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Walbro WB-37 carburetor information

by Had Robinson

The parts kit is the K10-WB.  Metering lever height is 1.25mm.  Pop-off pressure is the same as the WG-8: 1.2-1.3 Bar (17.5-19.0 psi).

Here is the service manual for the WB series carburetors.  It is not a detailed manual like the WG-8 manual from Walbro.  It is serviced (not tuned) the same as the WG-8 EXCEPT for the high speed adjustment and the metering lever height.  The service manual gives the basic high and low speed settings which are HS 1 1/4 turns (lean for most paramotors) and LS 1 1/4 turns.  I would start with HS 1 1/2 turns.  Always start with these and then fine tune things to the manual that came with your motor.  These settings are a bit rich which is safer for the engine.

Rather than a fixed high speed jet like the WG-8, this carburetor has an adjustable high speed jet.  Be EXTREMELY careful adjusting the high speed system and only follow your paramotor manufacturer's instructions (if any).  It is easy to BURN UP your engine!  Generally, if you do not have a CHT (cylinder head temperature gauge), do not change the high speed adjustment from the factory defaults.

The Minari has instructions on how to adjust the WB-37 on the 180cc engine but it is only for that engine and those flying at sea level.  If you have a different Minari engine or fly at higher altitudes you can still follow the instruction on this page.

A. Basic carburetor setup

Use the steps below to adjust your carburetor if your user manual has no information OR you operate your engine at high altitude.

These instructions do not require a CHT but you are crazy not to have one – my workbench is littered with burned-up engines which did not have this critically important instrument.  Running a high performance engine without a CHT or a tachometer is like not having any warning lights or gauges on an automobile engine.  You probably will never need these indicators but, then again, you might.  Not having the indicators of a about-to-happen destruction of an engine is an expensive mistake.

Minari note: BE EXTREMELY CAREFUL THAT THE CHT DOES NOT EXCEED 200-220ºC (392-428ºF)!

It is always safer to run an engine a bit rich (so it runs cooler) than lean (so it runs hotter).  There is a 30-60 second lag in the CHT gauge so always throttle up slowly, watching the temperature at all times.  It is very easy to quickly overheat an engine by running it lean.  If you find that the instructions below result in your engine overheating, simply enrich the high speed mixture.  Turn the high speed jet (#1 in the photo below) counter-clockwise 1/8th of a turn then check the temperature.  Continue to turn it 1/8th of a turn counter-clockwise until a safe running temperature is achieved.

1. Open the high speed screw #1 (1.5) turns from fully closed as a basic setting.  Open the low speed screw #2 (1.25) turns.  Start the engine and let it warm up.

2. Turn the idle speed screw #3 in or out until the idle speed is 1,900 RPM to 2,100 RPM.

The next steps are potentially very dangerous.  You must have the engine on your back and then stand with a tree or post directly in front so you will not move.  The alternative is to secure the engine to some immovable object like a tree, car hitch, or telephone pole/post.  The latter takes more time to do but it safer.

3. (This step and #4 may be skipped if you do not need to adjust the high speed system.)  Go to full throttle and close screw #1 until there is a slight decrease in RPM.  The engine is now running in a slightly lean condition at full throttle.  Do not run at full throttle for more than a 10-15 seconds, maximum.

4. Release the throttle and open screw #1 (3/4) of a turn.  This must be done carefully.

5. With engine idling, open/close screw #2 until maximum speed is obtained.  Suddenly apply full throttle.  Go to step a. or b., as the case may be.

  1. If the engine begins to stall, open #2 screw (1/16) of a turn.  Quickly apply full throttle again and see if the engine begins to stall.  If it does, open screw #2 another (1/16) of a turn.  Continue this step until the engine no longer stalls when full throttle is quickly applied.
  2. If the engine does not begin to stall, close screw #2  (1/16) of a turn.  Suddenly apply full throttle.  If the engine does not stall, continue to close screw #2 in (1/16) increments until the engine begins to stall when full throttle is applied.  If the engine begins to stall, back out the screw (1/16) turn.  This is the correct adjustment of the low speed system.

You may have to re-adjust screw #3 in order to achieve the correct idle speed.  In the photo below screw #1 is the high speed jet screw adjustment.  #2 is the low speed jet adjustment.  #3 is the idle speed adjustment.

WB-37

B. Midrange performance tuning

The WB-37, like the WG-8, has midrange problems that cause engines to run rich – and roughly.  The air/fuel mixture can be modified in the midrange to fix this problem.

DO NOT MAKE ANY CHANGES TO THE CARBURETOR UNLESS YOU HAVE INSTALLED A CHT AND LEARN HOW TO READ IT.  IT IS EASY TO BURN UP THE ENGINE.  BE EXTREMELY CAREFUL.  NOTE: MODIFICATIONS TO THE CARBURETOR WILL VOID ANY WARRANTY BY THE ENGINE MANUFACTURER AND THEIR U.S. DISTRIBUTORS.  USE ONLY PREMIUM 100% SYNTHETIC OIL AND ETHANOL-FREE GASOLINE/AVGAS.

Parts list

walbro wb-37 carburetor exploded parts view

Venturi diameter 25.4mm (1.00")

* Kit – Repair kit #K10-WB
** Kit – Gasket/Diaphragm D10-WB

1 * Ring – Screen Retainer
2 Ring – Throttle Shaft 16-71
3 Ring – Packing 16-75
4 Lever – Outer Throttle
5 Cover – Metering Diaphragm 21-199
6 Cover – Fuel Pump 21-1195
6A ++ Cap Kit –
7 Shaft Assembly – Throttle 30-800
8 Valve – Throttle 34-102
9 Shaft Assembly – Choke
10 Clip – Throttle Shaft
11 Swivel Assembly 52-540
12 Valve – Choke –
13 * Valve – Inlet Needle 82-75
14 Seat Ass'y – Check Valve
15 Governor Kit –
16 * Plug – Welch
17 * Plug – Welch
18 Ball – Choke Friction
19 ** Gasket – Metering Diaphragm
20 ** Gasket – Fuel Pump 92-149
21 ** Gasket – Circuit 92-185
22 ** Diaphragm – Check Valve
23 ** Diaphragm – Fuel Pump 95-65
24 ** Diaphragm Assembly – Metering 95-515
25 Screw – Lever to Shaft
26 Screw – Metering Lever Pin 96-156
27 Screw – Idle Adjust Kit
28 Screw – Circuit Plate 96-270
29 Screw – Throttle Valve
30 Screw – Choke Valve
31 Screw – Metering Cover – [4]
32 Screw – Throttle Clip
33 Screw – Pump Cover – [4] 96-543
34 Spring – Needle (idle) 98-160
35 Spring – Needle (power) 98-160
36 Spring – Choke Friction
37 Spring – Metering Lever 98-299
39 Spring – Throttle Return 98-308
40 Needle – Idle 102-3113
41 Needle – Power 102-531
41A Needle & Cap Kit
41B Washer – Needle 136-106
43 * Screen – Fuel Inlet 140-33
44 * Screen – Check Valve
45 * Screen – Check Valve
46 Pin – Metering Lever
46A Bushing – Throttle Shaft
48 Plate – Circuit 157-587
49 * Lever – Metering 166-35

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