03 YZ450F stalls / loses power when airborne

[s14unimog]

I posted this over on All Things Moto, but I wanted to post this on Thumper Talk as well just to see if there were any other ideas. The ideas that were offered over there, I will have an opportunity to look at this week and hopefully put them to bed.  Those were: possible damage to the insulating boot around the plug (which may also be fluid/oil build up in the plug valley - although my weep hole is clear to drain), possible pinching of the vent lines from the carb when the swing arm droops and verify if the baffle around the jets is in place (which I believe it is but will verify).  Outside of these good ideas, I wonder if anyone else has had direct experience with a problem like this and could shed any additional light.  At the very least, if the problem is a product of one of the ideas mentioned, then this thread will simply become a good problem/solution thread to archive on the forum.  Thanks for any incite or helpful advice.

 

The problem:

A simple description is that once this bike leaves the ground, it practically shuts off, lands and then stumbles back to life. The symptom is very repeatable and renders the bike almost useless as its just a matter of time before it pitches me over the bars. What I can say, is that the bike runs quite well when on one or two wheels and will do this problem only when completely airborne.

What I've addressed for this issue specifically:

• The rear wheel spins freely - brake and bearings are not dragging
• I'm not unknowingly standing on the rear brake - I've adjusted clear out of reach just to prove this to myself.
• Numerous carburetor adjustments including JD Jetting kit (170 main and 45 pilot - I live at ~1000ft elevation with 95*F average summer weather), JD Jetting Red hot weather needle, JD Jetting brass air/mixture screw with new o-ring (2-ish turns out), o-ring mod to AP, QS3 tunable leakjet on AP (3 turns out), carb cleaner stall test on intake and manifold boots (no leaks), full rebuild clean/clearing of all jets and ports.
• new OE exhaust gasket
• running with and without TPS plugged in - runs better with and has no affect on issue
• confirmed/measured accurate float height
• petcock fuel delivery has good flow and float valve works fine.

All of these adjustments have made vast improvements on the general performance of the bike. It always starts up and runs like a banshee without breaking up. The sudden power loss on take-off leads me to believe the issue is electrical, but the bike never ran so great when I first got it, so the aforementioned carburetor changes helped there. All that is left is this one stupid symptom otherwise the bike runs brilliant.

Please help me figure this out. I am forced to trail ride the bike only in the mean time and anyone with a 450 knows, they need open space. I don't want to sell it and pass the problem off b/c that is not right and I will not be foiled by something I just didn't consider.

[tsoutherlin]

Is the little cup that sits around the jets still in the carb. I have had several do it yourselfer customers clean there carb throw the cup away because its plastic and then experience the same symptom your having.

Its number 57 in this diagram.

[s14unimog]

Is the little cup that sits around the jets still in the carb. I have had several do it yourselfer customers clean there carb throw the cup away because its plastic and then experience the same symptom your having.

Its number 57 in this diagram.

 

This was proposed and I do believe mine is in place, but I will need to have another look to be sure.  Thanks for confirming exactly what that part does though.  I am looking to break into any ideas tomorrow evening - work has kept me away from home.

[s14unimog]

Well, just an update.  The baffle is still in place and I trimmed up my vent lines a little so they were just above my wishbone, to rule out any possibility they were being pinched.  Aside from that, I pulled the plug and replaced that.  I also thoroughly cleaned the coilpack as best as possible in an attempt to break up any leakage tracks that may be there - everything down there wasn't exactly clean.  I also thoroughly coated all insulating surfaces with dielectric grease to be damn sure that electrode has no chance of bleeding voltage.  It got a little late last night, so I couldn't exactly take it out jumping, so I plan to see where I stand this afternoon.

 

If this does not fix it, I am flat out of ideas....

Edited July 10, 2014 by s14unimog

[s14unimog]

Welp, no luck.  The bike runs a tad better with the new plug but still does just as it did.  Another thing I noticed, even simply popping a log on a trail will cause the symptom I've described.  I'm starting to think its an un-loaded engine issue.  Essentially, if there is no load on the motor, this will happen.

 

 

Now on this topic, I asked a fellow 450 owner about the issue and he quickly referenced cleaning the #100 pilot air jet as if he knew that would be the culprit.  Now mind you, mine is free and clear of any debris, but why did he go straight to that?  Could I have a blockage in the side port next to that jet on the carb inlet?  what exactly does that jet do and why would he quickly think it was that? I never got an answer from him.

 

 

So to rule out...

 

It's not a voltage leakage issue with the plug boot or coilpack.  It's not a missing bowl baffle starving the engine of fuel during weightlessness.  what else could there be?!?!

[Thommo_400f]

The most likely problem (based on my similar experience) is the gasket between the carb body (not a generally serviceable item) is most likely perished from too much carb cleaner at some stage.

What happens is... when you change the fuel level/angle dramatically, fuel somehow leaks across into the air passages and stops normal vaporization momentarily.... then coughs back to life when fuel settles.

Try a carb off another bike to confirm.

If it is the problem it's probably easier to toss this one and get one from a wreckers.

[ekulb14]

hmm yeh just a quick read but i would also tell you to shelf that carb and buy a "new" one. Anther place to investigate would be the ecu but you seem to have covered that.

[s14unimog]

hmm yeh just a quick read but i would also tell you to shelf that carb and buy a "new" one. Anther place to investigate would be the ecu but you seem to have covered that.

 

The most likely problem (based on my similar experience) is the gasket between the carb body (not a generally serviceable item) is most likely perished from too much carb cleaner at some stage.

