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Selecting the correct Pilot Jet and setting the Fuel Screw

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  • William1
39,853 posts
Location: Virginia
Garage View Garage

Posted September 10, 2010 - 09:00 AM

This is probably the easiest jetting adjustment you can make. But it still intimidates some. The same methodology works for lawn mowers to carb'd Ferrari's, motorcycles too!

Fuel Screw/Pilot Jet
Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.
Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.
*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***
Turn the screw in until the idle becomes rough or the bike stalls.
if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.
If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.
Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.
If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.
If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.
If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 to 1,900 rpm, but go by the spec in your manual.

Be sure to keep notes of your settings. You may find you need to make a change, say if you're riding in the mountains or down at the beach. Returning home, it is a simple matter of referring to your notes to restore things.

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