Ralphbf

My Feedback: (27)

I came across this and figured most people here could use it.
It's from a pocket bike forum but the info is still good.

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
__________________

SDC Walbros above 13.00mm
All of them

source: http://wem.walbro.com/walbro/family2.asp
__________________

captinjohn

I seen 3 new SDC carbs...in throat was 36....it looked like for a 60cc engine?????? Brand new $10 each at flea-market!!! Capt,n

tim220225

My Feedback: (4)

Ralph,

I created a spreadsheet using the same factors you are discussing to build the formulas. Works great for what we do. I have found that the .70 to .75 constants work the best. Any smaller or larger constants will just under or over feed the engine. Some still think that a bigger carb is better but it isn't. I have tested many carbs on engines and this formula works very well. Another interesting thing I figured out is that if you take the carb venturi size and multiply it by 2.5 then round to the nearest whole number you will get the exact size of the intake which is cast into the carb body. I have only found 2 carbs as I recall that this didn't work for. Walbro told me that happens rarely but there are OEM's that spec something different for whatever reason. Walbro also told me that they don't publish the size of the intake and that is what caused me to dig way back in my cluttered mind to figure a quick way to calculate the number. I actually use the formula backwards to figure out the venturi sizes of carbs I come across at auctions and such since I don't carry the service manual I have. You do need to be realistic on the RPM of the engine too as you say. I generally settle on 7200 for my singles as it gives me enough air and fuel reserve when the prop unloads in the air and the RPM's rise.

Boosted Weedwacker

I building mini supercharger for my mac 32cc engine I new I would have this problem the carb can't put out enough gas so I was hopeing yall could help me out? maybe a higher wt model will work but I don't know for sure so heres some pics.

nitro joe

My Feedback: (2)

I’m not sure why your not getting replies.
You may be in the wrong forum for your project.

There would be much more traffic over in the
“Gas engines forum.�Maybe start your own thread there.

I for one,would love to hear more.

Boosted Weedwacker

just a littel update im strating to make a b.o.v. for my supercharger im in a early stage of it I have a few pics of it but any ways do you know a better forum or thread I can post my project on?

av8tor1977

My Feedback: (6)

Interesting project. Keep posting here! The blower housing looks heavy. Is it just a mock up for one later made of plastic perhaps? How will the supercharger be driven? I've often thought a supercharger would be just the trick for my low compression Honda 31 four stroke....

I think making a pressurized box around the carb would be the way to go. Solves a bunch of carb metering problems. Of interest; I took a Husqvarna saw apart the other day. It had part of the cooling air blast from the magneto ducted into the air cleaner chamber to boost power. Reminds me of years ago when Smokey Yunick used a smog pump to gain a few mph top end on a Bonneville car!

AV8TOR

nitro joe

My Feedback: (2)

You are indulging in a controversial subject.

Even though it has been done,many/most will say it just is not practical on a two stroke.
Is your design somehow unique? You obviously know your way around machine tools.

It’s unlikely that most folks in this forum would have experience with a forced induction two stroke.

Is that a pop-off valve there your machining?

captinjohn

Here is more on the supercharger experiment. I do not think a walbro type carb will work. Take a look here for more. http://www.rcuniverse.com/forum/m_51..._2/key_/tm.htm Capt,n

gkamysz

My Feedback: (19)

What are you going to use this on? A blow off valve is not needed with a crank driven supercharger. What kind of pressures will the supercharger develop?

As far as sizing the carb you need to figure out what your flow is going to be, then you'll know what size carb to use. Are you putting the carb before or after the supercharger?

captinjohn

If the carburator was placed before supercharger, with the type of engine he is using would cause a very hard to start engine. To pull the gas/air mix that far may not work. Not usre though. Capt,n[:-]

Boosted Weedwacker

actually I used a drill press and dremel for machineing. I bought a harden steel rod & tube for the b.o.v. but for the supercharger I just used forstner bits. [link]http://www.proshoppublishing.com/tips_user.html[/link]

Boosted Weedwacker

[link]http://www.vortechsuperchargers.com/index.php[/link]maybe a b.o.v. is not required for such a small application but It's the proper way to build it .

gkamysz

My Feedback: (19)

Boosted Weedwacker

I don't want to put the carb before the supercharger because it probably won't start like you said and just think of the gas shooting out the blowoff valve a bit messy you think and the gas will also shorten the life of the supercharger or even melt the impeller. so I was kinda thinking of putting the carb after the supercharger but have a trottle plate in front of the supercharger to get a clean source of vacuum instead of getting a pulse but then that means I would have two trottles unless I remove the trottle on the carb but I'm pretty sure that wont work so... any suggestions?

