I build rebuild kart racing engines and I'm in my peak season now. I thought I would show a couple things I do with a Walbro wa55 that is similar to some chainsaw carbs. This carb is only used in a restricted class for kids up to 12 years of age and on a kt100 Yamaha 2cycle engine. The popoff is critical and is set by shortening or compressing the spring. It's fairly tedious but worthwhile. There isn't much that can be done to the carb to increase air flow but I have a couple tricks. Some carbs flow better than others so I can tell the good ones before they every get on the dyno or track. The 1st pic is a carb being checked with a popoff gauge. The second pic shows a carb on the flow bench. You can see my dyno in the background, it has the round crank handle.
Very cool! What pop off pressure do you shoot for or is top secret? What kind of power gains do you see?
I set popoff the wa55 at 17# psi with a stock spring they are above 25#. I have seen 1/2 hp increase on an engine that uses the wa55 and close to a full hp on engines with the larger wb3 carb going from a stock carb to a blueprinted carb. The rules are pretty tight on the engines being stock but there is a huge difference in the castings from one engine part to another so blueprinting shows significant gains.
Actually that gain is only is extreme cases. Carbs are all over the place afa air flow and that is the key to getting a good one. Fuel signal can be regulated with fulcrum arm height, popoff and needle settings. On the wa55 the venturi and throttle bore are untouchable because they come close as you can get to the limit so all you can do is change to a low profile butterfly screw to help airflow. On the wb3 carb there is enough extra material that the venturi can be machined or broached and the throttle bore can be machined to maximize airflow. I usually broach the venturi to .948 to pass a .950 no-go rule and then flow. If it's not a good flowing carb I'll chuck it up and bore the carb bore to 1.008. New ones come at 1" with a limit if 1.010, and is tedious to mount perfectly on center and not remove too much material but again worthwhile. The needle settings are nowhere near the factory settings. We control most of the fuel with the low needle and the high is only slightly open with the wa55 and closed to just cracked on the wb3.
