Much talk of heartbreak from Modders trying to run the stock carb with Big-Bore Kits as large as 82cc led me to pull an '86 Aero PA 29PWAF carb from its Dip bucket tonight. Clean as the proverbial Whistle, it was. The Float Valve closed and sealed tight merely by inverting the carb whilst blowing heartily through the fuel inlet, so a functional unit. I was prepared to see if Legendary Wheelman "skills" could disable it.
One of the problems with the PA series carbs is that the Pilot circuit is nothing but a hole cast into the carb body alongside the main jet tower. No removable Nada, so hard to upsize. And Upsizing is per-cisely what's needed when the displacement and/or RPM of the AF series engine is increased by Big-Bore kits and/or ministroker cranks. You can get it to run - if poorly - up to top RPMs where a bigger main jet takes over primary fuel atomization duty, but what happens when the throttle closes? The engine is still making a ton of heat through compression, but the tiny Pilot hole is very stingy with the supply of cooling fuel vapors. The result is an off-throttle Heat Spike, and often seizure of all that shiny Eye-Talian (or Taiwanese, for the less well-heeled...) metal. What to do?
Legends are told of a few intrepid modders brave enough to tackle the task of Pilot Enlargement on the one carb they care about, so I decided to give it a try: First off, a #50 Indexing drill is a snug fit into the Pilot standpipe. This allows a firm grip around both Pipe and drill with hearty Vise-Grips. A couple of patient twists later, and the Pipe Pops out. This exposes... The Legendary Pin-* O' Light Ooooh! Even Wheelman can capture its dim glowwithout pesky standpipe blocking most of the light. The hole is offset with respect to the pipe, for reasons known only to Honda-san himself.
That done, it is now possible to measure the Pilot Hole, with my smallest indexing bit. Turns out to be about 0.85mm.
Turning back to my drill set, I decided to enlarge it 10 percent. The closest bit I had was a .92mm, so I made a couple easy passes with my hemostat serving as a bit handle. Al-You-Minnie-Yum drills like buttah anyway, so it took only a minute.
That done, the Standpipe pressed back in with another twist, and the carb reassembled.
Full Disclosure:
One proverbial Fly in the Ointment of this particular carb. I noticed a sort of cylinder protruding 0.5mm proud of one of the flange mount holes. It was a thread insert, apparently installed - poorly- by some Previous Owner who had ham-fistedly stripped the threads in the Al. If anyone tried to install the carb like this, the carb would never seal to the Insulator and an inevitable vacuum leak would result. I suspect that was the reason the scooter "Toby" was orphaned and ended up in WHAC's care.
Lacking a more apt insert to replace it, I decided to ream some threads but the closest size I could use was a whopping 5/16x18 tap. Well, at least it won't be as easy to strip... One other option might be to bond a thin 6mm stainless nut to the backside and screw the stock mount bolt into that. The M6 threads on the other side looked fine.
Fuller Disclosure: I have no idea if my guess at .92mm is correct, but surely this carb will supply a bit more mist off-throttle than the unmodified PA can. And for sure it's clean!
![Smile :)](./images/smilies/icon_smile.gif)