Myrons Mopeds

== carb ID# ============  M O P E D S  ===========
6. PA 08B-A  1977-78 Honda NC50 Express xxx    30 mph TS
6. PA 08B-B  1977-79 Honda NC50, NA50  xxxx    30 mph TS
6. PA 08C-A  1978-79 Honda NC50, NA50  xxxx    30 mph TS
6. PA 08E-A  1980-80 Honda NC50 Express xxx    30 mph TS
6. PA 08E-B  1980-80 Honda NC50 Express xxx    30 mph TS
6. PA 08D-A  1980-80 Honda NA50 Express II x   30 mph TS
7. PA 12A-A  1981-81 Honda NCZ50 Moto Compo
7. PA 15A-A  1981-81 Honda NC50 Express xxx    30 mph TS
7. PA 16A-A  1981-81 Honda NA50 Express II  x   30 mph TS
7. PA 13B-A  1981-82 Honda NX50 Express SR x   30 mph TS
7. PA 13B-B  1982-83 Honda NC50 Express xxx     30 mph TS
7. PA 13E-A  1982-83 Honda NU50 Urban Express 30 mph TS
7. PA 25A-A  1982-83 Honda NC50AA, NU50AA      25 mph TS

 

 

 

 

 

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6. Keihin
size     carb  model code  year 00 frame number range  000  model
∅12 PA08B-A or B #147 1977 NC50-1139281 to 0000000 NC50 Express (US)    30 mph
∅12 PA08B-A or B #147 1978 NC50-2000013 to 0000000 NC50 Express (US)    30 mph
∅12 PA08E-A or B #147 1979 NC50-2100011 to 0000000 NC50 Express (US)    30 mph
∅12 PA08E-A or B #147 1980 NC50-2200005 to 0000000 NC50 Express (US)    30 mph
∅12 PA08B-B    on #163 1979 AB01-5000001 to 0000000 NA50 Express II (US) 30 mph
∅12 PA08D-A    on #163 1980 AB01-5200001 to 0000000 NA50 Express II (US) 30 mph

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Flange Mount: 38 mm hole spacing. 2 x 20 (24 od) intake o-ring.

Cable choke: This carburetor has a throttle cable and a choke cable coming straight up and looping around. It has no vent hoses, vacuum hoses, or oil lines going into it. The oil injection goes into the intake manifold. The float reservoir vent is just a small tube slanting downward.

As with most Honda carburetors, fuel is never allowed to spill onto the engine or anything else. Instead an overflow tube inside the float chamber allows any overflow fuel to flow out and down the drain hose and onto the ground below the engine.  

The 1977-79 Honda NC50 and NA50 each had two carburetor versions, PA08B-A and PA08B-B. Regardless of the year, or the VIN number, they could have either version. So the customer had to look at the model number stamped into the carburetor, and tell the Honda dealer that number, to get certain parts correct. 

Oil injection hose location: The 1977-79 Honda Express had the oil hose going into the intake manifold, not the carburetor. The 1980 and later models had the oil injection hose going into the carburetor at the throttle slide area. There must have been some benefit, like maybe throttle slide lubrication.

 

 

 

 

 

 

 

 

 

