Jump to content

Pilot Jet...to Go Big Or Small, And Which Is Which? It's All So Confusing!


ridgrunr
 Share

Recommended Posts

I leaned out the needle and main jet on my '08 Yamaha powered SY250r, but never messed with the pilot. I'm getting ready to some winter time maintenance and thought I'd mess with the pilot.

Bike runs great, but I get some back firing occasionally while starting the bike between sections, and the bike seems to use more fuel than most of the others, so I wonder if it's running really rich with the stock #40? Looking at what's available, it seems there's a #38, and a #42, #44 and #46.

Now my pea brain tells me should lean it out (I operate between 3000 and 7000 feet) and the #38 would be leaner than my stock #40, but my buddy swears it's different with pilot jets and the bigger the number, the leaner they are, which is why they offer more bigger (leaner?) jets than the one smaller (richer) jet.

Anyone know what the scoop is with the pilot jets?

post-57-0-01688600-1416589355_thumb.jpg

Edited by ridgrunr
Link to comment
Share on other sites

 

 
 
 
 

It is a bit of a mystery. On the modern bikes we go larger at altitude. It`s been this way awhile. Although 7000ft is not real high. It has to due with atmospheric pressure, totally backwards of reason. The TK was a strange cab though.

Where were you when billyt was wittering on about his 4rt ?

  • Like 1
Link to comment
Share on other sites

 
  • 3 weeks later...

Tk pilot jets are strange. There is a bore hole but also 3 or 4 holes at the top. Its these 3 or 4 holes that increase in size with bigger number jets. I think more fuel is sucked through the bore hole as these holes increase in size. Definitely bigger number is richer on tk

Link to comment
Share on other sites

 
  • 2 months later...

How did your jetting issue go? My bike also seems to run on the rich side. I replaced the needle and nozzle with new stock items, needle on the top clip, I am running a 38 pilot and still feels a bit rich. I even fitted new reeds. I also use almost double the fuel as the rest of my crew.

A bit annoying to always be worried about running out of gas.

Link to comment
Share on other sites

 
 
 
  • 9 months later...

With some help from lineaway, I finally got the SY250R jetting sorted out.  Stock jetting was 40 pilot, 140 main and the needle on the meddle clip.  Am now running a 42.5 pilot, 135 main and needle clip is set on the second from the top slot. Air screw is out 1.5 turns.  Bike starts second kick, and the air screw.  Make sure you use TK (Teikei) jets.  It makes a difference.  The Delorto jets fit in there, bu they seem to be calibrated different.

 

Link to comment
Share on other sites

 

The first things to check before deciding to alter your carburetor jetting:

1. Is the air filter clean and the airbox intake unobstructed?

2. Is the carburetor and the jets and all passageways clean?

3. Is the carburetor float level correct?

4. Is the air boot from the air filter tight to the carburetor and not leaking.

5. Is the intake boot from carburetor to cylinder (or Reed valve cage) tight, sealed and not leaking?

6. Is the exhaust gasket properly sealing and any exhaust pipe connections tight and not leaking?

7. Is the proper heat range spark plug installed (and is it new or clean)?

8. Throttle cable runs free and slide returns to the carburetor stop?

Until you can confirm (and there are more but this is a start) that these basic needs are met you can waste a lot of time and can become extremely frustrated chasing what symptoms you think might be improper jetting (and then "correcting" that) and getting even further from the goal.

Backfiring may make you think the jetting is lean but it could be an air leak in the Reed cage gasket or a low float level or your throttle cable is hanging up.

You might look at your spark plug and think it's running rich but it could be a dirty filter, a pinched air boot or float level out of whack.

Back off the throttle and it backfires? Could be a bad exhaust gasket or air intake leak or partly blocked pilot jet or?

Most carbs have some pretty well thought out jet settings for sea level and are basically a good place to start. Float level plays a larger then you might think influence on rich and lean conditions.

I have purchased bikes that have had the float level altered as much as 7mm from stock which throws your whole attempts at jetting out the window until it is back within the proper tolerances. Make sure it's correct.

This site has a good information page on jetting sequence and altitude / temperature correction.

http://jetsrus.com/FAQs/FAQ_rejet_elevation_and_temperature.htm

http://www.jetsrus.com/FAQs/FAQ_rejetting_101_how_to_rejet.htm

I'm not associated with this site but have bought many jets and parts from them and have been very satisfied with my purchases.

Happy Jetting!

Link to comment
Share on other sites

 

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
  • Create New...