04 450 wont start after new topend


30 replies to this topic
  • 2000yzf426

Posted May 12, 2008 - 04:04 PM

#1

recently got the frame powder coated and new top end (cylinder and weisco hi comp piston) after getting everything back together it wont start. I've checked the timing a few times its bang on, mark on both cams and flywheel line up perfectly. Checked the coil and magneto and they are all within specs in the manual. Had the carb off and cleaned the jets. All that happens when I kick it over is the odd back fire other than that not a thing, I am not sure what to check next.

  • georgedoggy

Posted May 12, 2008 - 04:24 PM

#2

check the following first
1. spark
2. fuel
if you dont have spark, then you know where to go, if you have spark but no fuel, then so on and so on. let us know how you go.

  • 2000yzf426

Posted May 12, 2008 - 04:32 PM

#3

ive put in a new plug and it seems to spark fine, ive also tested the coil with a volt meter and the specs are within what the manual says they should be. also had the carb off and it seems to be fine, squirts fuel out when you open the throttle. plug is alittle wet when i pull it out, all it does is backfire

  • KJ790

Posted May 12, 2008 - 04:41 PM

#4

Did you check the timing with the tensioner in or out? If you checked it with the tensioner out then you intake cam is off one tooth. If you checked it with the tensioner in then your timing is right.

If your timing is right, you say it backfires sometimes. This would make me believe that it has spark and is getting some fuel. Could be a carb problem (plugged jet, etc.). Maybe cleaning the carb didn't get everything out.

  • 2000yzf426

Posted May 12, 2008 - 05:15 PM

#5

I checked the timing with the tensioner out, have checked it a few times since just to make sure its right and all the marks line up. im leaning towards the carb also, ill be getting another can of carb cleaner tomorrow and see what happens.

  • KJ790

Posted May 12, 2008 - 05:41 PM

#6

I checked the timing with the tensioner out, have checked it a few times since just to make sure its right and all the marks line up. im leaning towards the carb also, ill be getting another can of carb cleaner tomorrow and see what happens.


That is most likely your problem. If you check the timing with the tensioner out then it is not right. When the timing looks right, the intake cam is actually one tooth off. Put the tensioner in and check the timing.

  • grayracer513

Posted May 12, 2008 - 07:39 PM

#7

While you are checking things, it won't hurt to verify that the flywheel is properly keyed by checking the indicated TDC with actual TDC as found with a probe through the spark plug hole.

  • rufusz

Posted May 13, 2008 - 05:10 AM

#8

Did you check the timing with the tensioner in or out? If you checked it with the tensioner out then you intake cam is off one tooth. If you checked it with the tensioner in then your timing is right.



Why would the intake cam be one tooth off when checking without tensioner??? The number of chain links from the crankshaft to exhaust cam and from exhaust to intake cam should be the same, regardless if the right part of the chain is tensioned or not (looking at the engine from the chain side and "counting" in clockwise order). When you tension the chain, neither the crankshaft, neither the camshafts rotate.
I think the crankshaft rotates anti-clockwise and that keeps everything tight. Isn't this right?

  • grayracer513

Posted May 13, 2008 - 07:42 AM

#9

Why would the intake cam be one tooth off when checking without tensioner??? The number of chain links from the crankshaft to exhaust cam and from exhaust to intake cam should be the same, regardless if the right part of the chain is tensioned or not (looking at the engine from the chain side and "counting" in clockwise order). When you tension the chain, neither the crankshaft, neither the camshafts rotate.
I think the crankshaft rotates anti-clockwise and that keeps everything tight. Isn't this right?

Some people overlook the fact that the engine turns in a particular direction, and that the front and top runs of the timing chain need to be drawn taut when checking the timing. If they have improperly set up the engine with the front side of the chain slack, the installation of the tensioner can and will cause either the crank or the cams to rotate well away from lining up correctly.

  • 2000yzf426

Posted May 13, 2008 - 04:13 PM

#10

well i cleaned the carb, timing was off but now im unsure which stroke of tdc i should be aligning the cams too? the cams seem to line up every second revolution of tdc, im still not gettin anything but the odd backfire

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  • grayracer513

Posted May 13, 2008 - 04:35 PM

#11

timing was off but now im unsure which stroke of tdc i should be aligning the cams too?

Old car guy, huh?

In a YZF, there's no difference between one TDC and the other until the cams go in. There's a spark before each TDC, and if the cams both line up on the same one, one TDC's as good as the other.

  • 2000yzf426

Posted May 13, 2008 - 05:16 PM

#12

well thats good to know, when I have the line on flywheel where it is supposed to be and the dot on the intake cam lined up the exhaust cam seems to be off abit, the dot sits higher than the intake but if i move it a tooth the dot is down too low. I replaced the timing chain and I am starting to think maybe I didnt put the flywheel on properly or something

  • grayracer513

Posted May 13, 2008 - 06:28 PM

#13

Do you have an automatic decompression camshaft? (a flyweight on the sprocket?)

  • 2000yzf426

Posted May 13, 2008 - 07:26 PM

#14

ya it has the auto decomp, doesnt seem to be much resitance on the weight when you push it out but i have no clue how that works

  • Gerry B

Posted May 13, 2008 - 07:47 PM

#15

Just out of curiosity, did you take the whole carb apart? Did you take the slide out? Sometimes when the slide goes in backwards it will make a backfire and not run right or not fire up at all.

  • grayracer513

Posted May 13, 2008 - 08:01 PM

#16

Well, we may have found your problem. The cam may very well be from a YZ450, in which case, the timing marks won't work correctly in a 426. The cam can be used, though, and they often are. If you can contact the previous owner, ask him if he used a YZ450 cam, or an after market cam made for a 426.

A 450 cam can be used by first timing the intake, the positioning the exhaust cam so that there are 14 pins of the timing chain between the two top marks on the sprockets. Timing the 450 cam off the marks will never work; the timing will be wrong, and there won't be enough compression to run.

Read:

http://www.thumperta...078#post3706078

Then:

http://www.thumperta...040#post5378040

  • 2000yzf426

Posted May 14, 2008 - 02:30 AM

#17

I never took the slide out of the carbs, just the jets. and the bike is an 04 450, the auto decomp cam came stock on the bike

  • grayracer513

Posted May 14, 2008 - 05:32 AM

#18

... the bike is an 04 450, the auto decomp cam came stock on the bike

Sorry about that. I was thrown off course by your user name. My bad. :thumbsup:

  • 2000yzf426

Posted May 14, 2008 - 01:36 PM

#19

no problem, bike still isnt running. are the dots on the cams supposed to line up perfectly with the head? if i align the mark on the flywheel and intake cam up the exhaust cam seem to be off abit

  • grayracer513

Posted May 14, 2008 - 02:54 PM

#20

With the tensioner installed, turn the engine forward to TDC and see where the marks are. They will not line up perfectly, and the question is whether it would look better or worse if you moved it one way or the other a full tooth. Visualize the mark being on the next spot over from where it is; what would that look like?





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