    quantity
o# ⇓⇓⇓⇓⇓  Keihin PA(a)08BA (b)08BB (c)08EA (d)08EB (e)08DA 
o# abcde price  Honda part#x  description xxx specs
10 1oooo  N/A 16100-147-673 
10 1oooo  N/A 16100-147-675 carburetor 77-79 NC50 PA 08B-A
10 o1ooo  N/A 16100-147-676 carb 77-79 NC50 NA50 PA 08B-B
10 oo1o0  N/A 16100-147-711 carburetor  1980 NC50 PA 08E-A
10 oo01o  N/A 16100-147-712 carburetor  1980 NC50 PA 08E-B
10 oooo1  N/A 16100-163-771 carburetor  1980 NA50 PA 08D-A
o1 10ooo $26 16010-143-620 gasket solo-set   1a o c d e f g
o1 10ooo $28 16010-143-305 gasket multi-set  1a o c d e f g h i
o1 01o1o $23 16010-147-004 gasket solo-set   oo b c o o f g
o1 o1111 $23 16010-147-711 gasket solo-set   oo b c d o f g
o1 o1111 $24 16010-147-505 gasket multi-set oo b c d o f g o i
1a 100o0 $$3 00000-000-000 top rubber washer 10 x 18 x 1
1b o1111 $$8 00000-000-000 top rubber washer 13 x 16.5 x 1
1c 11111 $$2 00000-000-000 intake o-ring 2 x 20 (24 od) 
1d 1o1o1 0$2 16176-551-004 jet or screw o-ring 1 x 3 (5 od)
1e 1o0oo $13 16197-063-004 jet support pad disk Ø7.5 x 1.8
1f  11111 $18 00000-000-000 float bowl gasket  Ø1.5 NC50-type
1g 11111 $$2 00000-000-000 drain plug o-ring 1.5 x 4 (7 od)
o2 1oo1o $21 16011-881-741 float needle valve  stirrup
o2 1oo1o $36 16011-881-741 float needle genuine stirrup
o2 o11o1 $22 16155-883-005 float needle valve regular
o3 1o1o0  N/A 16012-147-004 needle set 3a 3b 3c 3d 08BA EA
o3 ooooo $22 16012-130-640 needle set 3a 3b 3c 3d PA 03A-A
o3 ooooo $22 16012-165-014 needle set 3a 3b 3c 3d PA 03B-A
o3 o1o11 $15 16012-147-014 needle set 3a 3b 3c 08BB EB DA
3a 11111 0$5 16156-044-004 jet needle w-clip
3b 11111 0$2 16157-044-004 jet needle e-clip 1.5 for Ø2.0 needle
3c 11111 $10 00000-000-000 jet needle Ø2.0 D348 thick 08B-A,
3c 11111 $10 00000-000-000 PA 08B-B, 03A-A, 00A, 01A
3c ooooo $10 00000-000-000 jet needle Ø2.0 D117 thin 03B-A,
3c ooooo $10 00000-000-000 PA 03C-A, 03D-A, 03H-B, 03M-A, etc
3d 1o1oo  N/A 00000-000-000 needle jet (emulsion tube) 08BA 08EA
3d o1o11  N/A 00000-000-000 needle jet & jet fixed 08BB 08EB 08DA
3d ooooo $10 00000-000-000 needle jet (emulsion tube) PA 03A-A
3d ooooo $10 00000-000-000 needle jet (emulsion tube) PA 03B-A
o4 1oo1o  N/A 16013-143-620
o4 1oo1o  $46 16013-099-691 float set 4a oo 4c 4d black stirrup
o4 o11o1 $28 16013-147-004 float set oo 4b 4c 4d white regular
4a 1oo1o  N/A 00000-000-000 float bare, solid black foam stirrup
4b o11o1 $21 00000-000-000 float bare hollow white plastic regular
4c 11111 $$4 00000-000-000 hinge pin substitute Ø2 x 18.5
4d 11111 $$3 0-93600-04006 hinge screw M4x6 sunk phillips

o5 1oooo $23 16014-143-620 top set substitute 5a 1a M20-1.0
o5 o1111 $22 16014-147-004 top set substitute 5a 1b M18-1.0
o6 1oooo  N/A 16015-147-004 chamber set A 6a oo 6c 1e 1f 1g for jet pad
o6 o1111  N/A 16015-147-014 chamber set B oo 6b 6c oo 1f 1g for no pad
6a 1oo0o  N/A 16015-147-004 float bowl A shallow with pad holder
6b o1111  N/A 16015-147-014 float bowl B shallow, no jet pad holder
6c 11111 $16 00000-000-000 float drain screw M6x20 special
o7 11o1o $15 16016-143-620 screw set 7a b c d idle air
o7 oo101  N/A 16016-147-711 screw set 7a b o d e f g idle fuel
7a 11111 $$4 00000-000-000 idle speed screw M4x19 special
7b 11111 $$2 00000-000-000 idle speed screw spring Ø5.6 x 10
7c 11o1o  N/A 00000-000-000 idle air screw M6-0.75 thick tip
7d 11111 $$7 00000-000-000 idle air or fuel spring Ø4.8 x 9
7e oo1o1  N/A 00000-000-000 idle fuel screw M6-0.75 thin tip
7f  oo1o1 $$3 00000-000-000 idle fuel screw washer 3.1 x 5.0 x 0.5
7g oo1o1 0$2 16176-551-004 idle fuel screw o-ring 1 x 3 (5 od)

