How to Dial in Your Jetting

Alright so you've made a modification that messes with your airflow like:

Exhaust, Cam, Intake, Big Bore Piston

...and now your Ruckus runs like crap. You need to dial in your jetting to get it running as good or better.

First, you need to realize that you aren't gonna have a huge increase in power once you get it dialed in. Stock, your Ruckus is perfectly dialed in so it's making 100% of it's potential 5 hp or so. When you do a mod like an aftermarket intake your potential hp might rise to 5.2hp but you likely won't get the jetting totally perfect so you'll only be making 95% of your potential power so once you get it dialed in you'll likely perform at lot like stock. My recommendation is to only get into jetting if you're gonna dive in deep because rejetting is a lot of work and it's not worth it if you just gonna make one little mod. I don't recommend messing with your intake unless you're gonna do the cam and maybe the big bore piston too.

Anyways, you went ahead and messed around anyways so here's how you can adjust your carb so it runs good again.

There are 5 adjustments you can make to your carb which are:

1 - Main Jet (gradually opens as RPMs rise)
2 - Slow Jet (always open)
3 - Idle Mixture Screw (controls how rich/lean your idle is)
4 - Idle Screw (controls amount of throttle it gets at idle)
5 - Shimming the needle (adds gas off the line)

The main 2 adjustments you're gonna be using are the main and slow jets. The other 3 are things you can do later to fine tune things but they aren't as significant as these 2.

Your main jet is what provides the majority of the gas to your engine. At idle it is mostly blocked by the needle which pokes into it and largely plugs it. As you take off this needle gradually rises out of the hole and you get more and more gas (because the needle is tapered) until finally the needle is all the way out and the jet is wide open (maybe 10-20mph). The slow jet on the other hand is always open.

In order to rejet, first it's helpful to understand what changes your modification did. If you just opened up the intake or added an aftermarket intake/filter then your Ruckus is gonna be running very lean off the line but only somewhat lean on the top end because there are other bottlenecks on the top end (ie. cam) that restrict the effectiveness of the additional intake flow. If you do an intake and the cam your Ruckus will still be leaner on the low end than it is up top but the difference will be less.

Alright now onto the good stuff:

1) The first step is to get the jets. You'll need to order the slow jet kit and a main jet kit from Battlescooter. The slow jet kit includes jets 38,40 and 42 (stock = 35). I recommend getting the main jet kit that includes 72,75,78,80,82,85 because the other kits are missing a lot of in between jets that are important.

2) The next step you need to take is to swap your slow jet to either a 38 or a 40 (from 35). Choose 38 for minor mods, hot climates or high elevations and 40 for cooler climates, lower elevations and major mods.

You don't need to get it perfect yet but it'll likely be one of these and it's important to get it close or it'll throw you off when you adjust the main jet. Keep in mind that the slow jets works in combination with the main jet so don't expect it to run good off the line yet...it should just suck a bit less. Well come back and dial in the slow jet later.

3) Next you need to dial in the main jet. The main jet is largely responsible for how your Ruckus runs above 20-30mph so don't judge it based upon it's acceleration off the line...that should be fine tuned by the slow jets which we'll get to later. To dial in the main jet ONLY CONSIDER the top speed of your Ruckus. Since you already guessed at the slow jet you should have a 38/75 (slowjet/mainjet) or a 40/75 setup. Now go for a LONG ride. You want your engine to fully warm up before you judge the jetting setup because a cold engine generally has a harder time vapourizing the gas so wait until it's warmed up which requires at least 15min of driving. If your jetting is so far off that you can't even get it going then you can head back for home but if you can even get to 30mph keep going because even though it may not be the right jet we can still learn a lot.

Once you get home, record what your top speed with that jet was on a long flat straight. It's important to consider the wind and the temperature too so make notes. Now get out your spark plug socket ( you should have one in your socket set) and remove the spark plug and have a look. If it's white or has white areas/spots then you're too lean so you need more gas. If it's black/sooty then you've got too much gas (rich). Adjust your plug ONE increment in the direction you think you should go. It's tempting to just a few increments to save time but it's important to check each one. A 75 and an 80 main might both run mediocre because a 78 is the ticket.

With your new main jet go for another test run. Let the engine fully warm up. It's a good idea to do this on the same day so the temperature and wind will be similar and so the previous performance is fresh in your mind. Try not to do this on a windy day because that really messes with your top end.

