starting technique


12 replies to this topic
  • rparobi

Posted January 28, 2010 - 10:07 PM

#1

Hi guys , i have a new 09 yz450 i was just querious what the best why is to start ( kick ) these bikes with the auto decomp system as im used to having two strokes and just kicking away.Should i just kick the bike without finding tdc or push through slowly until i find the compression resistance ( tdc ). The reason i ask is because sometimes if i slowly find TDC then reset the kick start lever and attempt to kick through this part of the stroke it requires a firm push on the kick start lever to gradually push through ( as if there is no decomp )- once through TDC all other kicks ( usually 1 or 2 ) in an attempt to start the bike are relatively easy - is this normal

  • Family Man

Posted January 28, 2010 - 10:30 PM

#2

By finding TDC I think you mean the compression stroke. Yes locating it does usually help. Yes it is normal. It is not always required but most of the time it should fire in a couple of kicks if jetted right.

  • rparobi

Posted January 28, 2010 - 11:45 PM

#3

Im was wondering why i can feel compression resistance when there is an auto decomp fitted to the engine ,shouldn't the auto decomp relieve this compression, or is this just what happens when you slowly sneak up on TDC ( in other words do i have to kick quickly for the decomp to work.

  • grayracer513

Posted January 29, 2010 - 08:20 AM

#4

The auto decompression system does not release compression, it reduces it to a level low enough that the engine can be cranked, but high enough that it will start. In order to understand it, though, you first have to understand that the "hard spot" you hit with the kick starter IS NOT TDC. That resistance is the point at which the intake valve closes and the compression stroke starts. TDC comes about 120 degrees later, when the piston reaches the top of the cylinder and starts back down. You can feel that with the kick starter too, if you push through the compression stroke, as about 25 degrees of crank travel where there is no resistance.

In the old days, we used a manual release to position the crank on past the power stroke for the starting kick. Then, we'd kick and we'd have 1 full revolution to build momentum to turn through the next compression stroke. But back then, we has lower compression ratios, less efficient ring sealing, and much heavier cranks (a typical British 500 single crank weighed 35 pounds easily), and that technique won't work on a modern MX four stroke.

Instead, with something like the YZ426, we push up against the resistance of the compression stroke (what you and many others erroneously refer to as TDC), pull the compression release, and advance the crank a little farther into the compression stroke, so that we're about half way into it, but not past TDC or the point of ignition. By kicking from there, we drive the crank through the ignition event and over the top, hopefully getting a big enough bang from the engine to drive itself through the next cycle and start.

Auto decompression does this for us. Without AD, the intake valve would close at around 120 degrees before TDC, and the compression stroke would start there. We would then have to compress the air in the cylinder about 9 or 10 to 1 to crank the engine. The AD unit reduces this by holding one exhaust valve off its seat until the engine gets to around 80-90 degrees before TDC, so that we only have to compress the air about 6 to 1 instead.

The way it works is much simpler than you might imagine. The auto decompression mechanism consists of a spring loaded flyweight on the exhaust cam. When this flweight moves in response to speed, it rotates a shaft in the cam center that retracts or extends a pin located near the base of the left exhaust cam lobe. The pin is extended at speeds under 700 rpm, and is positioned so that it lifts the left exhaust valve off the seat at around BDC as it passes over the lifter, and rolls off the edge of the lifter at about 60 degrees BTDC, letting the valve reseat. This shortens the compression stroke by about half, leaving enough compression that the engine can start, but cutting it down enough that it can be rotated without releasing compression.

As far as kicking slowly making the unit inactive, the reverse is actually true. An electric starter turns the engine at about 600 rpm, and the AD unit rolls off at 700. You can actually kick the bike so hard that it turns faster than 700, that will deactivate the AD, making the bike harder to start if you kick too hard.

You can kick the '03-'09 YZ450 from any engine position without any setup at all, but it's more likely to start in one kick if you roll it up against compression first, then kick. If it doesn't fire, you can do the same thing again, or just thrash away at it like a two stroke. Try to make the kick itself a full, smooth stroke all the way through, rather than a sharp jab.

Also read this. You'll run into this eventually:
http://www.thumperta...979#post5314979

  • rparobi

Posted January 29, 2010 - 05:10 PM

#5

Thanks for the detailed reply, i guess i just have to kick harder - i was a little concerned that there may be someting wrong with the auto decomp.

