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Help understanding choke

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Here's how I understand things, and here's why I ask:

My 2005 CRF450R when you pull the choke (full) it decreases the air, causing the mix to be richer, causing the idle to be higher, create more heat all in expectation to warm the engine up.  Even if the bike is totally warm and you've ridden it for 15 minutes, if you pull the choke, the same thing occurs (no reason to pull it, but just stating how it works for me)

My 2005 CRF150F when you select full choke, the bike would never start.  No choke or 1/2 choke seem to make no difference.  If the bike is fully warm from riding and you go full choke, the bike stalls and once again wont start.

My 2006 TRX400EX (quad/atv) has the same setup as the CRF150F and behaves exactly the same.  Full choke stall, half choke and no choke seem to behave the same, no increase in RPM.

I have taken the carb off my CRF150F and completely cleaned it.  The pilot jet is clean, the main jet is clean, the air pipes are clean, everything looks good to go.  You can clearly see how full choke closes the valve completely and 1/2 choke leaves a little open, and no choke its right in the center, for maximum air flow.  So the choke itself is "choking" the system.  However, it seems why it just stalls out when choke is on is no air can get in?  Just guessing... totally not sure.

Are the carbs technically of a different design between the CRF450R and the CRF150F/TRX400EX?  Or should I expect the same behavior from the 150F and 400EX as I get with my R?  If so, what could the root cause of the problem be?

Edited by Agr0

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Technically, the CRFR carb (FCR) does not have a 'choke'. No butterfly or slide is involed when you pull out the knob.  It has an 'enricher' circuit.

What happens is more air and more fuel at the same time, at a specific ratio based on the 'power jet' in the carb.

It's like cracking the throttle, but much richer.


There is no reason to think that not needed the choke to start is a bad thing.....

Most bikes do not need choke to start, unless it is under 55 degrees; the cold air makes the density of the air higher, so the choke is needed to correct the mixture.

You can correct the mixture by adding more fuel and air (enricher circuit, like on the FCR), or by restricting air (like on the butterfly chokes on the low performance engines).


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The information on the enricher was really informative.  I have looked it up and understand a lot more.  Also thanks for the extra info on the temp, etc.



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