Stock PA Carb Pilot Mod

[Wheelman-111]

Greetings:

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:

4PACarbPilotTube.jpg (55.33 KiB) Viewed 31291 times

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...

4PACarbPinPrick.jpg (39.06 KiB) Viewed 31291 times

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.

4CarbPilotIndex86.jpg (25.53 KiB) Viewed 31291 times

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.

4PACarbPilot92.jpg (15.46 KiB) Viewed 31291 times

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!

[mousewheels]

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

I guess it's winter and thinking about parts vs riding is king around here...What if fuel metering is implemented somewhere else? Suppose the tube is a pilot jet.

Cutaway Spree carb showing air path from left (air filter side) -- to air screw and pilot jet.

Carb_Pilot_and_Carb_Body_0.JPG (49.35 KiB) Viewed 31224 times

Comparative view of hole in Spree carb body to venturi vs pilot jet.
Drill the pilot vs drill the carb passage to make the circuit richer? Which is closest to changing the pilot jet in an adjustable carb?

Carb_Pilot_and_Carb_Body.JPG (146.85 KiB) Viewed 31224 times

Wheelman can capture its dim glowwithout pesky standpipe blocking most of the light

There is that offset hole in the carb casting plus the pilot jet orifice. Light must get through the carb hole, bounce off the angled lip of the pilot jet (see photo above) , reflect off the carb body, then get past the pinhole pilot jet, the smallest passage in the carb. Removed from the carb and larger than life, it's still a small * of light.

Pilot_jet_light.JPG (16.67 KiB) Viewed 31224 times

[Wheelman-111]

Greetings:

Wow, Mouse! Incredible job depicting this complex system. I have but one thing to say:

You don't get out much, do you?

Sinceriously, you've helped me understand what's going on in there a whole lot better. Thanks!! Looks like I'll need to gauge the narrow part of the standpipe and drill that out too.

[mousewheels]

You don't get out much, do you?

Looks like I'll need to gauge the narrow part of the standpipe and drill that out too.

Try around 0.013 - 0.015 as a gauge for a Spree pilot. Pic below is a 0.015 wire vs Spree pilot jet. Wire would not pass through, but is close. Drill gauge for stock is approx #79-80.

Spree_Pilot_Jet_orifice_sizing_0_015_wire.jpg (118.58 KiB) Viewed 31144 times

[Wheelman-111]

Greetings:

Well, I finally got around to re-operating the Aero carb. I gauged the Pilot standpipe and found its throat was considerably more spacious than the measurements you got for the Spree: 1.37mm passed freely. 1.40 would not. It would appear we have found the Critical Difference between the Spree and the other PA carbs.

Anyway, following my own 10% rule, I picked an index drill that measured 1.51 and opened up the brass pipe's constriction to that dimension. It's buttoned back up waiting for the best Poet among the Forumistas whut needs 'em a PA carb.

[mousewheels]

Well, I finally got around to re-operating the Aero carb. I gauged the Pilot standpipe and found its throat was considerably more spacious than the measurements you got for the Spree. 1.37mm passed freely. 1.40 would not

That is *a lot* bigger - Spree pilot was around 0.015. Wheelman-111's measurements --> 1.37mm = 0.0539in, 1.40mm = 0.0551in

The carb tag "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"

I am reading Aero in your thread -- yet photos in your thread show a Spree carb Is the Spree just a photo model?

[Wheelman-111]

Greetings:

Let this be a lesson to all y'all:
Never try to fool Mousie-san.

Sinceriously Good Eye, Mr. Mouse! Verily the bystarter well resides outward, a clear Sign of Spreeishness.

I pulled this carb from my Big Box of Aero remains, so assumed it was of that ilk. The 82 pilot that was in it reinforced the error.
Maybe from an '83??

OK, so now it fits a Big-Bore Spree too.

[noiseguy]

Nice work on this guys.

Has anyone managed to find a source for replacement pilot jets for the PA carbs? They must be produced somewhere in the world...