2008 yz 450f will not start after rebuild

Hello, my buddy has a yz 450f and he just rebuilt the motor, new piston rings and gaskets. The bike will not start now. It doesn't have proper compression, but the timing it definitely on, and the rings are installed properly. What could the problem be? We tried everything. Auto decompression is not stuck. We have lots of spark. Please help

if its not even trying to fire ,take off timing chain turn crank around one time till marks line up put chain back on.even though everything is lining up the bike is trying to fire on wrong up stroke.ive had the same problem and this works every time.

if it trying to fire or has rough idle or starts and dies then you have timing off recheck.

if its not even trying to fire ,take off timing chain turn crank around one time till marks line up put chain back on.even though everything is lining up the bike is trying to fire on wrong up stroke.ive had the same problem and this works every time.

if it trying to fire or has rough idle or starts and dies then you have timing off recheck.

Better re-think that one. :thumbsup:

It's the cams that decide which TDC is compression/power, not the crank. The ignition fires on every stroke, so there's no such thing as the wrong TDC when it comes to setting up the cam timing. Without the cams, one's the same as the other.

Better re-think that one. :thumbsup:

It's the cams that decide which TDC is compression/power, not the crank. The ignition fires on every stroke, so there's no such thing as the wrong TDC when it comes to setting up the cam timing. Without the cams, one's the same as the other.

I agree 100%...Better just recheck that timing

the crank has two points which it is tdc .it only fires once . on my hondas this has happened a couple times .just pull the plug and watch how often it fires compared to how many times the crank goes around.if your not on the right stroke the bike will try to fire at the wrong time .

most of the time when we take our timing chains off we dont rotate the crank around and everything is fine ,but i have a couple times and about went crazy trying to figure it out why the bike wont fire.if the timing is off you should know as soon as you kick it,but it will fire and start just run like shit.

if it not even trying to fire this is whats wrong.

unless yama fire in more then one time this is whats wrong.

ya im new to yama but a four stroke motor is a four stroke motor

think about it in one cycle the piston goes tdc twice does it fire both times.

think about it in one cycle the piston goes tdc twice does it fire both times.

On a 2008 YZ 450 it fires on every revolution including the exhaust stroke. The 08 is of course carburated vs a EFI bike that fires every other revolution.

I agree with Gray and the others that the valve timing is off since the crank position only has one TDC and has no bearing on timing as long as its at the top.

And yes it fires on every stroke on a carburated bike.

Jason

the crank has two points which it is tdc .it only fires once .
Wrong.

The small coil located on the stator assembly is the trigger for the ignition, and the system creates a spark on EVERY time past TDC, regardless. The spark that occurs during the overlap stroke does nothing because the intake valve has only just opened at that point, and there is not even close to enough fuel present to ignite.

And I hate to be the one to tell you this, but the Honda CRF's are the same way. :thumbsup:

The idea of the spark being "180 out" is carried over from the automotive world, but car engines are fundamentally different in their layout. The ignition timing was until recently almost always controlled by the positioning of a distributor, almost always driven by gearing it off the camshaft. The cam was timed independently, then the distributor inserted and aligned with the TDC mark for #1 in the firing order. In that kind of setup, with the distributor turning at half speed along with the cam, it was possible to choose the "wrong TDC", that at which the #1 cylinder was ending the exhaust stroke, and time it "180 out". With the YZF, or the CRF, and a good many others, you just can't actually do that. There is no wrong TDC.

But a single tooth off one way or other can prevent the engine from starting, and that is the problem you corrected when you retimed it.

On a 2008 YZ 450 it fires on every revolution including the exhaust stroke. The 08 is of course carburated vs a EFI bike that fires every other revolution.
The EFI bikes fire every revolution just the same, and for the same reason. In order to discriminate between one TDC and the other, the system would need to have a camshaft position sensor, and it does not.
The EFI bikes fire every revolution just the same, and for the same reason. In order to discriminate between one TDC and the other, the system would need to have a camshaft position sensor, and it does not.

So why do you set the spark pulse per rev. (PPR) on most EFI bikes to .5 instead of 1 on hour meter tachs such as my KTM 350?

