alright guys so I just bought my first motorcycle being the s40, and I love it but I am having a hard time getting the bike to run right.

I bought the bike with around 25k miles and it runs and starts great except for the initial throttle from idle, the bike starts to sputter and even has died on me one time at a stoplight.

The bike has a voodoo aftermarket slip on exhaust and a raptor petcock however is still factory jetted

initially the bike was running really rich the spark plug was straight black, which I thought was odd due to the exhaust I would have though it would run lean which makes me think it may be something else... However I got the carb sonic cleaned and messed with the fuel mixture screw and it seems to have leaned out as there is no more black smoke coming from the exhaust however it still sputters at initial then starts to run good. I havnt pulled the sparkplug out again to confirm the rich running to go away however I wanted to know what else I could check to fix the sputtering if my carb seems to be ok

I even checked and the spark plug is getting a strong spark, the bike idles good starts right up just decides to sputter at first.

I did notice when spraying carb cleaner at the intake where the carb and engine meet if enough carb cleaner is sprayed on the intake the bike will die so maybe a vacuum leak???

I am trying to fix it myself so any advice on what to check would be greatly appreciated

lastly I did try to increase my pilot jet (before knowing it was rich) and the sputter went away however there was a terrible rough idle and a hanging idle....

Tomorrow I will remove my air filter to see if that helps, however I am hoping it is not the valve clearences or cam chain however if I need to check those I will

again to restate the bike idles and has great midrange and full throttle (may need a bigger main jet as it a\oulls a little harder when i let off the throttle a little) however when idling and then giving it 1/8 to 1/4 throttle it sputters before catching up

also when cold there is no sputter only when warmed up!! any advice would be great!!!

Instead of spraying flammable stuff hunting a leak try soapy water. It will change how it runs and you can see Where it's hitting, unlike shooting something that has invisible vapors. Most squirt bottles can be adjusted to work like a squirt gun instead of a shower.

oh also just checked thew fuel mixture screw it is 3 1/2 turns out the way I adjusted that was turned the biked idle really low before it would die then adjusted the mixture screw to where it would idle the highest. It would die or run rough if too far in and then eventually would run rough if too far out so i tried to find the happy medium then increased the idle to where it ran smooth. I dont have any way to exactly check the idle speed just going by ear and where it sounds smooth as I know these bikes are supposed to idle faster for the oil pump or what not !!

Again thanks for the fast replies before I joined the forum I had been looking at about nearly every post haha

I tried to switch the pilot jet out to the next size bigger as I didn’t have 2 sizes up and the bike stutters even worse now and just sounds like it has wayyy too much fuel I did get the fuel screw to 2 1/2 turns out but the bike just sounds super choppy and won’t stay running now once you start to ride and let off the gas it just dies so I don’t think it needs a bigger pilot jet(I could be wrong but it’s also shooting out black smoke on the initial throttle more then it did before) I’m thinking replace the intake rubber gasket and o ring since when I spray carb cleaner there the bike does maybe that will clean up the sputter? After these gaskets/ boots are replaced I’m not so sure what to check after that hopefully this helps any ideas would be greatly appreciated  

Stock air filter and even with removing the filter it still bogs a little better but doesn’t cure it. Then the vacuum has like a gas line  attached  to it but isn’t capped looks like the previous owner just out a gas line or vent line on it

BB57y5e wrote on 10/02/21 at 19:07:19:

Then the vacuum has like a gas line  attached  to it but isn’t capped looks like the previous owner just out a gas line or vent line on it

Now we’re getting somewhere. That line has to be capped off. For the time being, stick a golf tee in that line and see if that cures the problem.

A crack in the carb boot introduces air - not fuel - into the mixture. So it causes the engine to run lean. This is not your problem.

Tell me more about the smoke. Perhaps it's oil. Once again, a black spark plug tells you little about the pilot circuit, but it does tell me that you need to clean or replace the plug. What jets are installed and what is your (approximate) altitude?

The smoke is black and only on the 1/8 throttle  which is why I figured it was fuel cause I thought white was oil, but I am at 338’ according to google and I am ringing the stock pilot  jet which is 52.5  and then my main jet is one up from stock so 147.5 maybe I should just go down a pilot size and call it good?