Problem I need help with. plz reply soon as possible

6 replies to this topic
  • sourant75

Posted September 28, 2011 - 05:53 PM


Okay I have a yz450f an its basically stock. The only performance mods are a pro circuit pipe and other than that, the motor, transmission, valves are all stock. A local dealer sold me a 42 mm fcr carb. I couldn't get it jetted right to save my life so I took it to the shop tht sold me the carb an asked them to jet it. They can't do it either. It's popping on decel, an has a slight bog when accelerating. Is this carb too big for the stock 04 yz450f? The shop has had it for 2 months. Should i switch it out for a stock size carb? I think this big one is giving it way too much fuel. Thoughts please? Thank you

  • Theeebalz

Posted September 28, 2011 - 05:58 PM


how accelerating? like rolling or snapping? rolling and your lean as possible there's probably no chance unless you find another way to lean it up alot. if it's a snap then it might just be your acelerator pump. have you messed with the fuel air screw at all? kinda helps sometimes.

  • sourant75

Posted September 28, 2011 - 08:46 PM


That's feedback from the dealer. I'll get it out ASAP an give a better description. But is it in heard of to have a big 42 mm carb on a stock motor like that?

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  • HNR_Engineer

Posted September 29, 2011 - 12:56 PM


Had that on my one to. Two things it took to solve it.

The rubber manifold that attaches the carb to the engine needed to be replaced as apparently this gets worn from vibrations during riding and looses its seal.

Then the pilot screw needed to be adjusted once that was corrected.

Would not do harm to check for leaks in the exhaust system as i also had a case where the rivets on the stock silencer came loose during a race and cause a huge problem under braking with zero throttle.

Could also be something very simple, if the breather from the head gets blocked it can also cause some strange engine things to happen. Only seen this happen with Yamaha.

  • 00yzryder

Posted September 29, 2011 - 03:58 PM


I know mine has alot of popping, after alot of research seems like there are a few causes of excessive popping. 1 is the jetting is off, 2 and exhaust leak. I should be getting my carb back from Zip Ty racing and ill let you know if that mod helps with the popping at all.

  • grayracer513

Posted September 29, 2011 - 06:44 PM


The Zip-Ty mod should help with the bog, but it doesn't address the popping.

The stock carb size for your YZ450 is 39mm. However, a 42 is stock on CRF450's, and Ron Hamp bores carbs even bigger than that for the YZ450 when he mods the engine.

A couple of things...

Carbs that are too big do not give too much fuel, they give too much air. You have a certain amount of air being demanded by the engine, so if that same air is drawn through a larger hole, will it go faster or slower? Slower, of course, and that reduces the strength of the vacuum signal at the main nozzle, which means that you'll need a bigger main jet to get the right amount of fuel.

Popping on deceleration is NORMAL for a healthy, properly jetted YZ450. SOME popping, that is, some of the time. It should be set up about a third turn leaner than what it takes to get it just to the point where it pretty much never pops. If it's going off more than that, check for an air leak in the exhaust first, because you'll never fix a leaking pipe with a pilot jet. If it looks OK, then proceed to get your idle dialed in. The idle circuit is the only element of jetting that has any significant effect on this. Float level can effect everything, so check it.

Always start by getting the idle pretty close to right, then get the main jet set correctly, then tune for the needle. You may have to go back over each of the steps you've already covered as you go, since everything overlaps to an extent. Here's some helpful links from Common Threads:

Confirming the pilot jet size

Guide to jetting without a dyno

If the carb came from another 450, the needle and pilot air jet should be pretty close. If it came from a bigger bike, maybe not so much. If the donor engine was significantly larger, you might need a needle with a smaller upper diameter.

The pilot air jet is responsible for keeping the jetting at less than 1/6 throttle accurate across the rpm range. If, for example, your idle and low rpm, low throttle performance is perfect, but your bike stumbles and shoots ducks when you crack the throttle a bit at 6000 rpm, your PAJ is likely to blame.

Always check all the basic stuff first, like intake boots, float level, etc. Don't overthink, either. There's an old medical saying: "If you hear hoof beats, think horses, not zebras."

  • sourant75

Posted September 29, 2011 - 08:46 PM


Thanks! And like I said ill ride it soon and update to u guys. It's always popped a little, I havent heard it run yet after the shop fixed an exhaust leak and adjusted the valves. They say it runs good but he didn't want to be done cuz it still popped and wast fixed to his standards.... He also said there is a slight hesitation on acceleration. I'll order new intake boots and try an fine tune the jetting a little more myself. Thanks for all the replys. I'll let u kno Wat happens cuz I'm riding it for the first time since I got it back tomorrow.

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