Thanks, Jason

no senor would be needed cause the stator has to go around 4 times to make one spark .thats why if you pull the plug and watch it spark as you kick the bike it doesnt spark nonstop .theres a pause between sparks.

on my hondas they will start if the timing is off and will fire .they start hard and dont run well but they fire .if the crank is out 360 bike wont start wont even fire nothing.

i had timing off 2 teeth and bike fires .

i spent days the first time this happened going through everything, checked timing a 100 times was dead on and was at my wits end with the bike .then a local shop told me this. took off timing chain turned crank around put chain on and it started first kick.if none of what im saying is true why did this work.

just last week put new top end in 450 and same thing wouldnt start did same thing as above fired right up.

all i know is this has worked for me ,if your bike wont start what do you have to lose ?

Check your valve shims and make sure they are all in place properly. When removing the head they can easily fall out or move causing the symptoms you are describing with incorrect compression.

Better re-think that one. :cheers:

It's the cams that decide which TDC is compression/power, not the crank. The ignition fires on every stroke, so there's no such thing as the wrong TDC when it comes to setting up the cam timing. Without the cams, one's the same as the other.

Completely correct.. :thumbsup:

no senor would be needed cause the stator has to go around 4 times to make one spark .thats why if you pull the plug and watch it spark as you kick the bike it doesnt spark nonstop .theres a pause between sparks.

on my hondas they will start if the timing is off and will fire .they start hard and dont run well but they fire .if the crank is out 360 bike wont start wont even fire nothing.

i had timing off 2 teeth and bike fires .

i spent days the first time this happened going through everything, checked timing a 100 times was dead on and was at my wits end with the bike .then a local shop told me this. took off timing chain turned crank around put chain on and it started first kick.if none of what im saying is true why did this work.

just last week put new top end in 450 and same thing wouldnt start did same thing as above fired right up.

all i know is this has worked for me ,if your bike wont start what do you have to lose ?

Completely incorrect..:thumbsup:

stay tuned .

im going to take my bike apart to show you guys it will not start if its out 360.ill post it on youtube and post the link here.may take a few days till i can get to it.needs a new cam chain anyway.

and youll have one more reason not to ride red.

both 250 and 450 crf do this .

I don't know how many times you need to be told you're wrong, but here's another one. Greyracer is one of the most knowledgeable people here on TT, I guarantee he knows better than you do. I know better simply from personal experience.

On top of that I had 2 crf450's (02 & 05) before the yz450f I'm riding now, take a look at my user name. Both the crf450 and yz450f use a "wasted spark" system, if you look into it you will learn something about your bike.

no senor would be needed cause the stator has to go around 4 times to make one spark
WHAT? :thumbsup:

That would mean it would only fire every second compression stroke.

Since you are totally intractable, let me say this once more for anyone else interested:

The ignition is triggered by the crank, once per revolution. In order for the ignition not to be triggered every revolution, something would have to signal the system as to which TDC was which, and no such device exists.

The only thing in a YZF or most other current MX four-strokes that determines which upstroke is compression and which is exhaust is/are the camshaft(s), and it makes absolutely no difference whatsoever as to "which" TDC you start with when timing the cams.

None.

Last word on that subject.

stay tuned .

im going to take my bike apart to show you guys it will not start if its out 360.ill post it on youtube and post the link here.may take a few days till i can get to it.needs a new cam chain anyway.

and youll have one more reason not to ride red.

both 250 and 450 crf do this .

Can you explain to me how your timing can be 360° out? I'm very curious.

Can you explain to me how your timing can be 360° out? I'm very curious.

The same way it can be 360 deg. in :thumbsup:

I dont mean to laugh at someone being misinformed but this is entertaining.

You can absolutely have one 180 degrees out all marks lined up and will not start, just backfires, proof right here, watch mark on flywheel (08 YZ450F subject) turn cams to 10 and 2 and marks line up TDC also through plug hole, now turn and watch flywheel mark line up and look at cams they are down and valves are open. I dont care what none of you so called experts say Ive had one 180 out and pop only trying to start, remove cams rotate to flywheel mark reinstall cams and boom first kick, enough said. And by the way Ive been doing this 25 years and it just happened to me for the first time, thank you and goodnight....

It's one thing to be misinformed, and another to ignore a thorough technical explanation of how it really works.

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