Well, I could use the Honda monkey bike parts for a $50 internal throttle but frankly it and the Vespa setup look a little spindly. Nonetheless, guys are using these parts, still available from Honda, on their daily drivers. I could drop between $80 and $300 for a proper internal throttle but there just aren't many out there for 7/8" bars and those are the most expensive. Using a 1" throttle on 7/8" bars would be the easiest and save some time but would compromise appearance. I would have to configure a lead-in tube of 1" just to mount it. Plus that would certainly not be in keeping with my subconscious masochistic need to say, "I built it myself."
Unable to find pics of original 191Xs internal throttles, I studied several commercially available models and, after several mental iterations, came up with a plan..... sort of. After halfheartedly starting, it became more apparent what all I would need. I ordered some bearings(in duplicate) and played with the parts I had made. This helped finalize my plan.
Here are some pics. I didn't feel like photographing all of it as it was of a few weeks and each step was unlikely to be used in the end. Indeed I had to remake the end plug and the outer grip(cut the helix the wrong way!).
Here I'm milling the 5/16" slot for the cable "plug" to slide in.
Machinery to cut helical grooves is outrageous and, unless used for commercial production, wouldn't be needed very often. Here's how I did mine. Twice! Once in the wrong direction! This allowed me some practice so I was a little more careful the second time. Not that the first one wasn't smooth enough - it was. Maybe someday I will use it for the left-hand grip for ignition or something.
Something not shown that is very important are witness lines. I did the first (incorrect) helix with two scribed lines. This one I did with one(shown). One center line was better for placing starter holes (I used a stiff center drill for that)
BUT I did go back and scribe two outer lines before I went past this stage. Why? You can't mill up to an edge you can't see and a smooth edge is where the bearing does its work. Best would be to scribe 3 lines. A center to start your drilling and two edges to make sure you don't go "outside the lines". The layout was very basic. First decide how much cable pull you need. I think I settled on 1" needed so I planned for 1 1/4" cable pull. Next decide how twitchy of a throttle you want. I didn't want a 1/4 throttle as the Briggs was likely to lag and, with the clutch, it could be difficult to modulate. I went closer to a 1/2 throttle. Basically, you scribe lines around the tube in two places representing idle and full throttle. This tells you the beginning and end of the groove. In my case they were 1 1/4" apart. Arbitrarily mark points on those lines 1/2 the circumference from each other(of less if you want a quicker response). Connect the intersections with a flexible rule and scribe your center line. Scribe your outer lines and you now have a guide to cut. Of course you must consider bearing placement, cable slide travel, etc. but 1/3 to center of the tube will be close. I stacked two tiny bearings on a post tapped into the cable slide. These bearings ride in both slots so a turn in the helix forces the bearing to roll down the straight groove in the inner tube.
I bored the ends of the outer tube to hold needle bearings and added a roller bearing on the end. This is held to a plug on the inner tube with a c-clip in a groove so the outer tube doesn't slide off the end(!).
Okay this is more confusing to write than to do so look at the exploded view and ask questions if needed.
I'll place a thin piece of sheet over the groove so the two small stacked bearings down slide off their post and wrap it in black cloth tape like you see on the old racers.
There are many ways to do each part but this is what I did. Total cost was about $20 in bearings and about a full day in the garage.
EDIT: Now, this being said, I have a day job and making these cannot possibly pay me the same. The time is too great and liability too risky. I have received umpteen dozens of emails asking for plans, CAD drawings, parts lists, and completed throttles. I enjoy hearing from readers but I do not have any of the above. The internal throttle was a one-off job and I designed it pretty much as I went along. I honestly cannot now remember the bearing sizes or dimensions of the other parts. Frankly, I would have to take it all apart to see how I mounted that inner tube to the bars. I do know that I found some online pics of production 1" models and simply thought out how it worked and redid it in 7/8". You can too!
.......... and it still won't be completed. The throttle is mounted on the handlebar but what to do with that cable? I made a "fairing" of sorts for the cabel to exit. I can see me at the first show I take it to. It looks wobbly so a guy tries to help and grabs the bars breaking this little piece off! It's held on by two countersunk 6-32 screws that I turned down from electrical plate screws. I always hated those things, always had a surplus, and saw them as useless for anything else so threw most away. I just didn't want the cable coming out through a rough hole. I don't know if I like it at all. I might, if fact I'm pretty sure, go with something simpler and smaller like a little sheet metal fairing. The cable housing is from vintage bicycle brake cable. A quick slice down the side with a knife and the colored coating comes right off. Of course there isn't much protection from grime but this won't see much road use.
I needed to build a standoff or two to keep the cable attached to but not rubbing on the painted frame. I could tape it up as many original photos show. Tape is quickly removed and replaced in a race setting and firmly holds the cable but why not waste an evening?! An easy solution would be the zip tie and fuel line standoff. That works great but isn't something seen on a BTR........ at least not on this one. I knew there was nothing out there to meet my exact needs (what's new?!) so I'll make my own.
Taking the idea from vintage bicycle cable clamps, I made the clamps from 18g steel strips 1/4" wide formed over a piece of 1 1/8" tubing then drilled and tapped to accept one of the 6-32 modded screws mentioned above. I made one with an extra looped piece to hold the cable housing. Works perfectly and the cable housing is nice and tight. Securing the cable housing well - especially at the carb - will keep the motor from revving when the handlebars are turned.
Here's how it looks on the frame.
