Honda PA Carburetor / Fuel

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Parts

For parts info, navigate to the Honda PA Parts page (see link at bottom of this page).

Misc Tips

Make sure that the small drain tube below the bowl is pointing to the right when the bowl is reassembled. It can be rotated 180 degrees and still fit, but then you'll have problems. (Source: Frankman)

Performance Reeds

Boyesen

The following text is from Boyesen:

We have a few reed options available for this reed cage. The reeds we have found are a very close match, however, not a perfect match. Depending upon the width of the reed cage, you may need to slightly sand the sides of the reeds so they will fit properly.

We have a light material dual-stage fiberglass reed set, part # 6108. The price for this reed set is $25.95 USD + shipping.

We offer a heavier material dual-stage fiberglass reed set, for a modified engine, part # 712. The price for this reed set is $25.95 USD + shipping.

We also have a single stage Carbon Tech carbon fiber reed available, part # CT110LT (low tension material) for better low to mid range, or part # CT110HT (high tension material) for top end power use. The price for either Carbon Tech reed set is $32.95 USD + shipping.

All reed sets include 2 reeds, enough for one reed cage.

With best regards,

Cheryl Boyesen Engineering 8 Rhoades Road Lenhartsville, PA 19534 USA tel # 610-756-6818 / 800-441-1177 fax # 610-756-4102 E-mail: info@boyesen.com

"Boyesen Installation Instructions"

1197817878 boyesen instructions.jpg

"Various Pictures"

Pic1

Pic2

Pic3

Pic4

Idle adjustment

  • Get the bike running and warmed up
  • Set the idle speed screw (left screw) to give you a nice steady idle (speed does not matter at this point)
  • Adjust the idle air screw (right screw) in (C) and out (C-C) to give you the fastest idle speed.
  • Turn idle speed screw (left screw) out (C-C) to give you an idle speed that is just slow enough to keep the variator from dragging.
  • Repeat proceedure as necessary

(Source: silverfox)

Key: (C-C) Counter Clockwise (C) Clockwise

Frame Disassembly (To Access the Carburetor)

Outline: Each of the three methods described below involve, to various degrees, the removal of the sheet-metal PA50 sub-frame that contains the engine from the tube frame to which the tank is mounted.

Hobbit disassembly.jpg

above: a Honda Hobbit jack-knifed for carburetor cleaning (from the Moped Repair forum)


Carb Removal / Method One

Working on a Honda Hobbit is a bit trickier than more conventional style mopeds, like a Puch or Sachs. To get to any of the good stuff on the Hobbit (the carb, intake manifold, air intake... and you will need to) can be difficult and intimidating at first. But, with some practice you can have the thing apart in less than 4 minutes (yes, I've timed myself). As you take things apart, keep all the nuts and bolts and such in a neat little pile or in a dish (you should do this with EVERY project you do) so you don't lose anything. Here's what you need:

-17 mm wrench

-14 mm wrench

-Flathead screwdriver

-Phillps head screwdriver

-Needlenose pliers

-Vice grips/pliers.

-Maybe some WD-40, depending how dirty/rusty your bike is.

NOTE From memory, wrench sizes are 17 and 14 mm. If not that exactly, they're close. Open-ended or socket is fine.

Here's what you need to do. These can be done in pretty much any order, except you'll probably want to take the sidecovers off first:

  • Take off your side covers.

If they're clean and smooth, you can probably use your thumb. If they're rustier and dirty, use a screwdriver of appropriate size, as not to strip the plastic screws out.

  • Unscrew the shocks

Use the 14mm wrench to remove the 2 bolts attaching the bottom of the shocks to the bike. These bolts are easy to identify, they are rounded at the ends.

  • Disconnect the thottle cable from the carb

Peer inside and you'll see where the throttle cable attaches to the carb. Unscrew the phillips head screw and pull the cable out with the needlenose pliers.

  • Unplug fuel line

Shut your fuel off, and unplug your fuel line from your carburetor.

