Crf450x hot start 'weirdo' !

Hi guys, have just joined what looks an extremely great forum after having a strange problem arise in my 450x (2005).

Haven't owned long, been on a few rides with no problems except the last, where I had some starting issues after stalling (!!) including occasional backfire/pop, which I put down to reinserting the 'baffles'.

Took the bike to a friend as throttle was sticking open from about 1/2 way.

He replaced:

Throttle cable

Clutch cable

Hot start cable

Clutch lever assembly

Since these changes, the bike will start cold but not hot.

Clean the plug, start for a few minutes and take out and the plug is black/sooty. Running rich.....

Start it with choke engaged (run even richer) and the bike starts. Kill the choke, bike dies.

Any ideas please guys ??

Cheers, Paul

Usually, a clogged pilot jet causes this. I'd start by cleaning it. However, since the hot start cable was replaced, it's possible it's not closing fully, causing a lean condition. Operate it with the bike running to make sure it's working by listening for a noticeable rpm drop when the lever is pulled.

These cures assume no other changes/adjustments have been made since it last ran well.

Pull and rebuild  carb. Its something that you will want to familiarize yourself with as a owner. 


While your at it replace leak jet with #50 and install the updated accelerator pump from along with the merge racing accelerator pump spring. 


Grab a 6 pack and get at it! 

I agree with the others, but also want to kick in:


1. On a modern 4 stroke, it 's tough to read a plug reliably.

2. Instead, look at the jetting thread at the top of the forum.  Find a setup that's close to what you have and then go with that.

3. On the hot start, check lock to lock that the cable is not getting pulled.

4. If your carb is tuned right, pulling the hot start slightly will cause the RPMs to rise.  Pulling it all the way should kill the motor.   This is because on a well tuned carb, you'll be slightly rich on the slow circuit.

5. You should find on a hot motor that you need the hot start to get it going.


On the carb rebuild:


1. Just replace the slow jet....their too tough to clean properly if really gunked up.

2. Use fishing line or guitar string to clean the passages.  The one above the slow jet over to fuel screw is tough.

3. Don't use carb cleaner; it will melt plastic and rubber.   Brake clean is OK.

4. Check the slide seal carefully and if you have any doubts, replace it.  A wave or slight nick is all it takes to give you some weird idle problems.


This is a good overview of the carb rebuild process:




Many thanks for your replies.

Carb is being stripped,cleaned, an rebuilt as per recommendations.

Will update on results.


Or not

Hot start is probably stuck partially open inside it's cavity, forcing you to use choke 


Remove it, polish the cavity, and make sure the nut is not cracked, which releases spring tension making it leak again

Make sure the slide was not put in backwards.

Also, check the valves. Many times a hard start and stalling can be caused by valves closing up.

My two cents


Make sure the slide was not put in backwards.

Also, check the valves. Many times a hard start and stalling can be caused by valves closing up.

My two cents



What he means is make sure the slide plate was not put in upside down.

The hole goes on the bottom.

I've done that! Very frustrating!

exactly. Thanks Krannie

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

  • Similar Content

    • By cmckay4
      Anyone having or had problems with the starter not turning over easily on the 450X???
      I have read about this problem on CRF's ONLY and not seen anyone come up with a fix.
      Problem being is that you think there is a problem with the battery, when infact the battery is OK. When you have a good fully charged battery in the bike and press the e-start, it turns over very slowly.
      I have checked the electrical circuit and it seems OK. I am wondering if it might be the starter itself - any suggestions?????
    • By Dust_Devil
      I'll start, if you see the "Fox Sticker" out in the Dez you know you crossed paths with ol' DD.....

    • By JohnnyAirtime

      Guys and Gals...
      Post your links to any of your pictures in this thread....
      (proper placement of pictures is in the "picture" threads)
      Post your links to any of your ride reports in this thread....
      (proper placement of your ride reports is relative to the area you live/ride)
      NO TECHNICAL issues, questions or comments in this thread.
      (it'll be deleted to keep this thread clean and with only the above listed).

      Please follow this format;
      Ride Report: Gorman, Tuesday 4-17
      (by Skoot22)
      Pictures from Ride: Baja 4.14.07
      (by Johnnyairtime)
    • By dakynz
      Guys, because of a member ban the jetting database has been removed. Let's start a new one to get this great information created into a sticky for everyone's benefit. Please stick to the information below.
      Temperature Range:
      Smog (AIS):
      Airbox Modified Top:
      Airbox Modified Sides:
      Backfire Screen:
      Air Filter:
      Pilot Jet:
      Main Jet:
      Needle Clip Position:
      Fuel Screw Type:
      Fuel Screw Setting:
      AP Modified:
      Leak Jet:
      AP Cover:
      Exhaust Type:
      Exhaust Modified:
      Pink Wire Modified:
    • By 2can
      I've studied the Jetting sticky's but still have a number of questions:
      I bought my '06 a couple of years ago and had a smog block-off kit installed and the pilot jet was upsized to a 48 and the air box opened up top only. The bike has appeared to run a little lean at times and got hot on a couple of occasions in elevation and on one other occasion. I'm looking at upping the main jet to either a 165 or 170 to help with the possible lean condition. The bike currently has a 142 main jet in it which I believe is the stock size. It looks like the most widely used Needle Jets are the JD Red, but it doesn't look like they are sold separately, is this correct?
      I was going to also get a 45 pilot to have options for further tuning. My assumption is that every bike is a little different and that where you ride is going to have a lot to do with jetting. We ride mostly, mojave desert, baja, and southern Utah with elevations from sea level to 9,000 feet but the higher elevations are generally for a short period as we generally continue through the peaks and head back down.
      I'm looking to convert from a stock exhaust to possibly an FMF Q4. I want something a little throatier but not overly loud and will provide a balanced flow of intake/exhaust.
      So can I get the JD Red Needle Jet by itself or can I get by using the stock and move the clip and be just as happy. I'm just looking to enhance overall performance of the engine and maintain reliability.