Most chainsaw carbs were close to the .65 constant. exceptions were like 346 242 262. most saw are peak power at 9 krpm. the saws listed are closer to 10k.
http://www.billetboard.com/showthread.php/9036-Carbs-Venturi-Size-and-Formula-For-Proper-Carb
I DID NOT WRITE THIS INFORMATION UP, I FOUND IT ON A FORUM IN WHICH THE INFORMATION CAME DIRECTLY FROM WALBRO.
I figured some may benefit from this info when trying to choose the proper carb for an engine build. This info is for the WA, WT and WYK carbs that have a 13.00mm Venturi and larger. Also I'm including a link for ALL series of Walbro carbs that include a breakdown as well as part numbers.
Common Walbro carbs, Wt-257, 603 and 668 all have the same size venturi it's 12.7mm and only good for a stock 40cc (47cc) engine, not sure if the bore and choke sizes differ but they are all pretty small. These 3 carbs are common in the scooter world and i believe they are stock on 23cc scooters and whipper snipper engines, pocketbike reatailers have been passing them off as performance carbs and in my opinion all these carbies are doing is giving walbro carbs a bad name. The proper way to determine the correct size Walbro carb for an aplication is to use this formula:
D = K x Square Root(C x n)
where:
D=venturi diameter, in millimeters
K= constant between 0.65 and 0.90 (determines the smaller and the biggest diameter to be tested at the specific engine)
C=cylinder displacement, in cubic centimeters
n=RPM at peak power/1.000 (be realistic, dreaming only will make you try carbs far bigger than the correct, with results below your needs)
Stock cags only rev to ~10500 and you can use 0.65 as the constant so:
0.65 x root(40.21 x 10.5) = 13.35mm, obviously the 11mm and 12.7mm walbros are not good enough even for a stock 40, on a hotted up 49cc it would have no chance. Cag carbs are 13mm stock, they can be bored or drilled to 35/64th or 14mm fitted with a V-stack and filter and will work well with most applications.
The carbies listed below are available from most mower repair, chainsaw places and probably some pocketbike shops may be able to order them, google may help. I know ADA have manifolds to fit WA, WT or WYK they also have cable mounting hardware that these carbs may be missing. Reed valve engines can normally handle a bigger carb than piston port motors like that of the midbikes, a midbike normally has the CVT to try and recover from a loss of bottom end, if you have a 13.5 then it will still be good on a mild engine, but the 15mm would be a better choice. With a reed valve you can generally go up to a bigger carb and keep the bottom end. Carb sellection depends on what mods have been done to an engine to let it breath, the engine size and of course the rpm it will be running at.
__________________
Walbro Venturi Sizes above 13.00mm:
WA
WA-121 = 13.10mm
WA-107 = 14.29
WA-186 = 16.00
WA-52 = 23.01..borrrttt
__________________
WT
13.40MM
WT-484,
13.49mm
WT-118, WT-201, WT-192, WT-194, WT-195, WT-109, WT-114, WT-116, WT-202 WT-215, WT-228, WT-229, WT-234, WT-246, WT-281, WT-282, WT-283, WT-285, WT-286, WT-291, WT-294, WT-296, WT-313, WT-326, WT-338, WT-414, WT-429, WT-443, WT-446, WT-451, WT-459, WT-46, WT-463, WT-465, WT-480, WT-493, WT-493, WT-498, WT-503, WT-515, WT-529, WT-532, WT-533, WT-535, WT-536, WT-540, WT-542, WT-559, WT-560, WT-561, WT-562, WT-582, WT-590, WT-599, WT-610, WT-679, WT-680, WT-717, WT-727, WT-76, WT-
13.50mm
WT-185, WT-223, WT-224, WT-303, WT-303, WT-313, WT-325, WT-336, WT-401, WT-475, WT-497, WT-508, WT-509, WT-512, WT-550, WT-555, WT-567, WT-571, WT-574, WT-593, WT-594, WT-61, WT-614, WT-616, WT-622, WT-641, WT-648, WT-692, WT-699, WT-701, WT-708, WT-722, WT-724, WT-737, WT-79, WT-89, WT-98, WT-99
14.27mm
WT-19, WT-467, WT-6,
15.85mm
WT-591 The secret carb PSWS sent to Snowboardgeek?
__________________
wyk
13.5mm
WYK-100, WYK-101, WYK-103, WYK-104, WYK-132, WYK-134, WYK-138, WYK-140, WYK-142, WYK-143, WYK-144, WYK-152, WYK-158, WYK-172, WYK-173, WYK-178, WYK-193, WYK-196, WYK-200, WYK-201, WYK-205, WYK-210, WYK-215, WYK-216, WYK-218, WYK-223, WYK-28, WYK-29,
WYK-33, WYK-36, WYK-4, WYK-42, WYK-44, WYK-45, WYK-52, WYK-58, WYK-60, WYK-63, WYK-67, WYK-76, WYK-84
15.00mm
WYK-106, WYK-112, WYK-117, WYK-118, WYK-119, WYK-120, WYK-121, WYK-123, WYK-128, WYK-141, WYK-149, WYK-150, WYK-153, WYK-159, WYK-176, WYK-179, WYK-180, WYK-184, WYK-192, WYK-198, WYK-219, WYK- 73, WYK-74, WYK-91, WYK-95, WYK-96, WYK-97
http://wem.walbro.com/walbro/family2.asp
http://www.billetboard.com/showthread.php/9036-Carbs-Venturi-Size-and-Formula-For-Proper-Carb
I DID NOT WRITE THIS INFORMATION UP, I FOUND IT ON A FORUM IN WHICH THE INFORMATION CAME DIRECTLY FROM WALBRO.
