistalled and at rest. Blue arrow shows roll pin...
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Choke fully on. I'll drill a second hole for half choke. But for now it works perfectly. Quite a few steps to produce a simple choke, but the end result is simplicity and function... just what the super cool chopper jock needs...
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Chopit'nrideit... Prof

Prof
Founder, Choppers Australia
Posts: 5625
Joined: Sat Oct 22, 2005 3:54 pm
Location: Willunga, South Australia
Contact:

Re: GSX750 Chopper #6 Choke and other bits...

Post by Prof » Tue Jul 25, 2017 10:30 pm

Jame's GSX is getting close now...
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Carbies were originally supported at the rear by the airbox... gone now on this classic chopper. James is still unsure of what air cleaner set up he wants, so I will make a bracket in the centre (blue line) off the rear support (red arrow)...
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A piece of 25x3 stainless flat bar is bent in the vice with a hammer and then trimmed to 18mm wide...
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We'll use the gap in the outer two carbs to get the right angles on the bottom of the support; here bent to clear the carby tops...
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A foolproof way to to get the accurate tight bend we need to hook under the rear carby bracket is as follows. Clamp the piece to be bent in the vice with a second piece of steel bar with just enough clearance for the bend...
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Tap to 45 or so degrees. Son't go overboard or you will bend the bar backwards...
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Put the two pieces in the vice and squeeze. You can make a fold any size bigger than the bar you are using with this method...
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Hook works perfectly; now the top fixing...
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The fuel tank has a hidden tab at the rear that catches in the top tube. We drill a 7mm hole in the centre and then measure to the top of the carby bracket with a piece of welding rod...
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Mark our hanger remembering to allow for the thickness of the bar when it is bent ie we reduce the length by 3mm...
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Bend in vice as before...
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Drill in 5mm hole and tap it to M6...
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It actually fits perfectly (which is nice!!). There is always 5mm leeway as carbs are rubber mounted at the heads and can move a little up or down with out any problems, but don't force them anymore than this. The stainless button head bolt has a flat washer under it held in place with grease while we insert it down into the hole...
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View from the front...
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From the rear. Will hardly be noticed, but when polished will provide a little spot of shine when the bike is done...
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Posted this exercise for you new fabricators out there, as doing tight folds can be a bit of a mystery...
Chopit'nrideit... Prof

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