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

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