Backfire on the throttle...cutting out

[hglumac]

Hi guys, I have been reading TT for quite some time and up till now have been able to find tons of info by searching.

I have a 03 yz450 with a strange issue, I have owned the bike for 6+ years and never had a problem. I figgured it was time for a little maintence and totally ripped it apart this winter. I had a friend do the top end, sent the frame for powdercoating, all new bearings...you get the picture.

OK so I get everything put back together and broke it in a little before going to the first race of the year. I run in a local XC/harescramble series. The bike never missed a beat for about an hour and a half then started to get a pretty nasty bog when I would roll the throttle on. At that point I could run through it and the motor pulled hard after you got past the bog.

so I pulled in the pits and looked at all obvious things oil, coolant, carb boots ect...all was normal.

Now the bike will run until warm then constanty backfires misfires and cuts out when you apply the throttle. It starts and idles fine

Pulled the carb apart found a little dirt here and there but nothing blocked that I could see....no change

Tried a different carb from a good running 05 yz450...no change

Thought maybe hotstart was sticking open...new seal cap and cable didnt help

Thought maybe exhaust leak...since decel backfires are caused by that...new gaskets and packing for stock pipe and muffler..no change

Tried disconnecting TPS and Neutral safety....same

A friend has an 05 450 that we tried swapping parts (CDI and coil) from and it behaved the same.

up until this year I have barely had to adjust the idle on this bike so Im not sure what would have changed.

Heres a clue maybe. I tried adjusting the fuel mixture, set bike at fast idle and turned the mixture screw all the way in and it did not cause the idle to choke out. maybe even idled a little better. but the strange thing is another carb had no change for the situation.

I hope someone can point me in the right direction.

Please help

Edited June 2, 2013 by hglumac

[zombiekillr.45]

good initial assessment,

was initially thinking carb but your technique of using a good running carb proves other wise

but if you had a severe intake leak at the rubber boot ( while running use rubber safe carb clean or brake clean and spray around that intake boot on the head side of the carb if your engine idles up you have a leak might have to raise rpm use a helper to hold rpm)

or cams are out of sync, you may have assembled the top end correctly but............ did you replace your cam chain tensioner and guides and cam chain? the bike can run great until you slack off the cam chain ( chop the throttle ) and re apply throttle and the chain can jump as those tensioners are questionable in the best of times , I was told to replace cam chain tensioner every top end rebuild. just a thought, would be worth pulling the valve cover in my eyes. good hunting

Edited June 3, 2013 by zombiekillr.45

[hglumac]

Thank you very much for the reply...

My mechanic/friend replaced the cam chain but not the tensioners, I was told not to bother with them but what you say seems like a good point.

I forgot to mention that I ruled out an intake leak with the method you mentioned also inspected the boots very well and tried the boot from the 05 bike as well.

Im thinking it has to be something timing, valve or cam chain related.

The exhaust note sounds very loud and flat when it starts acting up, hard to explain on the computer.

Its sitting in the garage, tank, shrouds, and seat removed waiting to have the valve cover torn off tomorrow night.

I cant really say I know what im looking for but I will consult my manual.

I also thought about a bad ground, maybe I didnt get all the powdercoat off where the motor meets the frame, but I was pretty particular while doing the assembly. The fact that the problem seems to get worse with a hot engine and it starts/idles so nicely sort of rule that out for me.

[grayracer513]

I don't see where you've tested or substituted the stator....

The cam chain tensioners are rarely at fault for problems with jumping time. Stuff like that is almost always traceable to a kinked chain, which I can usually trace to flaky oil change habits.

[ekulb14]

My brothers 450f also had some wierd problems mostley backfiring and cutting out when the throttle was chopped. Turns out the fuel line was collapsing when under a vacume. So it sounds dumb but the problem may be the fuel line so for 6 bucks i would change it out.

[hglumac]

I don't see where you've tested or substituted the stator....

The cam chain tensioners are rarely at fault for problems with jumping time. Stuff like that is almost always traceable to a kinked chain, which I can usually trace to flaky oil change habits.

I have not yet tested or swapped the stator, is there a test procedure that would save me from tearing apart the good running 2005? is it tough to remove a stator? I have never pulled or installed one.

I thought about the fuel line because during my rebuild I also installed a ims 3.1g tank. I thought this whole mess was a result on not betting enough fuel into the carb but I have since switched back to the factory tank with a fresh piece of fuel line for testing...also much easier to remove the stock tank since I have had to tear it apart so much lately.

[hglumac]

Before anyone busts my balls I do have a manual and I will read to see about the stator testing procedure

[grayracer513]

Stator removal requires a flywheel puller and a #30 Torx bit, but otherwise, it's a 15 minute job. The flywheel puller should be widely available and cheap (it's like the first or second most common Japanese FW puller there is).

When you're testing per the manual, wiggle the wiring some, and try rapping on the cover (prudently) to see if the vibration causes the meter to fluctuate. This can mean a broken wire.

