Hi guys!
I have read that if the pilot screw has to be turned more than 2.5 turns out for best idling, the next smaller size pilot jet will be needed. I have my pilot screw set 4 turns out, because I ran too lean. But what is the logic of this? I thought the leaner you run the bigger jet you will need.
BTW I have Suzuki LTF 250
Thanks!

 

Anything over 3 turns out on that carb typically doesn't make that much more of a difference. If you are using 4 turns out, you ought to try a larget jet. You are correct...leaner calls for larger jet.

 

I think it depends where the screw is located. If its on the engine-side of the carb, then its called a "pilot screw" and turning it out richens the mix. Therefore, if you turn it out too far, you need a bigger pilot. However, if the screw is on the airbox-side of the carb, its called an "air screw". In that case, turning the screw out leans the mix. And turning it out too many times requires a smaller pilot jet. Many of Mikuni's aftermarket roundslides have air screws.



Copied from: http://www.mikuni.com/pdf/vmmanual.pdf

 

I spoke before I finished reading...I am used to working on my Eiger, so I was talking about the Keihen carb. Not sure which carb you have on the LTF250...sounds to me, based on JustRandy's post, you may have the Mikuni carb, so I would go with his info...sorry for the quick trigger

 

Thanks both of you for the useful information.
the LTF250 has a Mikuni BS29 carb and the pilot screw regulates the fuel (not air). It is located on the engine-side. So it makes sense that I should go for a bigger jet.
But what could happen when I keep this jet opened for 4 turns? May it cause some trouble?
The factory setting was 3 ˝ out but then it popped in the exhaust pipe when let off from WOT. That was the reason why I turned 4 out. Since then there has been no popping.
Another issue is that I am going to change the main jet too, as I would expect more HP on WOT. Probably my bike is running very lean. The place where I ride (southern Slovakia - Europe) has condition which prove this my speculation.
Altitude: 500 ft above sea level,
Humidity: 50%
I think these bikes are jetted from factory for some other conditions (mountains, etc.)

 

is your bike stock or does it have any mods? these will help alot with answering question about jetting your machine.
only 2 stroked have an air screw, 4 strokes have a fuel screw.
and your jetting depends on throttle position, if you had a popping upon deceleration or 0-1/4 throttle (which your pilot controls), your pilot was too small. if you have popping from 1/4-3/4 throttle, you need to richen up your needle. from 1/2 throttle to full is controlled by the main jet. so, if your plug is lean looking, you most likely need to fatten your main jet.
remember, troubleshooting jetting is based on THROTTLE POSITION!
here is a quick link about the working of a 4 stroke carb, it is a basic instruction, but it can be applied to most all 4 stroke carbs...hope this helps!
http://www.atvriders.com/articles/jetting.html

 

My bike is stock without mods. It used to pop upon deceleration. Always when I drove at maximum speed and then let off the throttle it popped into the exhaust. I solved it by setting the pilot screw 4 turns out. Since then I haven't had any problem in 0-1/2 range of the throttle. Lot of power and no popping has been in this range. The idle has been always fine. The only thing is the 3/4 - full range. Although there is no popping, but I would expect more power compared to other LTF250. Once I went riding with a friend´s LTF250 to the mountains and it drove like crazy at full throttle. It was at 2000 ft above the sea level. I live at 500 ft. So I am almost sure that my bike is jetted leanly for the environment where I live and I will need a bigger main jet for better power at full throttle. The question is if I really need to change also the pilot screw if it runs fine at 0-1/2 range with 4 turns out? And if I will need to do something else except changing the main jet (changing the needle position? air filter type? exhaust pipe? Spark plug?, etc...
Thank you for the useful link...

 

For the record, 2 strokes usually have the air screw, while 4 strokes usually have a fuel screw. Running the fuel screw that far out is an open invitation for it to fall out. Going up one size on the pilot allows more fuel, so the fuel screw can be leaned out a bit. Conversely, if the fuel screw can be left most/all the way in with no noticeable effect to performance, then you need a smaller pilot.

 

i would change the pilot because like scooter said it may fall out. i am surprised that you need to mess with the jetting this much with a stock bike, regardless if you are only at 500 feet. i would check and make sure that there are no airleaks at any of the intake boots. the only real way to check your jettiing without an exhaust sniffer is to do a plug chop. it is easy to do, just takes some time, and you need a few brand new plugs. since you seem like you have your pilot worked out, you need to test your main. you will need a long straight run to do the test. start by warming up your bike. when it is warm, put in a new plug. start it up and run it FULL THROTTLE through at least third gear. when the gear is up, quickly pull in the clutch and kill the ignition. pull over, now pull the plug and read it. this will give you an idea of how your jetting is at FULL THROTTLE. if it lean like you describe, it will be very light tan or even white. if the test confirms it is lean, you need to up the main jet and repeat the test WITH A NEW PLUG until you get the main jet figured out!

 

Yes, the screw could fall out is about the only concern. A longer spring or the addition of some small washers may prevent that.

I would download and read http://www.mikuni.com/pdf/vmmanual.pdf Lots of good info and charts there. Just keep in mind the air/fuel screw differences.

Btw, Adrian, your English is pretty good! I would have never guessed you're in Slovakia.

The amount of vacuum seen at the venturi is what affects which jets you need (along with the density/humidity of air - as you already know). The exhaust, intake, airfilter, valve timing, compression,,,, etc, etc determines that vacuum. Even the same engines can require different jets if one of these parameters are different. One thing I would recommend is checking and setting your valve lash. Also, if your piston rings are worn or not seated properly, then your vacuum will be a bit lower; therefore, you'll need a bigger jet since the vacuum is no longer strong enough to pull the gas up thru the smaller hole. There are just SO many variables.... Carb tuning is an art as much as a science.

What I usually do is set everything lean and work my way richer. Its easier for me to tell a lean miss than it is a rich condition (since the rich condition usually just results in a power loss). The fact that plugs are black doesn't tell me if I need to adjust the needle, the main jet, or the slide cutout. Plug chops work if you have a lot of new plugs to use. Once they get black, its hard to read it a second time. So, if you just start lean, at least you know which direction you need to go and you'll not waste as many plugs. The problem with being lean is of course engine damage. So, be careful. Oh, and I've read that unleaded gas makes plug reading inaccurate.

Another thing to consider.... Does you needle look like this?

If so, you need a new one and a new needle jet.