Ask Dr. Dirt: Smaller Carb On Honda CRF450

Dirt Rider’s Chief Physician AKA Dr. Dirt is Scot Gustafson. If you have a question about bike maintenance, set-up, tuning, or if you need help diagnosing a problem or just want to know how something works, send your question, along with your first and last name and your city and state, to We may edit your letter and might also use it in Dirt Rider Magazine’s letters section.


Dr. Dirt,

I have a 2007 Honda CRF450 and the hot start is stuck/broken. I want to buy a new carb just to fix the whole issue. The 2007 has a 41 mm Keihin carb. I found a different carb that I can buy from another Honda CRF450 from 2003. The thing is that that carb is 40mm instead of 41mm. Will it work? What other things would I have to do if I buy that carb that is 1 mm smaller than my stock one? Is it worth it?

Thank you very much,

Nelson Merello

Before you go out and buy another carburetor you should be able to remove the stuck hot start plunger by drilling a small hole in it and turning it free with an easy out. Some penetrating lube and heat around the area (with the carb removed and gas drained of course) will help. If this fails, the older carb will work on your bike. For optimum performance you will want to fine tune it with some minor jetting (fuel screw, main jet) but you will find that the smaller carb delivers smoother power. The larger carburetor that was spec'd on your bike delivered more top end power and was finicky to jet in certain situations. Finally, don't forget to lube your hot start cable from time to time.