YZ450 Carburetor Issues

hello everyone!

hey I'm trying to help a buddy of mine get a few little problems fixed on his 2003 YZ450F. Now its kinda tricky cause we don't know everything about the bike( he got it used) But to start it has been bored out to like 470-480cc I don't remember exactly. It has a FMF Factory 4.1 full system and numerous other engine upgrades. Honestly its got alots of fancy parts/upgrades but it's still a POS. So here's the problem or at least one of them hahaha(I'm glade this things not mine!) You give it gas and it goes, it goes fast but when you let of the throttle it doesn't really decelerate it keeps going like you're still giving it gas. I'm really only used to my own bike which slows down when you let off the throttle('06 CRF450R) so it just seems kinda weird. We have also been chasing some exhaust leaks as well. It pops and backfires when you let off the throttle and I'm sure the packing is toast.

We have a Haynes manual that I've got to say is kinda weird but anyhow

Here's what we have done so far:

-checked the jetting and needle -- we brought it into the one local shop and talked to "The carburetor guy" and tried his recommendations for the bike/exhaust/altitude - Colorado 7,000-10,000 feet

- replaced the choke(plunger starter) cause it wouldn't stay pulled out on its own while starting the bike

- installed a new exhaust gasket - cause there wasn't one

- cleaned the float chamber out cause it was full of dirt

- checked the clearance of the little.... ah hell I don't even remember what it does but its a little adjustment screw where the throttle cables attach to the carb.

- checked the engine idle, adjusted it to what the manual says and then fiddled with it some more and cant get it to idle right either way

- checked and adjusted the throttle free play

- applied a small amount of exhaust sealant to were the head pipe meets the muffler

- cleaned and oiled the air filter

- removed the Scott steering damper(not that this has anything to do wit all this but he wanted it off)

- changed the fork seals and oil(fork works a lot better now hahaha)

- we HAVE NOT checked anything in the clutch besides lever free play cause we have been just to lazy to get a new gasket for the clutch cover - I dunno why they didn't use an O-ring for that but ohwell. And with that it seems to be workin fine it engages and disengages fine as far as we can tell.

Alright I think I've covered everything any suggestions or recommendations would be great.

Thanks. :excuseme:

so ur saying basicly it dosent have engine braking? rekluse clutch? tight valves could cause popping, its probably lean jetting though, did you adjust the air screw at all?

well kinda? I'm not super familiar with and auto clutch I'm pretty positive it doesn't have one. Honestly as for the valves I don't think I'll go there because its not mine. I'm fine with doing all the engine work on my own bike for 2 reasons. 1 its mine and 2 its a Honda and from watching my other friend work on his WR250f it seems like a pain in the ass. I'll let him take it to the shop for that one.

So imagine this riding on some single track that's kinda open almost clay like with pinyon-juniper and oak brush. There's lots of ups and downs nothing to crazy but you can really find whatever you want. So you get to a tiny bit of downhill where you need alot of rear brake so you let of the gas and pull the clutch in and it feels like it still wants to keep going. Sometimes almost like the power delivery is kinda sporadic. Sorry this ones kinda hard to describe.

We brought a pretty good amount of jets(pilot and main) out with us on sat for some testing. We couldn't really get in dialed in this things being strange.

I'm not super familiar with the air screw I assume we did as we started by setting it up per the manual?

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