Walbro WB-37 carburetor information and adjustment
by Had Robinson
updated June 23, 2020
Follow this link for general notes on Walbro carburetors concerning design, inherent problems, fixes, and general information that may be helpful to those rebuilding or troubleshooting the WB-37.
Warming up – when is a paramotor warmed up sufficiently and ready to fly? When the cylinder head temperature reaches about 70ºC, the fuel/air mixture is sufficiently hot enough to ensure that it is 100% vaporized and that the engine will not be fuel starved and potentially overheat. Overheating can happen very quickly.
It is recommended that pilots study the official Walbro WB-37 service manual and instructions for rebuilding and servicing the WB series carburetor. For part location, name, and part number see the WB-37 diagram below.
Rebuild the carburetor if it has been more than a year or after (25) hours of operation, whichever is sooner. If gasoline with ethanol is being used, all bets are off and maintenance intervals may have to double i.e. every 12-15 hours of operation. Alcohol wrecks carburetor guts quickly.
WB-37 parts kit #K10-WB
Metering lever value 1.19mm - 1.59mm (3/64" - 1/16")
Pop-off pressure 1.2-1.3 Bar (17.5-19.0 psi)
Rest pressure 0.68 Bar (10 psi) or greater
Rather than a fixed high speed jet like the WG-8, the WB-37 carburetor has an adjustable high speed needle jet. Be EXTREMELY careful adjusting the high speed system and 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 more than 1/8th turn. It is unwise to fly without a CHT installed.
The WB-37 is serviced the same as the WG-8 EXCEPT for the high speed adjustment and the metering lever value. The service manual gives the basic high speed (HS) and low speed (LS) settings: HS 1 1/4 turns (lean for most paramotors) and LS 1/4-1/3 turns. Always start with these values and then fine tune things according to the manual that came with your motor. These settings are a bit rich which is safer for the engine. If you live at high altitude the stock values will be less.
In any case, it is always best to manually tune the engine for your flying environment. Things like ambient temperature, the type of fuel/oil, and humidity effect the carburetion of any engine.
Performance tuning – Most paramotors run rough in the midrange, the range that we spend most of our time flying in. Southwest Airsports will soon have a comprehensive solution to many performance issues, the FSM. Hopefully, it will be ready summer 2020 but that may change because of the current political and health crises.
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 the 200cc, a different engine, or fly at higher altitudes follow the instructions on this page.
A. Basic carburetor setup
Use the steps below to adjust your carburetor if your user manual has little or no information OR you operate your engine at high altitude.
These instructions do not require a CHT but it is foolish 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....
Minari note: BE CAREFUL THAT THE CHT DOES NOT EXCEED 200-220ºC (392-428ºF) FOR MORE THAN A MINUTE OR TWO!
It is always safer to run an engine a bit rich (runs cooler) than lean (runs hotter). There is a 30-60 second lag in the CHT gauge so always throttle up slowly, watching the temperature. It is easy to quickly overheat an engine.
"Open" = turn the needle counterclockwise while facing it.
"Close" = turn the needle clockwise while facing it.
Never fully close needles with any force. If you do, you will probably damage the needle seat and/or the needle.
1. Open the high speed (HS) needle (#1) 1.5 turns from fully closed as a basic setting. Open the low speed (LS) needle (#2) 1.25 turns. Start the engine and let it warm up.
2. Turn the idle speed needle (#3) in or out until the idle speed is 1,900 RPM to 2,100 RPM.
Steps #'s 3-4 are potentially very dangerous if the paramotor gets loose while at full throttle. The steps may be skipped if you do not need to adjust the high speed system. It is safest to secure the engine to some immovable object like a tree, car hitch, or telephone pole/post. Be extremely careful getting your hands near the spinning propeller! Thankfully, the REALLY dangerous parts of a spinning propeller are the tips and not the area next to the hub. Alternatively, you can test the engine in the air, land, make an adjustment, re-launch, etc. This method is much safer and less unnerving. If you think it does not matter, see this photo of what the tips can do...
3. Go to full throttle and close the HS needle 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 the HS needle 3/4 of a turn. This must be done carefully. If you find that these instructions result in your engine overheating, simply enrich the high speed mixture. Turn the HS needle counter-clockwise 1/8th of a turn, go to full throttle, and check the temperature. Continue to turn it 1/8th of a turn counter-clockwise until a safe running temperature is achieved.
5. With engine idling, open/close the LS needle until maximum speed is obtained. Suddenly apply full throttle. Go to step "a" or "b", as the case may be.
- If the engine begins to stall, open the LS needle 1/16th of a turn. Quickly apply full throttle again and see if the engine begins to stall. If it does, open
the LS needle another 1/16th of
a turn. Continue this step until the engine no longer stalls when full throttle is quickly applied.
- If the engine does not begin to stall, close the LS needle 1/16th of a turn. Suddenly apply full throttle. If the engine does not stall, continue to close the LS needle in 1/16th increments until the engine begins to stall when full throttle is applied. If the engine begins to stall, back out the LS needle 1/16th turn. This is the correct adjustment of the low speed system.
You may have to re-adjust the idle needle in order to achieve the correct idle speed. In the photo below #1 is the high speed power needle adjustment. #2 is the low speed needle adjustment. #3 is the idle speed adjustment. (image courtesy of Fresh Breeze)
B. Midrange performance tuning
The WB-37, like the WG-8, has midrange problems that cause engines to run rich – and roughly. The carburetor can be modified in the midrange to fix this problem. We hope to fix this problem and others with the FSM. Please be patient while we get the bugs out of it.
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 IF POSSIBLE.
C. Parts list and diagram of the WB37
Venturi diameter 25.4mm (image above courtesy of eReplacementparts.com
* 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 – not present on paramotor models
7 Shaft Assembly – Throttle 30-800
8 Valve – Throttle 34-102
9 Shaft Assembly – Choke - not present on paramotor models
10 Clip – Throttle Shaft
11 Swivel Assembly 52-540
12 Valve – Choke – not present on paramotor models
13 * Valve – Inlet Needle 82-75
14 Seat Ass'y – Check Valve
15 Governor Kit – not present on paramotor models
16 * Plug – Welch
17 * Plug – Welch
18 Ball – Choke Friction - not present on paramotor models
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 -not present on paramotor models
31 Screw – Metering Cover – 
32 Screw – Throttle Clip
33 Screw – Pump Cover –  96-543
34 Spring – Needle (idle) 98-160
35 Spring – Needle (power) 98-160
36 Spring – Choke Friction - not present on paramotor models
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 - not present on paramotor models
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