I have a 5hp kart with some mods done and im having an issue with a flat spot during drag racing. I race a 1/10 mile and when I launch it takes off hard then just looses power but after I get rolling about 15-20mph it takes off like a rocket. I richened the carb which helps but it feels like it has a transition issue at mid rpm. id guess around 2800-3000 its flat then comes in hard to 6200rpm. my carb is stock with the twist removed. is there any way to improve this issue by drilling the jet? if so what size? it seems to idle rich and if I richen it more it doesn't like to idle and dies.
flat spot on accel
- Thread starter briggs206
- Start date
Jon Brogden
Member
It's difficult to get that carb to transition well off the bottom , it really wasn't designed to do what your asking it to. It probably is a lean condition. About all you can do is run it as rich as you can get by with , maybe increase the idle some, and probably the most important for what your doing is to stall the clutch higher to try and get past that lean spot before the clutch starts to hook. We slip stocker's 3500 to 3800 for an oval, for a 1/10 drag race you need at least that and depending on your mods maybe a little more. Jon
yes the clutch grabs early and id like to get a higher idle out of it. I drilled the stock jet from .032 up to .034 and seems to help but my question is when I drill the jet out im richening the idle and high speed fuel correct? is there a way to increase fuel on just the high side or not? when I race my opponents get very far ahead of me for the first half track but once my engine cleans up I catch up fast. we run low cost clutches with 2000-2300 stalls. what size jet would you recommend? my engine has a shaved head, arc rod, 3hp flywheel with 6 dgs advance, a header, its ported and relieved and im doing cam swap soon.
cubicinches
New member
Take the flap out of the carb and drill a hole in it on the jet side. It'll let the carb idle leaner and allow you to run a bigger jet. I had the same problem with my flathead, either it was way too rich to run clean or way too lean to run. That should help it on the low end exponentionally. Just pop the cap off the back, stick a screwdriver down the hole and it'll unscrew and come out, it's tricky getting it back in but it works.
ok so drill a hole in the butterfly? what size? I see the high and low snake eyes but im wondering how drilling the brass jet out helps when the needle blocks the fuel anyway. how is drilling the jet out any different then just turning the needle out more? yes its gas and a stock cam. im swapping to a mild cam this weekend. its a dyno cam 00-4
cubicinches
New member
Drilling out the jet makes a difference, but don't go too big. Just get a pin drill set and go one size up at a time testing it in between until you like the result. As for the hole in the flap, just start with about a 1/16" hole, I'm not sure of the exact science on that, I just kinda drilled a hole in it and it worked. You're just looking to get more airflow into the carb to mix with the fuel at a low speed before it fully opens, so you may have to experiment a little.
Jon Brogden
Member
The idea there is to let the shutter go most or all the way shut so the metering hole is covered and the drilled hole will give it some air to idle, start small, a little bit of fuel will get by it so it will run. I don't think your gonna get by with making the jet much bigger than .036 on gas, a turn of the needle has a pretty big affect on gas. If you run it too fat you'll lose top end. The butterfly and screw are quite restrictive, thinning and cutting the screw down will help, do your mods and then retune the jetting as all that will change if you get the airflow up any. Jon