Suggestions for jet size for new 200x build.

[RUNMEDOWN]

I am building a 66mm, non-stroker 200x with a Bassani pipe and Uni filter. It will have a Wiesco 10.25:1 piston and using a XL185 (longer duration/higher lift than stock 200x) cam. I just received a new stock carb but the carb came with 40/110 Jets, since it is bored to 66mm (+1mm from stock), has the exhaust and Uni, what are some good sizes to start with to get in a ball park with jets. I am in the midwest (Altitude reference) and would be using this mostly for low and Mid RPM riding on trails. I know it will take some fine tuning when it is toghther but where is a good start point?

[yaegerb]

buy a few sizes from 112, 115 118. You can do so from jets R us. Start with a 118 main and go down from there. 40 pilot should be good enough. Below is the method I use to jet.

These jetting specifications are designed as a rule of thumb. They are in no way absolute. Variations in air density, specific gravity of fuel, altitude and other engine modifications play a large part in jetting. Newly built engines need rich jetting during break-in. Do not jet for power until an engine is broken-in.

Myths
1. Plug reading may/may not work.
2. Revving the engine while it’s sitting in the garage doesn’t work.

Other than Dyno testing, the steps below is the simplest way you can jet your engine.


1. Find a gentle slope that you can ride in 2nd or 3rd gear. Look for something that will put a reasonable load on the engine. This will be your “dyno”.
2. A basic outline of which jet is active at a particular throttle setting: Pilot Jet = 0 to 1/4 throttle. Needle = 1/4 to 3/4 throttle. Main Jet = 3/4 to Full Throttle. Changing the Main Jet size won’t affect how your engine idles or
runs at 1/4 throttle. Engine RPM isn’t what determines which jet is active - throttle position does.
3. Make the recommended jetting changes. Always start rich and work leaner.
4. Start and warm up the engine, then ride your ‘dyno hill’. Any point where you feel the engine stumble or hesitate indicates a tuning problem. Note the throttle position and modify the corresponding jet (1/4 to 1/2 throttle = needle. 3/4 throttle or more = main jet).
5. Only change jetting by 1 step at a time, and 1 circuit at a time (don’t change needle and main at the same time). Re-check after every change.
6. Once the engine runs smoothly throughout, you’re jetted!

If you ever notice an abrupt loss of power, or engine sounds change, shut it down.

[Jmoozy27]

Couldn't describe it better myself. I check the plug initially when dialing in a friends bike. Just to see how far off it is from tuned. But after that it's all about the senses.

Sound- listen to what the machine is telling you, if it's popping when decelerating, it's lean at that circuit. If it is sputtering like its flooded, then it's rich in that circuit. Also if the motor is warm and when you rev it take a long time to return to the initial idle speed, that's also a lean condition.

Smell- if your smelling gas when it's idling then either the air/fuel screw or the pilot need adjustments.

Sight- look for blue smoke, fire, or fuel vapors from the exhaust, that indicates rich conditions.

Touch- you have to just learn to feel for certain symptoms, a lean motor runs like crap and is cold natured hard to start. A rich motor will run good until it gets hot and then it runs like crap.

Taste- don't taste anything, it doesn't help!!!

[RUNMEDOWN]

Thanks Yaegerb, I will start with 118 and go down from there, I also just found the stickey on carb tuning......my bad.

[barnett468]

I am building a

non-stroker 200x
Bassani pipe
Uni filter.
Wiesco 10.25:1 piston
XL185 (longer duration/higher lift than stock 200x) cam.
new stock carb
bored to 66mm (+1mm from stock)

I am in the midwest (Altitude reference) and would be using this mostly for low and Mid RPM riding on trails.

A specific altitude would be better but as long as it's under around 1500 feet it doesn't matter much.

If you haven't bought a cam yet, I would look at some others first like the 1st and 2nd cams on Web Cams site.

http://www.webcamshafts.com/


For your app, you dont really want a cam that revs farther than your current one unless you want more rpm's than you currently have, but the higher an engine revs, the less bottom end power it will have.

[barnett468]

.
Here's another option or different approach to get you close.

Start with the 118 main jet as suggested and the needle in the middle position.

I would not use synthetic oil for engine break in.

After break in, ride in second gear on level ground or slight uphill as suggested with the throttle between 1/3rd and 1/2 way open then suddenly mash it all the way open and hold it there for a few seconds . . Repeat this process maybe 3 times with each jet change so you get a feel for it.

As mentioned, see if it burbles/sputters even the tiniest bit when doing this . . If it does, the main jet is too big.

If it just feels like it runs out of fuel and hesitates for a second and THEN accelerates, the main jet is too small.

After doing this test but BEFORE changing jets, remove the spark plug and take a photo of it . . Do this after every jet change so you have an additional bit of info to help determine what is going on.

Clean the spark plug with a brush and carb or brake cleaner after every test.

As mentioned, change jets by one size at a time UNLESS, the plug is totally black OR almost totally white with no black anywhere on it . . If it looks like either of these, you can change the main jet by 2 sizes to save time.

One you get it to run properly with this test, the fine tuning can begin and will be easier.

Lite blue smoke is oil . . Dark gray to black is excess fuel which is too rich .

[YTZ drew]

I have a 200x with the XL185 head, cam, and jug, an XR185 10:1 piston (63mm), stock 200x exhaust, stock carb, stock airbox with a uni-filter in it. Running a 40 pilot, and a 108 main. Mine almost needs a 42 pilot (idle mix screw is 2 turns out) but the main is perfect. Everyone else's jetting tips are good, and should get you in the ballpark. Good luck!