Starting Problems After YZ Timing Change



6 replies to this topic
  • Jerry57

Posted April 13, 2002 - 05:13 PM

#1

Am I the only one who has noticed an increase in starting difficulty after changing the valve timing to YZ spec? I can't seem to get the engine through a full stroke with the kickstarter now. When I choke up and kick it, the starter stops just short of passing TDC a second time.

What are your experiences?

Jerry

  • Bryce_Senff

Posted April 13, 2002 - 05:55 PM

#2

Hi Jerry,
Didn't have a problem at all starting after timing mod. You did rotate the exhaust cam
clockwise one tooth I hope? Some have turned it counter-clockwise and have had poor running bikes. I do have more problems starting when warm since I rejetted. I now usually have to use the hot start. will probably go down a size on the pilot jet.
Good luck to ya,
Bryce

  • Bill

Posted April 14, 2002 - 02:11 AM

#3

Jerry,

Sounds like you have a problem worth pulling the valve cover and rechecking your cam positions and number of pins between marks.

Bill

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  • YAMAKAZE

Posted April 14, 2002 - 03:59 AM

#4

Also check your throttle cables and ensure that your throttle isn't be held slightly open. I too have had that problem from time to time,Since changing to YZ timing, especially when it's cold. I have found that NOT Priming the motor and checking the throttle and clearing the engine with the Comp lever engauged will resolve the issue. After warm up the problem doesn't reoccure.

Bonzai :)

  • Scott_in_KC

Posted April 14, 2002 - 02:46 PM

#5

Yama hit on a good point, I noticed the same thing. It does NOT want any squirt of the pumper when starting cold. Just choke and kick...BAM 1st time usually. Now, in the winter when it was much colder it wanted a little juice, but not much.

How is it running once it's RUNNING??? Notice a difference?? If not, then recheck the timing as suggested.

  • Jerry57

Posted April 14, 2002 - 04:56 PM

#6

Thanks for the comments.

The bike runs great once started. I worked with my local Yamaha dealer to get the jetting set and it seems to be great. After the timing change, I noticed a significant improvement in front wheel lift from mid to high RPM. The extra poop at midrange is very nice when riding the rocky, rutted deserts of SW Idaho. I lifts easily in second and third gears.

My only problem is starting. The engine seems to want to stop turning just before passing TDC while cranking. It is as if the gear ratio on the kickstarter needs to be slightly higher to get a little more turn on the engine with each kick.

New Comments ???

Jerry

  • Mark_Cantrell

Posted April 14, 2002 - 05:37 PM

#7

Jerry,

I misread your post the first few times, looks like most of us did. I am so used to people complaining that it won't start that I didn't notice that yours is just hard to kick.

OK, first the basics (everyone gets tired of my longwindedness, please just skip on by if not of interest).

The kickstarter is geared for just about exactly two revolutions which is one pass through all four strokes. If you roll just past compression with the compression release, then let off to get a full kick, you will hit a second compression stroke about 3/4 of the way down. This time you will have momentum of the engine already turning with you to carry it through.

With the throttle fully closed, the intake stroke pulls a hefty vacuum through the closed throttle slide but doesn't pull much air. This means that you get 12.5:1 against a cylinder maybe half full (at the bottom of the intake stroke the air just pulled in is less dense than back in the airbox, i.e. has a vacuum relative to atmospheric). This vacuum eats up maybe half of the compression stroke so you only have to kick through 1/2 the normal compression.

Now if you open the throttle, the slide goes up out of the way, kick it and a cylinder full of air at atmospheric pressure rushes in. When you get to the compression stroke, you have to overcome maybe twice as much compression with kickstarter force. That will cause black and blue insteps.

That is what I think is happening to you. I think your throttle cables are holding the throttle cracked (it only takes a very little to feel this effect, maybe 1/16th throttle, get a friend with a working bike to try it for you). If not the throttle cables, then the idle is set too high. You can test this theory. Start the bike (wear your best steel insole boots or bump start it) and see if the idle is high. If it is, check the throttle cables and roll back the idle adjust screw. The choke makes this worse because it raises the idle speed also (i.e., lets in more air at idle).

YAMAKAZE was on the right track, maybe.

Good luck and let us know how fast it idles,
mwc




 
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