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YZ450f ***? pleasehelp

By Atcr1,
in YZ400F/426F/450F/450FX

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Atcr1

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So I have a 2007 YZ450f I use just for the dunes. While away on a dual sport trip I had a new top-end put in because it was burning some oil. They put in new pistion,rings, gaskets, and stainless valves since my titanium ones were shot. Took it out today, fires up 1 kick but after idleing in the garage for just 2 minutes it starts to piss coolant out the overflow??? Never did this before, you could leave it running for an hour with no boil over. So I check the coolant and its still above the fins so I ride. Everytime I stop and give it a few revs within seconds out comes coolant. Its also popping upon decel like a machine gun, so its gotta be running super lean right? Only thing is last season I had it jetted and it was running great. I checked to make sure the choke and hot start are not stuck open. They didn't touch the carb so why now is there an issue. I guess my question is two fold. First is there anything they could have screwed up not including the carb to make it run the way it is? and is my engine being very lean leading to the coolant issue? or is that another problem altogether? When riding the bike pulls and seemsto have no fuel/jetting issues at all except for the popping. Please something to look for?

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grayracer513

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The unusual thing is that it was ever able to idle for two minutes in still air without boiling. But that part aside, I would look into the pilot jet, and/or the pilot circuit passageways. Some of the heating is not unusual for a brand new piston but that should pass quickly. The decel backfire, assuming the pipe is properly installed, indicates a lean idle.

Sitting for as little as couple of weeks can result in varnish deposits with some of today's fuels.

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Atcr1

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Do a quick search and you will find its 100% normal for a modern 4st to puke out coolant if you let it sit and idle.

yeah but after 2 minutes? I can ride for hours at the dunes, damn near at the rev limiter then sit on the top of a hill for 5-10 minutes with it running waiting for someone who fell to catch up and it has never boiled over. So doing it in my garage tells me somethings really wrong. Not to mention that it now sounds like a machine gun and is running super lean.

Between my KTM 530, Wr250 and the YZ450 I have never had one puke coolant on any rides. Even riding in Phoenix in august.

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Atcr1

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Like lumpy advises - With the bike cold, top up & let it idle. If it starts to "puke out" after a minute or two & the coolant isn't boiling hot, this would indicate that the system is pressurising.

Ill give it a shot, but the puking is only a side effect. The real issue is the bike running super lean. Its not a carb issue I ruled that out today. Its also not a leak in the gaskets or intake tubes leading into the airbox, carb, or cylinder. I used carb cleaner to check. So something they did inside the top end is messed up?

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Atcr1

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Well they did a leak down test, checked a few things and eventually replace my rad cap and rejetted the bike to a richer setting. I had my doubts picking it up that rejetting was just putting a band aid on a much bigger problem. My fears were justified. I got it home started it up and let it idle for 5-10 minutes while I got my gear on. No boil over. So thats a good start. Took it out on the trails and within a few minutes it started to pop on decel. This gradualy got worse and worse until the bike started to lurch in every gear like it wasn't getting any gas, then it would just die. After 15-20 kicks I would get it running again. It would start off again nice and strong, and witin 2 minutes pop, pop,pop lurching then dead. This occured 4 times with Ten miles and several times I thought I was hiking home for the truck. Luckily I nursed it home and took it right back. Whatever is going on has nothing to do with jetting because it all began when they replaced the top end. Any other thoughts?

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Atcr1

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Next time it does this, loosen the gas cap and see if that fixes it. YZ's use a check valve in the vent line marked with an arrow as to which way it goes. It could be backward or clogged.

Actually I have the stubby moose vent tube. The end piece is the valve and can only go on one way. I drained the float bowl screw and kept the gas on to make sure it was flowing, no debris in the bowl either and all the jets and float were just inspected and cleaned the day prior to me picking it up.

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Atcr1

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They went over the carb a third time, the float was off just a little, which they corrected. They tore apart the top end again, check seals, valves guides etc. Everything was good. Still popping. Then they noticed that my fuel-air screw had rattled out. Replaced. Problem solved? not quite. Got it home. Now it doesn't shut off on you, but it still pops on decel more than it should for a jetted bike. Took it back, checked the plug. Definatley running rich, so how could it need more gas? We are starting to think electrical too, but that doesn't seem to make since since its not misfiring.

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