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Discussion Starter · #1 ·
2007 YFZ450 my customer bought from a guy.
Ran great for about 2 days. Started easily, just an absolute perfect bike for him he thought. Customer was ridding it, normal ridding, then dies. No noises, just died. Wont start. (Have not tried pull started it yet, thats not my concern, concern is why wont it start normally lol)

Now its in my shop and I am stumped.. Ive been doing this stuff all my life, 2013 YamaPro graduate from MMI Orlando, but i cant seem to figure this one out :lol :nerd::wtf:

So, Ive done everything you could possibly do without telling him to buy a new one lol.

Bike turns over, starts spinning fast, slows down slightly, starts spinning fast again.. like its "trying," then just trying to help it out a little by just barely , and i mean BARELY a little throttle bump to give it just a tiny shot of fuel and BOOM backfires through the carb and exhaust. Caught the air filter on fire earlier LOL.

We have good hard spark, will jump a 1" Gap.
Ive timed it With YFZ and YZ (cam mod) timing, no change
Battery 13.4VDC
120 lbs of compression (with auto decompress activated) Realistically probably around 160 to 170 Without it
Cleaned the carb 2X, even let set in cleaner for 2 days (but i mean it ran the other day perfect, i witnessed it, no way the carb is stopped up lol).
Accelerator Pump operational
45P, 2 Turns out @ 700Ft Temps 50 degrees. No lid
Ex .008" IN .005"


The stroke on these engines are so short.. So its 100% impossible (unless you have HULK hold the breaker bar for you(you will lose teeth, seen it happen @ MMI Lol)) to do a leak down test on the cylinder just to verify no less than 10% loss. But once again, valves are in spec, timed correctly, lobes 10 and 2.

Valves seal fine, filled ports up with water and stood for 5 min ZERO leakage

I did him a favor and went ahead and re-ringed and resealed the top end, so that's all new.



So, let me clarify... Im looking for professional help.. So lets go ahead and establish: We have spark, we are getting fuel, Obv we are getting air, we have compresion, battery charged (considering hooking up to 24V circuit just to see what it will do, only done that on the Hondas for GNCC), Valves are in spec.

There are no "fuses to check" in the ignition system, only fuse is for the starting system (sorry, i have to weed out all the 16 year olds who believe they own this forum), Yamaha uses an AC-CDI system which means it produces its own electricity Via P.M.S (Permanent magnet system (Magneto)).. The battery is only for lights and the starting system, has nothing to do with ignition.

All the wires are connected.... or else I wouldn't be getting spark..
This isn't Fuel Injection(dont get me started on that), so nothing electrically would alter fuel.

TPS is at correct resistance.

Yes , the key is on......... No there is no "seat sensor".. (you guys have no idea how many times ive been told this..:|)

PS: Im really sorry if i sound lazy and misspell some words, Its 3AM Lol. Sorry if i seem rude, Im really nice :) and i love to help others.. I just cant stand a troll on a serious matter..

I can honestly say this one has me stumped. Give me some answers please, Yall have a good one !
 

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Does it fire off for a few seconds with a shot of starter fluid spray? That would at least narrow it down to a potential carb issue. I didn't realize the FCR MX carb has a set of mid-body gaskets until I found some videos on a full FCR carb rebuild. The mid-body gaskets on mine were completely shot when I opened it up. They looked crushed (more so than just being tightened down), almost melted in appearance. I can only attribute some of that to the ethanol fuel that was left in the carb for several years to evaporate.

Long story short, the '05 I purchased and went through thoroughly before attempted to even start it, wouldn't completely fire up until I reset the "cam mod" back to stock timing on the cam marks. Original post noted that was already attempted, but with a combo of the cam mod hard starting issues and possibly a carb problem, it can be a major pain to troubleshoot.
 

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If the magneto/ stator is bad and the cdi uses ac from the stator wouldnt that cause it to have spark from the battery but not run under its own power? Im currently troubleshooting a similar issue
 

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I know this is an old post but maybe this will be helpful...
My 08 yfz 450 was firing and would start for a little with throttle going from wide open-closed-wide open-closed.
It was backfiring from the exhaust and through the carb... like whole flames lol.
After fixing the wire harness issues and properly routing the stator wires that the previous owner had all mangled i figured it was either valves or timing... turns out it wasnt timed properly.. adjusted that last night and now she purrs and idles on its own, even though its not jetted for the full aftermarket exhaust.(new jets on the way)
 
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