Boosted Weedwacker

I finely started on the gearbox but I ran into a littel problem with the the drive bearing is a 16th inch to big but besides that it turned out fine even though I steal wish I could have put oil seals on it. I don't konw maybe later hmmmm.........[sm=72_72.gif]anyway heres some more pics.

nitro joe

My Feedback: (2)

If nothing else,I gotta say, that is some impressive drill press work.
What clearance did you set the gear mesh to?

captinjohn

You should only need one throttle plate...the one in carb. Once engine starts and the RPM gets high enough, the engine will start to get boost. The biggest problem I for-see is getting the right drive train from engine crank to the supercharger. Air from the supercharger would go to a box that surrounds the carb. You could have a base plate between carb abd engine that would give a place for the top part of the box to seal too. Use long enough bolts at each corner to hold it together. You could make this from clear 1/8 thick plactic and seal with clear silicone rubber. Kinda seal it like a fish tank. I would use a electric blower from maybe a vacume cleaner just to test it. Better yet a electric blower you can vary the RPM...that creates boost. just some ideas...it is your project, we just want to help. Good Luck capt,n

captinjohn

Here is more from another thread. .................................................. .................................................. ........................








Posts: 6108
Joined: 1/25/2002
From: Hesperia, MI, USA
Status: online I have no idea id this would work...but here goes! Use a carburator with a float. Build a box just big enough to enclose the carb. The output from your blower will push air into the box via a connector. You may be able to "Test" this therory by using a high output electric blower (ac or dc powered) to simuate boost. A tube hooked to box would show the ammount of boost. The amount of boost woud have to be controlled by electric blower controls or a waste gate to relieve to much boost. May be kinda fun! Capt,n

___________________________





cosmospho








Posts: 110
Joined: 11/27/2002
From: Helsinki, FINLAND
Status: offline I was just wondering, why don't you run a ducted fan engine with a brushless engine setup linked to the throttle channel and some kind of mixing to regulate it the same as the throttle or the amount you need....

With the latest technology in eletrics, that would be the way to go, all you would need is a lipo, a DF blowing into your intake or to manifold.... I like your perseverance, inspite of all saying there are better ways.... This is the way things get invented and go forward, because of crazy nuts like you :-).....

Boosted Weedwacker

Never mind my previous statement a reed valve will solve my problem. my orignal problem was getting a clean vacuum insted of a pulsing vacuum (in & out).

Boosted Weedwacker

Sorry for the wate I'v been working on my car I had to replace the sub frame, transaxle, and engine. the car was handed down to me after my brother ran in a curb at 40 mph while racing it besides that I'm about 75% done with the bov I turned a spacer for my old transaxle for my sunfire on my drill press I used 1 tooth of a 10'' miter saw blade and wet dry sand paper 220-1000 grit. but the next step I'm going to be more careful and take my time. I got to bore a hole on the side of the bov witch will probably leave jaged edges and Im going to make a preload adjusment and top plate for the bov to make a 100% so anyways heres pics of what progres I got done.

Boosted Weedwacker

hey every one I got a littel more progres done now I'm 90% done with the bov I turned a 1/4'' piece of aluminum on my drill press and made a top for my bov and I also made a preload adjustment for my tiny bov and yes I have more pics.

Boosted Weedwacker

I just ordered a cog belt set one cog is 1-1/2'' with a 3/8'' bore and the other one is 1'' with a 5/16'' bore which should sent the supercharger to spin around 30,000+ rpms. (7,500 est rpm x4.3 to 1=33,750 rpms.)

T.L.A.R. eng

Will the engine have enough power to overcome the blower drive system? []

av8tor1977

My Feedback: (6)

Before I chose gearing, I think I would have tried to rig up a way to run the blower and find out how much air it produces and what rpms would actually be needed...

The other day I fixed up a Husqvarna chain saw for my nephew. I noticed it was set up to pressurize the air filter box chamber with bleed air from the cooling system air provided by the magneto fins. I would have loved to see how much power that gained them, but my nephew needed his chainsaw so I couldn't experiment with the engine... []

But it is an interesting project you are doing. Keep at it.

AV8TOR