o8 11111 $38 16022-143-620 valve set 8a, 8b
8a 11111 $22 00000-000-000 throttle slide Ø13 #15A
8b 11111 $12 00000-000-000 throttle spring original Ø9.5 x 42
8b 11111 $$7 00000-000-000 throttle spring substitute shorter, stiffer
o9 11111 $25 16046-147-004 choke set substitute 9a 9b 9c
9a 11111 $15 00000-000-000 choke top substitute M12-1.0
9b 11111 $$4 00000-000-000 choke spring substitute
9c 11111 $12 00000-000-000 choke slide substitute

11 11111 $2.00 16118-166-004 rubber boot substitute Dellorto
12 22222 $1.00   93500-04012 bowl screw M4x12 pan phillips
13 22222 $0.20   94111-04000 lock washer M4 split
14 11111 $1.00 95001 – 35003 hose substitute 1/8 x ¼ 3.1 x 6.3

15 11111  N/A 95002 – 02070 drain hose clamp B7 (for 7 not 6.3)
16 10o0o  N/A 99201.081.0780 push-in jet 78 08BA
16 10o0o $23 substitute jet, #70 drilled to 78 08BA
16 01111  N/A fixed needle jet & jet pressed in 08BB EA EB DA
this jet is 0.78mm 0.0307″, cleaned with a #69 drill 0.0292″     

00 22222 $1.00 mount bolt, nut and washer M6 x 20

 

Float: The white float with non-stirrip needle for PA08B-B and the black float with stirrup type needle for PA08B-A do not interchange. The white float is too “tight” on the PA08B-A. Something restricts its movement. The black float is too “loose” on the PA08B-B. It moves a lot before moving the needle.

Main Jet: On PA08BB, EA, EB and DA versions the main jet is a tamper-proof tight press-fit type, in the center. The jet cannot be removed, and cannot be replaced. It is 0.78 mm – 0.0307″, and can be cleaned perfectly with a #69 drill 0.0292″.    

On a PA08BA the main jet presses or slides in and out, and is supported by a rubber disk. If the disk is missing, the jet can slide downward, allowing fuel to get past the jet o-ring, causing a rich fuel mixture.

Idle Hole Cleaning: Near the engine side of the carb body is a brass tube that dips down into the liquid gasoline. That tube, about 1.5 mm (0.060″) inner diameter and 20mm long, leads to the idle or pilot jet, which is about 0.30 mm (0.012″). The idle jet gets clogged from sitting, then the engine will run but won’t idle. Nothing but a very thin (0.010″) steel guitar string will clean it out. See below about jet drills and cleaning tiny jets. Ultrasonic parts cleaners, carb spray, carb dip, and compressed air are not usually effective. Another way to clean out the idle jet is to 1) place a drill bit that fills the hole (so it does not get crushed), 2) clamp it with a small vise grips, 3) rotate back and forth while pulling, 4) when it comes out, the actual hole is easy to see and clean, 5) press it back in the same way it came out.

Strong solvent is needed to dissolve varnish or tar residue left from years of sitting. Carburetor spray, parts dip, or acetone all help clean out the tar, sometimes with a lot of scraping, brushing or rubbing. Always remember to remove all rubber things first, to not let the rubber touch the solvent. Strong solvents will swell soft rubber, deforming it. Never soak the whole carburetor in strong solvent. In this way, Shaun (at Royal Cyclery and later at Myrons Mopeds) has successfully cleaned out over 100 Honda Express carburetors, without ever replacing any of the rubber gaskets. Part of the reason was they were only sold at Honda dealers, but nonetheless, they are not needed in a normal service or clean-out.