  • Disconnect wiring

This can be especially tricky. All your wiring SHOULD be color coded, which is nice, in a nice little bulk of wires that come together on the left side of your bike. All the wires snap in and out of each other, just pull them and the brass fittings will come right apart. Remember where all of the wires connect to! Make a mental note before you start taking them apart. Most are pretty obvious.. red to red, green to green. But there are a few blacks with different identification marks on them. Be sure to distinguish between the black w/ a white stripe and black w/ a yellow stripe.

  • Disconnect decompression cable

Use the needlenose pliers or flathead screwdriver to pry the decompression cable from it's springy thing at the cylinder head.

  • Removing the main pin

Use the 17mm wrench to unbolt the main pin, and pull it out with vice grips or pliers, or tap it out with a wrench.

  • Disconnect rear brake

Unscrew the tightening nut on the rear brake, and take off the little metal cylinder and spring. The brake cable goes through some metal holes and sheathing before it gets to the rear wheel. Thread it through the holes so it doesnt get caught on anything on it's way out.

NOW, the frame half of your bike will lift right off of your engine/rear wheel half. From here, you can unbolt the carburetor/intake manifold. In your little pile of nuts and bolts that you have been keeping in a dish is: 2 14 mm shock absorber bolts, 1 main pin w/ appropriate bolt, 1 brake tensioning nut, brake cylinder and spring. The engine/rear wheel half will stay stationary on its kickstand. Lean the frame half up against a fence, or if the tank is empty, you can just lay it down. NOTE: It is helpful to have a partner while disassembling, but it is not necessary. I took my Hobbit 3-4 times a day for about a week by myself. It's a bit of a balancing act, but with some practice it becomes second nature.

To put the bike back together, just do all the steps except in reverse. This is also a bit of a balancing act, so I try to put the main pin and shock bolts back in first. That way the bike is stationary to do all the other little things.

Carb Removal / Method Two

This is offered as an alternative to removing the frame. Before you try this option, look at your manifold fasteners. My 1978 used phillips head screws on the manifold, and I HAD to remove the frame the first time to get them out. I've since replaced the screws with bolts. My 1979 had bolts on the manifold from the factory. If you have bolts - this can work for you!

Here's what you need to do. These can be done in pretty much any order, except you'll probably want to take the sidecovers off first:

  • Take off your side covers.

If they're clean and smooth, you can probably use your thumb. If they're rustier and dirty, use a screwdriver of appropriate size, as not to strip the plastic screws out.

  • Unscrew the shocks

Use the 14mm wrench to remove the 2 bolts attaching the bottom of the shocks to the bike. These bolts are easy to identify, they are rounded at the ends.

  • Prop up the back-end

Now - on both of my hobbits today, I don't have a rear fender - but even when I had a rear fender this worked. I used a standard gallon paint can and "wedged it" between the tire and frame/fender. Be careful when you do this. This rotates the entire frame up several inches more than normal. Not a big deal unless you go too far, and the frame hits the decompression valve/spring (I bent my valve once - learned the hard way). I would put my foot on the rear tire, and lift on on the seat or frame, and wedge in the can. I've used blocks of wood, too.

  • Disconnect the thottle cable from the carb

Peer inside and you'll see where the throttle cable attaches to the carb. Unscrew the phillips head screw and pull the cable out with the needlenose pliers.

  • Unplug fuel line

Shut your fuel off, and unplug your fuel line from your carburetor.

  • Remove the carb / manifold

Remove the four manifold bolts. I normally use a "swivel joint" on my ratchet. After the bolts are out, and depending on your moped, you may need to lift up the front end of the manifold about a 1/2", then move the carb forward some. This step hasn't always been very easy, and I've had to stop on occassion to jack up the frame a little more so that the carb can be removed.

To put the bike back together, just do all the steps except in reverse.

Carb Removal / Method Three- The Easy AND BEST way!

Forget removing the rear shocks and rear brakes, here's the easiest way to get your carb out to clean on a PA50! A hydraulic car jack is recommended. Or really just anything that you can pick up the front and stick it on (seat, blocks of wood, etc.)