[hglumac]

<Edited see botom of this post if you are in a hurry>

I started into testing the resistance on the stator this evening and found an interesting result.

using a Fluke digital 87

Not sure if im reading the meter right but the pickup coil should have a resistance of 248-372 ohms mine is reading 260ish THOUSAND or K ohms

sorce coil 1 should reas 720-1080 ohms mine reads 1.6 Million? or M ohms

Source coil 2 should read 44-66 ohms mine reads 1.05 Million or M ohms

on these parts the resistance climbed the longer I left the clips attached...seemed to slow way down after 30 seconds or so but still went up slightly.

I wanted to be sure the meter was reading right so i tested the spark plug coil that I know to be in good order

both the primary and the secondary were within range and the meter with the range in auto mode threw out the correct prefixes.

Does this mean I need a $300 stator assembly?

keep in mind the bike still starts and idles great...

I may tear the tank off the good running 05 to test the stator just so I know im not reading these prefixes wrong.

EDIT>>>>>>>>>>

Further testing reveals that I'M AN IDIOT>>> and was testing the wrong side of the connector when testing the female side of the plug that actually goes to the stator all of my resistances were within range. Sorry for the drawn out explination of doing it wrong ...moving the wires around didnt seem to change anything but it was hard to maintain the connection and move wires at the same time.

Is it still worth swapping the stator to see if mine fails with heat? It seems to run ok until the engine gets warm.

Edited June 4, 2013 by hglumac

[hglumac]

I was about to pull the valve cover and remembered I got a new spark plug for after the rebuild...I just told the guy at the parts counter what bike I have and he gave me a CR9E, I thought nothing of it and ran it....browsing in the manual I noticed my bike calls for a CR8E. Would the different plug be enough of a difference to cause this problem?

[grayracer513]

The CR9E will almost certainly foul, yes. Could be at at least part of your problem.

[hglumac]

I picked up the proper plug today, since I have the bike semi apart I'm going to check valve clearance and timeing...will report the results.

Grey, I most certainly appreciate all of your input. If the valves and timeing are within spec I fear I may have to enlist the help of a good technician. Any names come to mind in south western PA?

[hglumac]

I checked valve clearance tonight with an american feeler gauge.

If you are sitting on the bike the 2 intakes on the right took a .003 but would not fit a .004 the left valve took the .004 snugly minimum clearance in my manual is .0039

The exhausts both took the .008 snugly as minimum clearance is .0079

Are those clearances too tight for a valve set with 10 years of use?

The timing marks lined up just right. I didn't verift TDC with a screwdriver but the cam lobes we're pointed out.

[zombiekillr.45]

puzzling,try the new spark plug, hopfully its that simple. maybe try a leak down test, maybe a valve isn't seating properly or damage to the seat..... would be able to idle but reving up would prove difficult like you describe.

[hglumac]

I'm going to the Highpoint national this weekend...maybe I will just drop her off at the factory tent and let the pro techs have a look! I wish...

[grayracer513]

I checked valve clearance tonight with an american feeler gauge.

If you are sitting on the bike the 2 intakes on the right took a .003 but would not fit a .004 the left valve took the .004 snugly minimum clearance in my manual is .0039

The exhausts both took the .008 snugly as minimum clearance is .0079

Are those clearances too tight for a valve set with 10 years of use?

The timing marks lined up just right. I didn't verift TDC with a screwdriver but the cam lobes we're pointed out.

The valve clearance is about what I'd expect. Yamaha uses valve shims in .01mm increments when they build the heads for these, and they set the clearance as close to .10mm/.20mm as possible. Correct the intake clearance to be within spec, and if you're sure about the exhausts, leave them alone.

[hglumac]

I'm going to buy a metric feeler gauge so I don't drive myself nuts trying to do the conversions would that little variation in the clearances I listed be enough to mess me up?

[hglumac]

I'm going to buy a metric feeler gauge so I don't drive myself nuts trying to do the conversions would that little variation in the clearances I listed be enough to mess me up?

I assume when tjings get hot the clearances become tighter.

[grayracer513]

...would that little variation in the clearances I listed be enough to mess me up?

It might influence things a little, but it's not the cause of your problem.

I assume when tjings get hot the clearances become tighter.

Not necessarily. The head expands at a greater rate than the valves do because it's aluminum. The valves do run hotter, but the distance between the valve seat and the camshaft expands slightly more than the increase in the length of the valve stem, so the clearance normally increases a little bit.

[hglumac]

at this point I have adjusted the valves and installed the correct spark plug....2 quick test rides letting the bike get hot and it has not backfired yet. its too soon to call it fixed but it looks like I wont have time to whoop on her until the weekend, hopefully mom will give me a couple hours off as a fathers day gift.

I sprayed carb cleaner all around the valves and in the intake to possibly break up any carbon buildup if maybe a valve was sticking hard to tell. I will get back with you after I get a good honest test ride in.

It sure starts good now, wasnt a problem before but seems like its running before my foot hits the bottom.