Keihin PA08C-A: This is approximately for a 1979-80 NC50, or maybe NA50. It is identical to a PA08B-B. It has the same white float, non-stirrup float needle, fixed jet, 18mm top threads, and same gaskets. The idle mixture screw type is unknown. No photo available. 

 

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7. Keihin
size  carb  model code  years       models
∅12 PA12A-A on #GC6 1981-81 Honda NCZ50 Moto Compo
∅12 PA15A-A on #190 1981-81 Honda NC50 Express (US) xxxxx xxxx 30 mph
∅12 PA16A-A on #193 1981-81 Honda NA50 Express II (US) xxxx xxx  30 mph
∅12 PA13B-A on #187 1981-82 Honda NX50 Express SR (US) xxxx xx  30 mph
∅12 PA13B-B on #GA6 1982-83 Honda NC50 Express (US) xxxxx xoxx 30 mph
∅12 PA13E-A on #GC1 1982-83 Honda NU50 Urban Express (US) xxxx 30 mph
∅12 PA13E-A on #GC1 1982-83 Honda NU50M Urb. Expr. Deluxe (US)  30 mph
∅11 PA25A-A on #GA6 1982-83 Honda NC50AA Express (Iowa) xxxxxx  25 mph
∅11 PA?????   on #GC1 1982-83 Honda NU50AA Urban Express (Iowa)   25 mph
∅10? PA25A-A on#GC1 1982-83 Honda NU50MA Urb. Ex. Deluxe (Iowa) 25 mph

 

 

 

 

 

 

 

 

 

16010-187-003 gasket set

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Flange Mount: 38 mm hole spacing, 2 x 20 (24 od) o-ring on NC50, NA50 (PA15A-A, PA16A-A, PA13B-B)
Flange Mount: 32 mm stud spacing, 2 x 18 (22 od) o-ring on NX50, NU50, NU50M (PA13B-A, PA13E-A)
Flange Mount: 36 mm hole spacing, 2 x 20 (24 od) o-ring on NC50I, (PA25A-A)

Vacuum-operated automatic choke: These carburetors feature a screw-in main jet, and a built-in vacuum-operated (not electric) automatic choke. When the temperature switch on the head is cold, it allows the engine vacuum to raise the choke plunger up, which opens a new fuel passage, to enrich the mixture during cold starting. As the engine warms up, it closes the vacuum, allowing the thin spring to pull it back down, for normal running. The more precise term for this is “automatic vacuum operated cold start fuel mixture enrichment device”. The 1981-83 automatic choke device itself is not replaceable.

The vacuum source hose has a check ball valve that only allows air flow into the carburetor venturi, and not back out. It needs this even though there is reed valve, because there is still some residual reverse flow caused by the forward and reverse motion of the piston.

The vacuum choke that came out in 1981 did not last long. After 1983, 50cc on-road Honda’s had electric chokes, because they were electric-start scooters with 12 volt batteries. 

Identical twins: The PA16A-A looks identical to the PA15A-A. It is on the 1981 Honda NA50 Express II, the final year of that USA-only model. They both have 38 mm flange hole spacing, #75 screw-in jet, 20 mm intake o-ring and the non-stirrup type B float needle. The only difference is the top cable curved pipe. On a PA15A-A it is permanently attached to the screw-on top cap with a swivel joint, while on a PA16A-A the curved cable pipe is detachable.

Intake studs: After 1981 these carbs had studs instead of threaded holes in the mounting flange, except for PA25A-A. PA13B-A and PA13E-A studs are spaced 32 mm with a 18mm intake o-ring. PA13B-B studs are spaced 38 mm, with a intake o-ring 20 mm. 

Iowa carb: The PA25A-A is like a PA13B-A, with M18-1.0 top cap threads, 13.0 mm slide, vacuum-operated automatic choke, choke jet in float bowl, tall 2.5 x 1.5 float bowl gasket, intake o-ring 20 mm, 38mm mount spacing, non-stirrup type float and float needle, screw-in main jet size 75. The only noticeable difference is the absence of studs.

Iowa cylinder: The exhaust port is restricted. Here left is 49-state, right is Iowa.