1. Use a needle nose pliers and unhook the decompression cable from cylinder head. Unplug spark plug wire.

2. Use a phillips head screwdriver, loosen and remove the throttle cable from above the carb.

3. Remove fuel line at the carb.

4. Use 17mm wrench and 17mm socket to remove the big engine mount bolt, one on each side. Remove bolt.

5. Use a hydraulic car jack under the front tire and slowly raise, being careful not to stretch the electrical wires above the carb. Go just far enough to access the 4 10mm carb bolts. (If you don't have a jack, raising the front by hand and placing something under the tire for lift would work as well) Be careful not to let tire swing left or right and fall off the jack stand.

6. Use 10mm socket (with extension for rear bolts) and remove the 4 bolts holding the carb to the reed block. Lift out and remove carb!

7. Installation is reverse. Lower the jack, insert engine mount bolt, put fuel line on carb, re-install and tighten throttle cable, use needle nose to put decompression cable back on, put spark plug wire back on and bingo! Done.

Carb Removal / Method Four

This method will only work if you have enough muscles to pick up your subframe/engine.

1. Find a WALL, not a pole because the bike will move and could fall. Put some cardboard or something underneath your bike where the subframe/engine is-- maybe a doormat. At this point your bike will be on the kickstand still.

2. Unhook your decomp cable and throttle cable. Unhook your fuel line from the carb.

3. Unbolt the main engine bolt with a 17mm socket.

4. Push the bike forward to drop the kick stand.

5. Boom, engine and subframe come down and will fall onto the cardboad/doormat/old carpet whatever. Now lean your bike against the wall so it doesn't fall down.

6. Now you have access to the four bolts/screws that hold on the carb. Remove those guys. Clean yo carb.

7. The tricky part is putting it back together. I guess you can place something underneath the engine to bring it to the appropriate level to bolt it back up.. otherwise lift it up with one hand, stick in the bolt with the other. You probably want to use your strong arm to pick up the engine.

Carb Operation

I'm not sure how to present this information, so I'll just throw something out there, and let everyone help make it better.

Main Jet / Full and Part Throttle Operation

I expect that everyone knows that the fuel is pulled up into the carburetor venturi area. The fuel travels up from the bowl, through the main jet, into the emulsifier where it mixes with air, and then into the venturi area. How much of the fuel/air mixture is drawn up depends on how much vacuum in created in the venturi area. The amount of vacuum is dependent on the amount of air flow controlled by the carb butterfly.

The air flow path that is provided to the emulsifier is shown below:

Emulsion Tube Air Supply Path 2.jpg

Idle / Tip-In Operation

This operation is a little harder to explain. We'll start with an air and fuel supply path first:

Idle and Tip-In Air and Fuel Supply 2.jpg

When the throttle is closed, a vacuum signal pulls both air and fuel through various passages, through the idle mixture adjustment screw. The amount of air is limited by a brass orifice, and similarly the fuel amount is limited by another brass orifice. The location of the orifices will be shown in the carb cleaning section.

When the carb butterfly is opened slightly (from its idle position to maybe as much as 1/8 open), a number of small holes provice additonal fuel/air. I call this first 1/8 of throttle Tip-In. The following picture shows the Tip-In holes (the two that are side-by-side) and also the idle mixture screw tip:

Idle and Tip-in Orifices in Carb Throat.jpg

Carb Cleaning

OK - first my philosophy. I think its best to blow air or cleaning fluid in the reverse direction of what is normal. Dirt comes in from outside the carb, and comes in a definite direction. If you clean in the forward direction, you could just move the dirt where you don't want it - further in the carb.

In addition, I recommend removing your welch plug on the side of the carb. You can get by without this, but removing it is better. In some of the pictures below, the welch plug is removed, and for others it is installed. I have found that welch plugs from Walbro work fine (WALBRO 188-171-8. 5/16" OD. STENS PART NO. 615-690). I bought some off of ebay - very cheap.

Having said that, let's get cleaning!

