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exmarkmetroguy

crf 450x rejetting

15 posts in this topic

i have a 2007 crf 450x. i love the thing to death but it is simply to loud for my california neighbors. so i have the stock tip but what do i have to do about rejetting the carb. ive never done this. i am very mechanically inclined and with a little advice and help im sure i can do this. so my question is what jet do i need and basically what is the process? i also have to check my valves soon so what are the clearances for both intake exhaust valves? thanks guys

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What pipe, airbox mods, and jetting do you have now.....

Stock pipe and snorkle only removed on air box would jet like this:

150 main

42 pilot

55 leak

NCVS needle on 2nd clip

(0 - 4000ft)

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Every bike is different, so no jet recommendations from me. I would get the main jet right first. Run W.F.O down the road in top gear, hit killswitch and pull in clutch at same time. Pull spark plug and read. Tan to light brown is what you are looking for. White to light tan is lean,dark brown to fluffy black is rich. Pilot jet is lean if it won"t run off choke after aprox. 30 seconds(may start and run fine with no choke if rich rare! rare!). If bike responds well right off idle your pilot is probably good. Once those are right play with needle clip position for best transition between the two. You may need to time/or mod accel pump after you work out baseline jetting.My( stock gutted exhaust ) bike was too loud initially,but my Q4 got it right.

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You can't do a plug chop on a four stroke with pump gas. It means nothing. Too many gas additives, and fourstrokes vary in tempreture too much for a plug chop less than one mile.

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Just checked my valves, so the clearances are fresh in my head..

Intakes 0.16mm +/-0.03mm

Exhaust 0.28mm +/-0.03mm

And the decompression is 0.35mm over the right exhaust valve measured clearance, FWIW.

Edited by billybaru13

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My two cents.

Make all your airbox & muffler mods first and put in a new plug. You then need to "isolate" the effects of the different jet circuits.

Find a place to ride 3/4 to WOT and hold it a few seconds without hitting the rev limitter. If the bike doesn't have a miss, then the main jet is close. If it misses, you are either lean or rich, Experienced folks can tell the difference. Otherwise, swap main jets a couple numbers longer and shorter and ride again at WOT. This will take a while the first time.

Next, ride the bike to ensure it is at op temp. Adjust the idle to about 1700 rpm. Adjust the idle mixture screw for highest idle rpm. Should be between 1 to 3 turns out. If it's more than 3 out, the pilot jet is too small. If it is less than 1 turn out, then the pilot is too big. Reset idle to about 1700 rpm.

Next, find a place where you can ride and hold the throttle constant at 1/8 to 1/4 open. If it misses, you are either rich or lean on the needle jet circuit. Dropping the needle (raising the clip) leans the circuit. Raising the needle, (dropping the clip), richens the circuit. Again, if you can't tell a rich miss from a lean miss, try each position on either side. Again, this will take a while the first time.

Last, is the accelerator pump circuit. This only has an effect when you are turning the throttle. That's why the previous tests have to be at constant throttle. You can try smaller leak jets(#50) which add more fuel (richens the circuit). Ride it with different leak jets on a trail to see which one give the most crisp response. You can also adjust the timing of the squirt and do the o-ring mod or Merge Racing spring on older carbs.

The JD jetting kit will start you very close for your conditions, if you study and follow the directions. You can also call for help. But it's expensive relative to just buying a few jets. If dollars matter, follow the advice of the settings from the previous posts as a starting point. Again, everybody's riding conditions, carb condition, muffler, airbox, and gas are different, so it's difficult to just do it well with a recipe and not test ride.

Edited by Sofiedog

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Krannie , I've always done my initial main sorting with a plug chop. I did it with my 2005 450x,and my friend's 250x. Short of a dyno, our bikes are as close as the average guy on this forum will ever get. I useually pin it in third and upshift( as it taps the rev limiter) until i get close to rev limiter in 5th. Then i chop throttle,pull in the clutch, and hit kill switch at the same time. Then I pull the plug(after seat and tank) and read. Short of installing an o2 bung and sensor or strapping to a dyno do you have a better way?

