Need pilot advice. '08 CRF450R at 5000ft.

Hello world,

I am rejetting my '08 CRF450R for altittude (4200-6500ft). I have decided, based on this forum and the owners manual on a 160 main jet and 2nd clip from the top on the needle. Stock is 178 main and 3rd from the top on the needle. My question involves the pilot jet. It seems no one changes the pilot but my bike has an intermittant bog when held at steady RPM around 1/2 throttle. I was told that this is the pilot circuit being to rich. The fuel screw is all the way in. I have a 40 pilot jet to replace the 42 stocker but I am curious, should I put it in while the carb is apart? Or will the main jet and needle changes possibly correct this problem? Any experienced help would be appreciated as this is my first 4 stroker and I have been out of bikes for over10 years.

No, who ever told you that is wrong. The pilot is for idle only. You set the fuel screw for a good idle and leave it be.

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.

Your 'bog' might be the AP but you say it is intermittent and only happens at 1/2 throttle?

Please be more definitive so I can more accurately respond.

First of all thank you.

Ok so the bog is strange. It isn't just half throttle but more that it doesn't bog out coming off of idle or lugging and it doesn't bog when wide open. I think a more accurate way to say it is that if I was to hold, say, 5000 RPM for a minute just crusing down a fire road or something it would cut out regularly every few seconds. But when doing general off gas/on gas riding I don't notice it. So it's a steady intermittancy, if that makes sense.

leave the stock needle on clip 3. 160 main jet is fine and so is the stock 42 pilot jet.

if you are going to do steady throttle road riding unplug the tps.

I am guessing here. Does TPS stand for Throttle Position Sensor?

Anyways thanks Eddie, I don't really do steady throttle riding, I just noticed it and started to experiment. I thought something was wrong and then I heard the thing about the pilot jet. Good thing I ran out and bought a 40 huh?

So here is the plan: I am going to jet according to Mr. Sisneros' guidance and then spend some time fine tuning along the lines of what William1 explained to me.

Thanks guys,


Right about the TPS. You'll do fine.

leave the stock needle on clip 3. 160 main jet is fine and so is the stock 42 pilot jet.QUOTE]

My 08 is set up like this and runs great. 5000' and up

Could you guys explain why it's good to unplug the TPS here? I don't question your advice, just would like to understand the reasons for it. Thanks!

To ensure a bad or misadjusted TPS is not causing the problem. Disconnected, the bike is in a default mode.

I see, thanks. So this is for troubleshooting only or would you ever want to leave it disconnected for good?

On most MX bikes, leaving it always diconnected will reduce max power/performance.

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