Idle / Tip-In Circuits

I am not including disassembly steps, just a method to clean the carb . . . . THE FOLLOWING STEPS ASSUME THAT YOU HAVE THE CARB ALREADY DISASSEMBLED.

Remove the Idle Mixture Screw, and leave in the screw which is just below (and behind) the Idle Speed Screw.

Use your cleaning fluid (I use Brake Kleen) and blow the fluid into the hole for the Idle Mixture Screw. See picture:

Clean through Idle Mixture Screw Hole.jpg

Try to "seal up" the end of your cleaning fluid tube (whatever) so that the force from the can really pushed through the drillings. What drillings? See picture:

Drillings behind welch plug.jpg

Your pushing fluid down behind the welch plug, out the tip-in holes, and out the hole to the left, which is the fuel and air mixture for the idle and tip-in holes.

After you've got this circuit flushed out real well, now put the idle mixture screw back in, and at its normal position. Then force the cleaning fluid in through the air supply for the idle and tip-in circuit. See picture (its the right hole):

Idle and Tip In Step Two.jpg

This action forces the fluid back through the circuit to the idle mixture hole, and the tip-in holes. You should see fluid coming out of these holes inside the carb body. In addition, fluid is being forced down through an orifice that limits the amount of fuel available for the idle and tip-in operation.

Next, remove the screw which provides access to the idle / tip-in orifice. See picture:

Idle and Tip-In Orifice.jpg

You can try forcing cleaning fluid down through the orifice (fluid will come out of the main jet area). See picture:

Fuel Source for Idle and Tip-In Circuit.jpg

This is a very small hole, only about 0.014". I've found it best to run a small drill bit or a small wire down through this hole. Get it good and clean! A plugged or restricted orifice means no idle, and a poor tip-in.

You're done with the Idle and Tip-In Circuit!

Emulsion Tube Air Supply

This one is easy. Force fluid through the hole on the left:

Idle and Tip In Step Two.jpg

Some will notice that I'm going against my philosophy here - but there is not an easy way to force the fluid backwards. When cleaning, you should see the fluid coming out from this area:

Emulsion Tube Air Supply.jpg

Float Bowl Air Vent

Another easy one. Force your fluid through the bottom hole in this picture:

Float Bowl Vent.jpg

Emulsion Tube

Like most emulsion tubes, the PA50 tube is hard to dislodge from its housing. However, it can be easily knocked out from above (through the main body of the carb) with an improvised tool, once the main jet has been unscrewed. One easy method is to take a straight piece of wire (such as from a coathanger), bend the last .25" at a right angle, and use it to knock the tube loose from inside the body of the carb.

Once it's out, just clean it really good. Clean the holes in the sides (these are air inlet holes), and the holes on the top and bottom.

During a recent cleaning, I found that using a drill bit slightly smaller than the hole works really good. The flutes on the bit work well to clean the sides of the holes.

Warning: when reassembling, be sure to align the tube back into the carb with the 2 big holes aligned with the 2 holes in the carb body, you can look into the carb body where the tube sits and see the 2 holes near the bottom (right above the main jet) If the holes in the atomizer and carb body are not aligned, you will not be able to idle.

Emulsion Tube One.jpg

Emulsion Tube Two.jpg

Emulsion Tube Three.jpg

Keihin atomizer difference.JPG

10mm carb emulsion tube, left has a groove, and is 26.5mm. 12mm, right, doesn't, and has a larger orifice, and is 26mm.


Main Jet

Another simple one - clean it good! The jet hole should be completely round and clean.

Main Jet.jpg

Fuel Inlet

Fuel Inlet.jpg

Force the cleaning fluid out through this hole. The fluid will come out of the fuel inlet for the carb.

Main Jet Sizes

If you are running a stock Hobbit, then changing your jets may not be necessary. If you are performance minded, then this information is for you. Fortunately for us - the main jet on your hobbit is also the Air Bleed Jet on Keihin FCR carbs. That means that these jets are available. Not only that, but these jets are also used in Honda lawnmowers.