I figured some may benefit from this info when trying to choose the proper carb for an engine build. This info is for the WA, WT and WYK carbs that have a 13.00mm Venturi and larger. Also I'm including a link for ALL series of Walbro carbs that include a breakdown as well as part numbers.
Common Walbro carbs, Wt-257, 603 and 668 all have the same size venturi it's 12.7mm and only good for a stock 40cc (47cc) engine, not sure if the bore and choke sizes differ but they are all pretty small. These 3 carbs are common in the scooter world and i believe they are stock on 23cc scooters and whipper snipper engines, pocketbike reatailers have been passing them off as performance carbs and in my opinion all these carbies are doing is giving walbro carbs a bad name. The proper way to determine the correct size Walbro carb for an aplication is to use this formula:
D = K x Square Root(C x n)
where:
D=venturi diameter, in millimeters
K= constant between 0.65 and 0.90 (determines the smaller and the biggest diameter to be tested at the specific engine)
C=cylinder displacement, in cubic centimeters
n=RPM at peak power/1.000 (be realistic, dreaming only will make you try carbs far bigger than the correct, with results below your needs)
Stock cags only rev to ~10500 and you can use 0.65 as the constant so:
0.65 x root(40.21 x 10.5) = 13.35mm, obviously the 11mm and 12.7mm walbros are not good enough even for a stock 40, on a hotted up 49cc it would have no chance. Cag carbs are 13mm stock, they can be bored or drilled to 35/64th or 14mm fitted with a V-stack and filter and will work well with most applications.
The carbies listed below are available from most mower repair, chainsaw places and probably some pocketbike shops may be able to order them, google may help. I know ADA have manifolds to fit WA, WT or WYK they also have cable mounting hardware that these carbs may be missing. Reed valve engines can normally handle a bigger carb than piston port motors like that of the midbikes, a midbike normally has the CVT to try and recover from a loss of bottom end, if you have a 13.5 then it will still be good on a mild engine, but the 15mm would be a better choice. With a reed valve you can generally go up to a bigger carb and keep the bottom end. Carb sellection depends on what mods have been done to an engine to let it breath, the engine size and of course the rpm it will be running at.
__________________
Walbro Venturi Sizes above 13.00mm:
WA
WA-121 = 13.10mm
WA-107 = 14.29
WA-186 = 16.00
WA-52 = 23.01..borrrttt
__________________
WT
13.40MM
WT-484,
13.49mm
WT-118, WT-201, WT-192, WT-194, WT-195, WT-109, WT-114, WT-116, WT-202 WT-215, WT-228, WT-229, WT-234, WT-246, WT-281, WT-282, WT-283, WT-285, WT-286, WT-291, WT-294, WT-296, WT-313, WT-326, WT-338, WT-414, WT-429, WT-443, WT-446, WT-451, WT-459, WT-46, WT-463, WT-465, WT-480, WT-493, WT-493, WT-498, WT-503, WT-515, WT-529, WT-532, WT-533, WT-535, WT-536, WT-540, WT-542, WT-559, WT-560, WT-561, WT-562, WT-582, WT-590, WT-599, WT-610, WT-679, WT-680, WT-717, WT-727, WT-76, WT-
13.50mm
WT-185, WT-223, WT-224, WT-303, WT-303, WT-313, WT-325, WT-336, WT-401, WT-475, WT-497, WT-508, WT-509, WT-512, WT-550, WT-555, WT-567, WT-571, WT-574, WT-593, WT-594, WT-61, WT-614, WT-616, WT-622, WT-641, WT-648, WT-692, WT-699, WT-701, WT-708, WT-722, WT-724, WT-737, WT-79, WT-89, WT-98, WT-99
14.27mm
WT-19, WT-467, WT-6,
15.85mm
WT-591 The secret carb PSWS sent to Snowboardgeek?
__________________
wyk
13.5mm
WYK-100, WYK-101, WYK-103, WYK-104, WYK-132, WYK-134, WYK-138, WYK-140, WYK-142, WYK-143, WYK-144, WYK-152, WYK-158, WYK-172, WYK-173, WYK-178, WYK-193, WYK-196, WYK-200, WYK-201, WYK-205, WYK-210, WYK-215, WYK-216, WYK-218, WYK-223, WYK-28, WYK-29,
WYK-33, WYK-36, WYK-4, WYK-42, WYK-44, WYK-45, WYK-52, WYK-58, WYK-60, WYK-63, WYK-67, WYK-76, WYK-84
15.00mm
WYK-106, WYK-112, WYK-117, WYK-118, WYK-119, WYK-120, WYK-121, WYK-123, WYK-128, WYK-141, WYK-149, WYK-150, WYK-153, WYK-159, WYK-176, WYK-179, WYK-180, WYK-184, WYK-192, WYK-198, WYK-219, WYK- 73, WYK-74, WYK-91, WYK-95, WYK-96, WYK-97
http://wem.walbro.com/walbro/family2.asp