Keihin PA12A-A: This carb from a NCZ-50 has a cable choke like the ’77-79 NC50 carbs. But it has a different part number, 16046-GC6-003 instead of 16046-147-004. Despite that, the NC50 replacement cable choke top and choke pieces fit the NCZ50.  

 

 

 

 

 

 

 

 

 

 

      quantity
o# ⇓⇓⇓⇓⇓⇓  PA (a)15AA (b)16AA (c)13BA (d)13BB (e)25AA (f)13EA
o# abcdef price  Honda part#x  description xxx specs
00 1o0000  N/A 16100-190-672 carburetor ’81-81 NC50 PA15A-A
00 o10000  N/A 16100-193-672 carburetor ’81-81 NA50 PA16A-A
00 o01000  N/A 16100-187-673 carburetor ’81-82 NX50 PA13B-A
00 o00100  N/A 16100-GA6-671 carburetor ’82-83 NC50 PA13B-B
00 o00010  N/A 16100-GA6-671 carburetor ’82-83 NC50AA PA25A-A
00 o00001  N/A 16100-GC1-671 carburetor ’82-83 NU50 PA13E-A
01 001001 $24 16010-187-003 gasket solo-set   1b o d g h k
01 110110 $24 16010-190-004 gasket solo-set   1b c d g o k
01 001001 $25 16010-GF8-305 gasket multi-set 1b o d g h k t u
1b 111111 $$8 00000-000-000 top rubber washer 13 x 16.5 x 1
1c 110110 $$2 00000-000-000 intake o-ring 2 x 20 (24 od) 
1d 111111 0$2 16176-551-004 idle fuel screw o-ring 1 x 3 (5 od)
1g 111111 $$2 00000-000-000 drain plug o-ring 1.5 x 4 (7 od)
1h 001001 $$2 00000-000-000 intake o-ring 2 x 18 (22 od)
1k 111111 $22 00000-000-000 float bowl gasket 2.5 tall by 1.5 wide
02 0010?0  N/A 16012-187-671 needle set ’81 2a 2b 2c 2d
02 001000  N/A 16012-187-731 needle set ’82 2a 2b 2c 2d
02 1100?0  N/A 16012-193-671 needle set 2a 2b 2c 2d
02 0001?1  N/A 16012-GA6-671 needle set 2a 2b 2c oo 2e
02 0000?1  N/A 16012-GC1-671 needle set 2a 2b 2c oo 2f
2a 111111 0$5 16156-044-004 jet needle w-clip
2b 111111 0$2 16157-044-004 jet needle e-clip for 1.5 clip groove
03 111111 $28 16013-147-004 float set white 3a 3b 3c
3a 111111 $21 00000-000-000 float bare white plastic regular
3b 111111 $$4 00000-000-000 hinge pin substitute Ø2 x 18.5
3c 111111 $$3 0-93600-04006 hinge screw M4x6 sunk phillips
04 1o0000  N/A 16014-190-014 top set NC50 fixed cable pipe M18-1.0
04 o10000 $24 16014-147-014 top set NA50 detachable pipe M18-1.0
04 o01110  N/A 16014-187-013 top set NX50, 82-83 NC50 M18-1.0
04 o00001  N/A 16014-GB2-003
04 o00001  N/A 16014-GB0-831 
04 o00001  N/A 16014-098-980 top set NU50 fixed cable pipe M18-1.0
05 110000  N/A 16015-191-034 chamber set deep bowl with choke jet
05 001000  N/A 16015-187-671 chamber set deep bowl with choke jet
05 0001?0  N/A 16015-GA6-671 chamber set deep bowl with choke jet
05 0000?1  N/A 16015-187-043 chamber set deep bowl with choke jet
06 000111 $32 16016-191-004 idle fuel screw set 6a 6b 6f 1d
06 111000 $32 16016-187-003 idle fuel screw set 6a 6b 6f 1d
6e 111000  N/A 00000-000-000 idle fuel mixture screw M6-0.75 special
6b 111111 $$7 00000-000-000 idle fuel mix screw spring Ø5 x 9
6f  111111 $$3 00000-000-000 idle fuel mix screw washer 3.1×5.0x0.5
07 111111 $0016028-187-003
07 111111 $15 16028-GC8-004 idle speed screw and spring 7a 7b
7a 111111 $$4 00000-000-000 idle speed screw M4x19 special
7b 111111 $$2 00000-000-000 idle speed screw spring Ø5.5×10
08 110000 $38 16022-143-620 throttle slide & spring
08 001011  N/A 16022-187-731 throttle slide & spring
08 000101  N/A 16022-191-004 throttle slide & spring
8a 11?? 1? $22 00000-000-000 throttle slide Ø13 #15A
8b 11?? 1? $12 00000-000-000 throttle slide spring Ø9.5 x 42