The jets come in sizes of 2 and 3. Here's what I mean. The stock PA50I used a 60. Up from there is a 62, then 65, then 68, then 70, then 72, then 75, then the stock PA50II jet (a 78), etc. Jets are available from 48 up to 150. The jets are stamped with their orifice size. The pic below is a 60 size jet.

MainJetSizeStamp.jpg

The Honda part numbers are 99101-124-0600 (for a 60) OR, the FCR part number is 99101-ZF5-0600.

I've found jets for about $6 each.

For those of us who want to try and save money, here's a chart of information to help you choose the right drill bit to use to drill out your smaller jet to a larger size. You will see that there isn't a very good alignment from numbered drill sizes to jet sizes.

The chart below seems to be hard to interpret for some. Let's say you want a 92 jet size. The chart indicates that a number 66 drill bit is between a 90 and a 95, like close to a 93. That is as close as you can get. For another example, if you want an 85 jet, the closest bit is a 1/32" bit, which is close to an 84 jet (Honda doesn't make an 84, but if they did, it would be closest).


Jet Size (cc per min) Jet Bore (mm) Jet Bore (inch) Inch or Wire Size Bit Metric Size Bit
75 0.7490 0.0295 0.75
0.0296
0.0297
0.0298
0.0299
0.0300
0.0301
0.0302
0.0303
80 0.7730 0.0304
0.0305
0.0306
0.0307
0.0308
0.0309
0.0310 68
0.0311
0.0312 1/32
0.0313
85 0.7970 0.0314
0.8000 0.0315 0.80
0.0316
0.0317
0.0318
0.0319
0.0320 67
0.0321
0.0322
90 0.8200 0.0323
0.0324
0.0325
0.0326
0.0327
0.0328
0.0329
0.0330 66
0.0331
95 0.8430 0.0332
0.0333
0.0334
0.0335 0.85

Also, this jet seems to be used in some pocket bike carbs. This set:

http://shop.hkmotos.com/product_info.php/cPath/47_45_22/products_id/133/osCsid/19adf54b06ce68b9a92f6e2b8edb90cc

works for hobbit carbs, but not the dellorto carb they are sold as fitting. I do not know what the sizes translate too, but I guess they would be cheap enough to use as blanks for drilling to needed sizes. If the sizes are equal to the honda sizes, then according to the tuner spreadsheet this is a pretty good neighborhood to jet a kitted hobbit in.

Refer to Honda PA Parts for purchasing jets.

random bit of info

honda express carbs will fit if you'd rather have a slide carb. They have 12mm venturis and the same oulet size but the carb's mounting holes must be slightly elongated.


Adding a Honda Express Carburetor

Here is what is involved.

1 Express vs Hobbit Intake Stud Spacing.jpg

The intake studs do not line up with an Express carburetor. You must elongate the holes toward the outside, about half a stud length

2 Make a hobbit template out of metal.jpg

Lets make a template out of metal to transfer the stud spacing pattern to the carburetor. I used a marker to mark where the studs touched the metal. Then drilled them out with the appropriate drill bit. Make sure the Hobbit intake fits the metal template!

3 Clamp your carb in a vice.jpg 4 Use template to bore out the carburetor.jpg

Now clamp your carburetor on the bottom of your template and use a drill or dremel to remove the material from the carburetor flange. I just eyeballed the spacing to be equal. After you remove the bulk material. Keep putting the carburetor on the intake and take a little bit off each side to get it centered as best as possible. I do not have any tricks to getting that right.

5 It fits.jpg

Now that the intake and the carburetor fit. Lets look at how it mounts in a Hobbit.

6 Slide carburetor install 5.jpg 7 Slide carburetor install 4.jpg

There is not much room if you do not modify the stock airbox and filter system.

Try a filter.jpg You can, try a filter. From a auto parts store, smushed down. NOT recommended

Hard to access.jpg

Helpful to modify the subframe to be able to adjust the pilot air screw. Just eyeball it :) Also cap off the strange vacuum choke system.

111carb.jpg

The throttle cable tube sits too high also. It will hit the frame. You need longer shocks.

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