I think I am the only person who loves a stock Gurtner carb. Well, not the Peugeot carbs. Or the AR2-10 because you can’t remove the diffuser. But that AR2-12 and the AR1 carbs, that’s me all day.
Years ago, when Handy Bikes was a wonderland of NOS parts and lore for those in the know, I found an unfinished AR1-14 carb body. All 3 of you with the deep Gurtner knowledge knows that’s not a carb. AR1-10 and AR1-12 came on various versions of the 7, AR1-13 came stock on the AV10, but a 14? I think it was a prototype (if you have any information about it hit me up, I’m still very curious). There was one box of carb bodies with no threading in the holes, and only stamped AR1-14, with a different type of idle adjust system (a J shaped hook that pulled the slide up from the bottom, not a side screw). I tapped it, and set it up on my VLX and rode it for years. It worked great, I loved it and I loved telling people about it. It is still around but not currently in service. Also I have no pictures of it right now but I’ll work on for those same 3 people who care.
One day I was tasked in rebuilding the motor on an AV42 of some year. Old I think, mid 50’s, can’t remember. The really old moby’s are so rare I honestly don’t have great knowledge of them. But a moby motor is a moby motor, so I rebuilt it. One thing that really stuck out was the factory crank seals were a type of rigid white plastic. It occurred to me it was probably so old they didn’t have a rubber that worked well for the seal. Classic moby design though, they never changed the dimensions of anything so new AV7 seals worked just fine. Upon firing it up it ran fine but with the stock carb all very stock but clean and rebuilt it was very rich on the jetting. Poor thing choked out at 15mph on fuel. Then it hit me, they probably just jetting rich from the factory to offset the crank seals that didn’t do their job perfectly. Now that it has more modern rubber seals it probably is just too rich.
Problem is the silly thing already had the smallest jet Gurtner made. I was also unwilling to swap to a different carb or do any modifications to the stock airbox or carb. My solution was to simple build an adjustable main jet!
I took the idle adjustment air screw and spring from a hobbit clone carb I had sitting around and ordered a tiny drill bit and a reamer (‘ll have to look up the sizes). At the time I had access to a really nice collet lathe equipped with hex collets. The old gurtner used the same jets as the later moby carbs so I sacrificed a new one instead of cutting up an old one. I drilled and tapped the end for the hobbit idle adjuster, drilled and reamed the center of the orifice of the jet, and honestly it was that easy (when you have all the right tools of course, this would be very hard to do without them).
It worked great on the bike. It adjusts all the way down to completely closed, to more fuel than this bike would ever need. I was able to adjust it to be a perfect adjustment for the bike and it ran quit nicely. I still had one little problem. The idle circuit gets its fuel from the main jet with no adjustment at all, just predrilled passages. So when I get it turned down enough to run smoothly at full throttle it runs the idle circuit a bit lean. Adjust the jet back up to make the idle smooth and the full throttle is too rich. I settled on a place that was a blend of the two and called it good. These old bikes still don’t go very fast but it was a nice little experiment in tuning around an old bike’s original hardware.
If I were to do this again I would need to sort out an o-ring or some other kind of seal on the adjustment screw. It leaked a tiny bit when running but nothing crazy. Still I considered it a win. It would be worth doing again if I were to run a kit with a Gurtner which is a very appealing idea for me so this might get revived again at some point. Until then, it’ll remain a one off, successful nonetheless.