  • rufusz

Posted February 01, 2010 - 06:15 AM

#6

And what is the mechanism behind the pin that doesn't allow it to be pushed back?

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  • grayracer513

Posted February 01, 2010 - 09:19 AM

#7

And what is the mechanism behind the pin that doesn't allow it to be pushed back?

The shaft that is rotated by the flyweight has what amounts to an eccentric cam lobe on its inboard end. This cam is engaged in a slot ground in the pin, and in the "at rest" position, the spring pulls the flyweight in, the flyweight rotates the shaft, and the cam extends the pin. The reason it doesn't get pushed back is that the cam is just past its own TDC at this point, so the force of the valve spring can't force the pin to retract because it can't rotate the shaft. Once the engine turns 700 rpm, the weight easily retracts the pin, and everything is back to normal.

  • rufusz

Posted February 02, 2010 - 01:39 AM

#8

My original question was referring to the fact that when you hold the exhaust cam in your hand, and the pin is at the default "at rest" position, you can't push the pin back by hand. But I've figured out the answer already (I think so) : it's due to the fact that in the out position, the force that is pushing against the pin is perpendicular on the axis of the shaft and thus is not generating any rotational force.

Like in this image : http://www.thumperfa.../03_ecam_02.jpg, there is a "cutting"/groove at the end of the shaft to which the weight is mounted.

Huh, sorry for this "explanation" but I don't know the right terms in English.

  • YamaLink

Posted February 02, 2010 - 08:35 AM

#9

I have great luck with this technique on all 4 strokes: squirt throttle 4 or 5 times, stroke kickstarter from tdc to bottom slowly about 5 times, get to tdc, no gas whatsoever (I even look at the throttle and my bike won't start), and one big healthy kick.

When bike is hot I put it in neutral (1% of time it'll start in gear with clutch in), pull in hot start, tdc, big kick.

  • grayracer513

Posted February 02, 2010 - 11:23 AM

#10

My original question was referring to the fact that when you hold the exhaust cam in your hand, and the pin is at the default "at rest" position, you can't push the pin back by hand. But I've figured out the answer already (I think so) : it's due to the fact that in the out position, the force that is pushing against the pin is perpendicular on the axis of the shaft and thus is not generating any rotational force.

Like in this image : http://www.thumperfa.../03_ecam_02.jpg, there is a "cutting"/groove at the end of the shaft to which the weight is mounted.

Huh, sorry for this "explanation" but I don't know the right terms in English.

I understood your question, and that's what I said...:ride: Next time your cam cover is off, take a look down the other end of the cam, through the hollow shaft, and operate the flyweight. It will be obvious to you immediately.

  • rufusz

Posted February 03, 2010 - 12:49 AM

#11

I have great luck with this technique on all 4 strokes: squirt throttle 4 or 5 times, stroke kickstarter from tdc to bottom slowly about 5 times, get to tdc, no gas whatsoever (I even look at the throttle and my bike won't start), and one big healthy kick.

When bike is hot I put it in neutral (1% of time it'll start in gear with clutch in), pull in hot start, tdc, big kick.


When I wanted to buy my '02 426 I was advised by "the people" in the scene, to not do it, because it's a pain to start and it's impossible to start when engine is hot and you fall. After owning it for 2 years my opinion is, they never bothered with adjusting the carb and more exactly the fuel screw. And also nowadays everyone just buys the bikes and rides (around my place), they never-never try to adjust the carb, even when they can't start it properly (especially true for people who have a magic button).

Some of them had interesting, I call them unorthodox methods :
- cold starting : kick several times with hot start button, lean bike over until gas flows out of carb hoses, lean it back, pull choke, cross fingers, say pray, kick-kick-kick
- hot starting : almost same technique, but pull choke and hot start together and who knows what

Maybe we could start another thread, so we don't hijack this one :ride: for these urban-legend techniques.

  • Lota Fun R

Posted February 03, 2010 - 04:15 AM

#12

Maybe you should get an electric start model? :ride: :bonk:

JK :lol:

  • Lota Fun R

Posted February 03, 2010 - 04:19 AM

#13

I've never used the Hot start knob on my 01 426. Just kick it a little past the hard spot and bamm it's running. No throttle, just kick. But then again, I've made sure it's working as good as possible. It' starts right up. Giving it throttle at all will mess up the whole deal. Mine generally runs rich as I think most of them do.





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