Repeat this cycle until you're spark plug is looking good when you pull it out. It should be tan/brown coloured. I like mine a dark brown but definately not black. The jet that yields this colour should also yield your highest top speed. If your top speed is not what you were expecting then there might be something else wrong like your expectations or if you have a filter directly mounted on the carb the air flow can be to turbulant for the carb. I've tried this and don't recommend it.

4) Okay, you've got your main jet dialed in and it's running good at the top. Your Ruckus might be running good across the board if your slow jet is right now's the time to examine that to ensure you're performance 0-30mph is optimized. You can't analyze your slow jet by pulling the plug so instead you need to compare how it performs in hot weather vs. cool weather. If you're Ruckus is still stumbling off the line go up one size but other wise just take the next couple times you go riding and note how it performs off the line and the temperature. On hot days the air will be less dense so your engine is getting less oxygen but the same amount of gas so it'll run richer. If your Ruckus bogs a bit and runs worse off the line in hot weather you'll need to drop the slow jet 1 size but if it runs better in hot weather you're likely a bit lean usually and you can go up one size. There aren't that many slow jets to choose from so trial and error generally works best. Just a slow jet mainly on your performance from 5mph-25mph because the idle mixture adjustment controls your 0-5mph launch.

5) Okay you've got the slow jet and main jet dialed. Now consider if your slow jet adjustment messed up your top end. The slow jet is always open so if you increase the size then you're gonna be running a bit richer on the top end too and vice versa. Generally, going up 2 sizes (ie. 38 to a 42) has the same effect on your top end as changing the main jet 1 size. If you just moved your slow jet up or down one size your top end is probably still okay but if you moved it up or down 2 sizes you'll likely need to drop/raise your main jet 1 size to adjust. A good way to figure this out is to have a look at your spark plug. I like to give my spark plug atleast 5 minutes of full throttle riding to change colour. Try and spend this time at full throttle going fast because if you're idling at lights it will throw the colour off. I like to go for a blast at top speed and when I get home I hit the kill switch to cut the spark so it's as accurate as possible.

6) Alright so your main and slow jets are dialed in and you're satisified with your performance from 5mph all the way up. Now you need to adjust your idle mixture. There is an idle mixture screw on the LEFT side of the carb which controls how rich or lean your idle is. Don't confuse this with the idle screw on the top right of the carb which determines how much throttle you're giving it at idle. The hardest part of doing this adjustment is getting the fancy screwdriver tool that fits the Idle Mixture screw. Once you've got that you want to adjust this screw for the highest possible idle because that means it's running the best. A tach is very helpful here but you can use your ear if you're good. As always, DON'T do this when your engine is cold because the engine won't be running as well and there's also a 'Start Enrichment Valve' that adds extra gas when your engine is cold and gives you a chaotic idle for the first 2 min. To adjust this go for a nice 15min ride and then let it idle in your driveway for a couple minutes and then go to work.

7) Once you've got your idle as high as possible via the mix screw you need to reset the idle so it's at 2000rpm (+/- 100). This is hard to do without a tach but basically you want it as high as possible WITHOUT the rear wheel engaging. If the rear wheel is spinning on the centerstand it's gonna be trying to do that also when you're stopped at redlights which means your clutch will be slipping and wearing out faster. It's possible that once you started messing with your carb the idle went to crap so you cranked it up but now that the mix is correct again, your idle is too high. Just relax and dial it down to 2000rpm. It's normal for your idle to rise and fall a bit...especially on a cold engine.

8) Alright you're pretty much done. Just a few other miscellanious thoughts:

- The 2006 Ruckus uses a different and improved needle. Consider swapping to this before you start rejetting if you have an older Ruckus.

- In my experience, shimming the needle is not necessary now that slow jets are available. Shimming the needle lifts it out of the main jet so it's blocks less of the main jet which gives you a bit more gas off the line but you should be able to accomplish the same thing with the right slow jet and you won't get the same dead spot around 10-20mph.

- Another carb adjustment is shortening the diaphram spring. This basically makes the spring weaker so it pulls out of the main jet faster so you get more gas sooner. This can give you a bit quicker starts but it's a seperate adjustment you don't have to make and personally, I've found it's not easy to improve upon the stock setup...it's really a shot in the dark adjustment.