wb3a mods

hi guys, as you can tell im working on a kt100 for hot lapping. what passages in the walbro should I enlarge to get more fuel to a high compression semi ported piped kt100, thanks, chuck
 
You could open up the low jet a tiny bit and smooth the casting seam in the venturi. Run your fulcrum arm a little high and set your pop off to 9.
 
You really don't need to do anything with the fuel passageways if running gas. The carbs capable of getting plenty rich , even with a radical SA from what I've seen. Now if you want to unleash all the potential of your port work, the carb needs more airflow. If you can, bore the venturi to around 1.00 and the throttle bore to handle a 1.100 shutter. There's a lot more to it than that but it will take a stocker from 65 cfm to 90. Hot lap ? Heck yeah.
 
You really don't need to do anything with the fuel passageways if running gas. The carbs capable of getting plenty rich , even with a radical SA from what I've seen. Now if you want to unleash all the potential of your port work, the carb needs more airflow. If you can, bore the venturi to around 1.00 and the throttle bore to handle a 1.100 shutter. There's a lot more to it than that but it will take a stocker from 65 cfm to 90. Hot lap ? Heck yeah.
thanks jon, great info. I will keep this info with my other kt stuff. chuck
 
That may be more than what you want to get into but since it's non tec , it's easy to pick up airflow even with no bore work. Take the throttle shaft out and carefully cut the topside off , the button head screw will set right on the shutter. Reducing the profile of the butterfly and throttle shaft assembly is a major factor when you try to hod rod these butterfly carbs.
 
That may be more than what you want to get into but since it's non tec , it's easy to pick up airflow even with no bore work. Take the throttle shaft out and carefully cut the topside off , the button head screw will set right on the shutter. Reducing the profile of the butterfly and throttle shaft assembly is a major factor when you try to hod rod these butterfly carbs.
thanks so much jon. that sounds much easier. I am looking forward to tinkering with my kt. what a great site! chuck
 
Here's what can be done with a WB3A. 1.100 shutter out of a WB20, anything bigger will break out. A Tilly 380 shutter at 1.125 will work with a little patching . You'll want to leave the venturi about .100 smaller than the throttle bore to have a strong venturi effect while positioning the V right at the high side dump for max signal. The very best flow was achieved with the old filter cup, with a full radius at the edge vs a velocity stack. Build a high flow shaft assembly and all that will get you 90 cfm with a stocker moving 65.
 

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Here's what can be done with a WB3A. 1.100 shutter out of a WB20, anything bigger will break out. A Tilly 380 shutter at 1.125 will work with a little patching . You'll want to leave the venturi about .100 smaller than the throttle bore to have a strong venturi effect while positioning the V right at the high side dump for max signal. The very best flow was achieved with the old filter cup, with a full radius at the edge vs a velocity stack. Build a high flow shaft assembly and all that will get you 90 cfm with a stocker moving 65.
thanks jon, those pictures of the carbs look like works of art. the second picture that carb looks much bigger to me. thanks chuck
 
Jon,

If you feel like experimenting.... an air cleaner cup that looks something like the rendering below is what I found to work better than anything I ever tried. It is based on the "vena contracta" theory. I made one back in the late 70's to run on my KT100 (before there were any real controls on the air cleaner cup), and it was worth about half to a full second a lap on a ~1:45 road race track over anything else I tried.

IIRC, that radius is calculated off the bore diameter, velocity of the flow, and "viscosity" of the medium (air in this case).

Unfortunately, it was banned as soon as a tech person saw it, and it was actually (and unfortunately) the impetus for tight dimension callouts on air cleaner cups in the IKF rulebook shortly after that. :cry:

(note: drawing below is not to scale -- I just whipped up a quick cad drawing so the general idea could be illustrated)

PM
vc.jpg
 
Nice Pete , I can see that working well with that big fat radius. Ha! That's where that came from a Muller rule.
Chuck, easier and cheaper definitely gonna be slower as you would expect. Here's a different way to handle the airflow needs of a SA KT . The manifold of course matches the 1.100 TB.
 

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The manifold matches but it won't fit the 34 mm carb lol. That 34 works pretty dam good on that stroker better than any wb3a for sure. definitely not slower and no pulse hole for the 34 later Chuck.
 
Yo Chuck this might be easier and cheaper later Chuck.
These 2 stroker engines will be for sale once I check them this weekend or next week. Unfortunately as many of you know we lost Chuck (nobozos) and his wife in a car accident and his family asked me to help selling his racing stuff. Plenty of Yamaha stuff. I'll keep everyone updated. It is tough for me to do this but I want to hrlp as much as I can.
 
These 2 stroker engines will be for sale once I check them this weekend or next week. Unfortunately as many of you know we lost Chuck (nobozos) and his wife in a car accident and his family asked me to help selling his racing stuff. Plenty of Yamaha stuff. I'll keep everyone updated. It is tough for me to do this but I want to hrlp as much as I can.
10-4 rainman
 
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