10 100000  N/A 16118-086-004
10 100000  N/A 16118-166-004 cable rubber boot
10 101110  N/A 16118-187-003 cable rubber boot
10 000001  N/A 16118-GB4-003 cable rubber boot
11 111111 $22 16155-883-005 float needle valve KL 18-4608
12 110110  N/A 16199-435-771 overflow hose 3.0 x 6.5
12 001001  N/A 16199-187-003 
12 001001  N/A 16198-HC0-405 overflow hose 3.0 x 6.5
13 001000  N/A 16608-187-000 control hose with check ball 5 x 10
13 110000  N/A 16608-193-670 control hose with check ball 5 x 10
13 000110  N/A 16608-GA6-670 control hose with check ball 5 x 8
13 000001  N/A 16608-GC1-000 control hose with check ball 5 x 8
14 222222 0$1 93892-04012-08 screw & washer M4x12 pan phillips
15 111111  N/A 95002 – 02070   hose clamp B7 vacuum source hose
16 111000  N/A 95002 – 02100   hose clamp B10 for control hose
16 000111  N/A 95002 – 02080   hose clamp B8 for control hose
17 111111  N/A 95002 – 02650   hose clamp B6.5 for overflow hose
18 111111  N/A 95005 – 35001   vacuum (source) hose 3.5 x 7 x 130
19 oo0010 $$7 99101-187-0700 screw-in jet #70 M6-0.75 PAMOA PA25A ? 78-80 Indian, 82-83 NC50I
19 110100 0$7 99101-187-0750 screw-in jet #75 M6-0.75 PA15A 16A 13BB 81-83 NC50, 81 NA50
19 oo1000 0$7 99101-187-0780 screw-in jet #78 M6-0.75 PA13BA 81-82 NX50
19 oo0001 0$7 99101-187-0850 screw-in jet #85 M6-0.75 PA13EA 82-83 NU50

 

 

Float Valve Seat: On these carburetors the float seat (the cup that the float needle valve slides in) is carved into the actual carburetor body, and is not replaceable. Polish it with a spinning Q-tip and polish, such as Turtle Wax. 

Choke jet: On all of these NC50-type carbs, both shallow and deep-float-bowl versions, there is a choke jet, pressed into a corner of the float bowl. When it gets clogged the engine will only start easily when it is warm.

Verify all jets and fuel or air passageways are open, using wires and or visually with carb spray (wear goggles).

Cable Differences: You can tell what year range a Honda Express is from a distance. A 1977 to 1980 has two cables, a throttle and a choke cable coming straight up from the carburetor.  A 1981 has just a throttle cable coming straight up, but also a vacuum hose for the choke and a float vent hose.  A 1982 to 1983 has a black plastic curved elbow on top, that redirects the throttle cable forward and down, a vacuum hose for the choke, and a vent hose. A 1982-83 also has a different frame, a low headlight, and a two speed transmission.

The Honda Express throttle cable curves rearward and loops downward to get below the carburetor, and then goes forward into the frame tube. The loop design adds friction, but it makes a good clean step-thru frame without any cables or wires hanging out.  

 

 

 

 

 

 

 

 

 

 

 

You can tell the difference between a carb with an idle air mixture screw, and a carb with an idle fuel mixture screw, by the shape of the casting at the top of the float chamber.

 

 

 

 

 

 

 

 

This entry was posted on Saturday, April 3rd, 2021 at 7:21 am and is filed under Keihin, Media. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.