Jump to content
Sign in to follow this  

General jetting practices for a big bore kit?

Recommended Posts

Hey everyone,

So I bought a dirtbike for a good cheap price in the hopes to fix it up for a bearable backup / buddy / camping bike. It's a 1992 Suzuki DR 350, that supposedly has been changed to be 380cc. After riding it, I'm inclined to believe them as many does this thing BOOGIE and have some serious tractor like grunt.

Anyways, the carb setup was seriously mucked up. The float level was way off (fixed), the needle and seat were toast (replaced) and the pilot jet was two sizes over stock.

When kick it, it was definitely too rich, and still is somewhat. After crashing I have to dump the float bowl in order to be able to kick it over again, and it's still not very happy...

When it does get running, it's pretty happy, but man is it unhappy and unwilling to start.

There could be more issues at hand, but I'd like to know what the general practice is behind adjusting pilot jets on big bore kits. I have been told a size smaller is actually standard as the change in CC's is actually creating more vaccum and thus pulling more air.

Share this post

Link to post
Share on other sites

Yeah, adding a big bore can require leaner jetting because the engine is 'sucking in a larger gulp'.

Do the pilot test/fuel screw adjustments

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 rpm, but go by the spec in your manual.

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this