Pop pop pop goes the xr650r

I still have some backfiring at 8500 feet, 2nd gear, high rpm throttle closed. That means I need to back the fuel screw out till it goes away, and if I get over 3 turns out, I go to the next size pilot?

No, it means either you have an exhaust leak or a non-functioning ACV coupled with a pipe with little to no muffling.

You set the fuel screw for a perfect idle and leave it alone. You do not try to make the idle circuit do what it is not intended to do.

Nearly new fmf q4, no leaks, and isn't the fuel mixture screw totally different than the idle screw?

Fuel screw and idle speed screw are different.

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.

Do not try to adjust the fuel screw to get rid of popping. You overly richen the pilot circuit and make the bike a bear to hot start.

Your pipe is considered an open pipe and not as efficient at quenching decel pops. You stock system did pop, just you could not hear it.

Nearly new fmf q4, no leaks, and isn't the fuel mixture screw totally different than the idle screw?

My fmf pipe would leak at the joints on my xr650r even though they looked solid.

Create an account or sign in to comment

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

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now