What happens is... when you change the fuel level/angle dramatically, fuel somehow leaks across into the air passages and stops normal vaporization momentarily.... then coughs back to life when fuel settles.

Try a carb off another bike to confirm.

If it is the problem it's probably easier to toss this one and get one from a wreckers.

 

Thanks for the responses. Gasket between the carb body?  Where abouts is this - does it call out or is it illustrated on the parts fiches? I wish I knew someone with another 450 close.  I might just need get another one in to test this idea out.  I will say one thing though, the bike doesn't seem to mind riding on the back tire, it's only when it leaves the ground and I suppose the load on the engine is removed suddenly.

Edited July 15, 2014 by s14unimog

[grayracer513]

It's actually not all that likely that the gasket Thommo refers to will fail in the FCR-MX carb with the regularity that they do on the older FCR that the 400's used.  IF you suspect it, however, be advised that it is NOT a regularly serviced part, and as far as I know, there is only one source for replacements: JD Jetting.  My advice would be to have one in hand before you take the carb apart at that point. 

 

Carb cleaner, BTW, is no harder on the gasket than gasoline, just so we put that one away.  Most of the stuff in carb cleaner is in gas to start with.

 

You say you have the float set correctly according to the manual, and that the main jet baffle is in place.  One thing you might look at is the cast-in tube that shrouds the pilot jet.  The "tube" should extend all the way down to the base of the main jet.  I've seen the engine facing side of these broken out, and if that's the case on yours, it could be the problem. 

[s14unimog]

It's actually not all that likely that the gasket Thommo refers to will fail in the FCR-MX carb with the regularity that they do on the older FCR that the 400's used.  IF you suspect it, however, be advised that it is NOT a regularly serviced part, and as far as I know, there is only one source for replacements: JD Jetting.  My advice would be to have one in hand before you take the carb apart at that point. 

 

Carb cleaner, BTW, is no harder on the gasket than gasoline, just so we put that one away.  Most of the stuff in carb cleaner is in gas to start with.

 

You say you have the float set correctly according to the manual, and that the main jet baffle is in place.  One thing you might look at is the cast-in tube that shrouds the pilot jet.  The "tube" should extend all the way down to the base of the main jet.  I've seen the engine facing side of these broken out, and if that's the case on yours, it could be the problem. 

 

gotcha - thanks for adding a bit more to that.  Just to help rule out carburetor air leaks, I went ahead and bought all of the gaskets that seal off the slide and intake (needle valve plate o-ring, AP cover gasket and seal grommet, intake manifold boot and the side cover gasket) - those should be here on Friday.  I will take another look at the cast tube you mentioned at the base of the pilot jet when I go to install those. 

 

I will say, the idle is a tad erratic and I'm still getting some popping on decel. 

[grayracer513]

Have you checked your valve clearances?  Tight intakes can cause some of the idle problems you're complaining about.  Popping on decel is normal and healthy as long as it isn't excessive.

[s14unimog]

Have you checked your valve clearances?  Tight intakes can cause some of the idle problems you're complaining about.  Popping on decel is normal and healthy as long as it isn't excessive.

 

Oh really?  I have not, but am capable of doing so.  I'll double check those while I have the tank off.

[Thommo_400f]

It's actually not all that likely that the gasket Thommo refers to will fail in the FCR-MX carb with the regularity that they do on the older FCR that the 400's used.  IF you suspect it, however, be advised that it is NOT a regularly serviced part, and as far as I know, there is only one source for replacements: JD Jetting.  My advice would be to have one in hand before you take the carb apart at that point. 

 

Carb cleaner, BTW, is no harder on the gasket than gasoline, just so we put that one away.  Most of the stuff in carb cleaner is in gas to start with.

 

You say you have the float set correctly according to the manual, and that the main jet baffle is in place.  One thing you might look at is the cast-in tube that shrouds the pilot jet.  The "tube" should extend all the way down to the base of the main jet.  I've seen the engine facing side of these broken out, and if that's the case on yours, it could be the problem. 

Yes...sorry, my mistake... I forgot this was the 450....mine was the old 400 FCR which the gasket failed.

[s14unimog]

Well, some good news and some no good news. I went through and replaced all of the o-rings/seals and vent lines as I said I would to the tune of about $80 after shipping.  Once thing I noticed when changing the lip-type o-ring on the "backfire plate"/needle valve (as Yamaha seems to call it), I noticed that the previous owner had actually installed it UPSIDE DOWN?  What a jerk.  Every time I got in there to service, I was always careful not to reorient it, but this time I took a look in my shop manual just to verify, and I'll be damn - it was in there wrong.  I suppose to the previous owners credit, it seems to fall into place a little more easily upside down, so I imagine he just didn't notice. Sure plays hell on the idle and operation of the bike like that though; lol.

 

The good news from all this is that now the bike runs awesome!  It idles great, starts up first or second try, tiny air screw adjustments can be felt/heard on idle now and the bike's power is much more linear.

 

The bad news is that is done almost nothing towards the air borne stall issue.

 

My plan now, unless we've got some more ideas, is to verify valve adjustment clearances.  It's been said before that it may play into this symptom and I have yet to check them.  I will follow up after the fact, but I just wanted to add some on the issue.

[TriedToPullOut]

has anyone found a solution to this problem? my bike does it too. but carb is jetted and has a qs3 on it

[s14unimog]

I ran into a brick wall on it and gave up.  Sorry.

Edited September 15, 2014 by s14unimog

[TriedToPullOut]

so I guess the bikes useless then.. any steep hill or really steep wheelie it does it.