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Krannie , I've always done my initial main sorting with a plug chop. I did it with my 2005 450x,and my friend's 250x. Short of a dyno, our bikes are as close as the average guy on this forum will ever get. I useually pin it in third and upshift( as it taps the rev limiter) until i get close to rev limiter in 5th. Then i chop throttle,pull in the clutch, and hit kill switch at the same time. Then I pull the plug(after seat and tank) and read. Short of installing an o2 bung and sensor or strapping to a dyno do you have a better way?

Hey, whatever floats your boat. Who am I to tell you what works for you.

Pump gas has additives in it that change the color of the burn, having nothing to do with jetting.

So, if you use 'Thrifty' gas with Ethanol, vs Texaco or Chevron with Techron, you will get a vastly different plug reading.

And it will be different from station to station, pump to pump, day to day.

A 2005 CRF450X comes with a 142 main.

Uncork the intake and exahust you and go to a 160/5.

Add a full header and pipe system with the top cut off the airbox, and you can go to a 165/70.

Go to a higher/longer lift cam, and you can go up to a 175. Radical port and polish, and you can go to a 180.

I have never done a plug chop on any four stroke. It's not like a two stroke where jetting is ultra critical in the main and needle to get power and clean running. We have the fuel screw, the apump, the air jet, and the exhaust header length and diameter to include in our fine tuning. Put too small a main jet on a four stroke, and it 'tells' you, it's too small: WOT is slower than 7/8 throttle.

Yes, if you pull the plug on a bike that has been running lean for a few rides, and never been lugged, it will look lean. But it will also run lean, which is to me a faster way of tuning.

It's not that complicated.

Now, if you are talking about a XR650R or DRZ400 pumper conversion, and you have no reference to start from, an simple AF meter and a flat section of road, and a steady throttle hand for 1/8, 1/4, 1/2, 3/4, and WOT, then that is faster, better, and quicker.

Especially a DRZ400 with a long intake tract, and low compression, and long exhaust exit, it's a real pain in the ass to jet by seat of the pants.

Edited by Krannie
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Krannie your main jet recommendations look spot on. My airbox top is completely gone,airbox side holes cut, backfire screen removed, and I have a full FMF system. I ended up with a 170 main by my methods(tried 165,170,172). I was running a 165 prior to the exhaust system change(already modded airbox and stock gutted silencer i inherited when I bought the bike) and the bike pulled harder at 3/4 throttle than wide open. I wasn't being a jerk, just always looking for new/better ways.

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Krannie your main jet recommendations look spot on. My airbox top is completely gone,airbox side holes cut, backfire screen removed, and I have a full FMF system. I ended up with a 170 main by my methods(tried 165,170,172). I was running a 165 prior to the exhaust system change(already modded airbox and stock gutted silencer i inherited when I bought the bike) and the bike pulled harder at 3/4 throttle than wide open. I wasn't being a jerk, just always looking for new/better ways.

You were not a jerk. You are not a jerk. You might be a jerk, but that remains to be seen!

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One of the benefits of this forum is that many people have had these bikes on a dyno, and those results have been passed along.

A 165 main at sea level in the summer is proven over and over on a dyno. At 6,000 feet, a 158. There's not much mystery left.

Every bike I have tested, which is three 450X's, have been identical. Zero variation between different bikes with different exhausts, one of them the stocker, one FMF Q4, and a T-4, and all had an uncorked airbox.

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very close to JD kit recommedations (they state airbox should be opened up):

#165 main for seal level to 4000 ft.

#162 for for 3000 ft to 6000 ft.

#160 main for 6000 ft to 9000 ft.

#155 main for over 9000 ft.

Edited by Sofiedog

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The instructions on my JD kit for 2008, under 6000 ft and stock exhaust said: 165 main, red needle, 3rd clip, 1-1/8 out on fuel screw. (stock pilot 45) They do recommend opening up the airbox. I've since gone to an FMF Q4 and had to go to around 1-1/2 out on the fuel screw.

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You were not a jerk. You are not a jerk. You might be a jerk, but that remains to be seen!

You may be a jerk too, but that remains to be seen(In my opinion, which means as much as yours!). Enough said.

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thanks for all your help guys i really appreciate it. my bike has airbox mod stock exhaust with an aftermarket tip and i today just checked the valve clearances. ive decided to leave it the way it is because it is more enjoyable! :smirk: but thanks so much

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