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Kicking from TDC

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Hello to who ever reads this forum,

 

I bought an XR400 back in the middle of February, and I have gotten a few rides in on it. I love the bike for its power and because it is a XR! But anyway, I finally broke down and let my friend ride it (he has a yz250f) and he isn't the smartest when it comes to bikes. He just expects them to start. So, he stalled my bike in the woods and couldn't get it to start since, as XR people know, TDC is where you kick them from. I am looking for an explanation as to why XR's should be kicked after finding TDC (he didn't understand the concept of finding TDC) and why they are kicked in this manner (unlike race bikes, such as a yz250f, and how you don't have to find TDC).

 

I hope my rambling made sense. Thanks.

Edited by travism99

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I understand how the XR engine works, and your explanation makes complete sense. But, I am trying to figure out is why Honda recommends that you kick from the top of the stroke (for the 250, 400, 600, 650) when on say a crf450r, you don't kick from the top of the stroke. You just keep kicking, not seeking TDC.

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Funny you say that. My XR doesn't care where you kick it from.  It just starts.  My YZ450F on the other hand,  If you don't stop and find TDC you might as well forget about it.  

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Our Xrs start the best,just past TDC.Like the kicker,1/2in down from top.Then bring it up and kick.On the other hand my 11 wr450.If its on TDC you cant move kick start with your foot. You need hit  it down,slight bit to kick.Better to use estart.

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I understand how the XR engine works, and your explanation makes complete sense. But, I am trying to figure out is why Honda recommends that you kick from the top of the stroke (for the 250, 400, 600, 650) when on say a crf450r, you don't kick from the top of the stroke. You just keep kicking, not seeking TDC.

 

Have you found some reference material from Honda that specifies starting it from TDC. I don't remember seeing any.

 

It's just something I learned about the XR long ago, always do without exception after owning my Y2K for since new and recommend to others.

 

To expand on BarterFreaks notion, I have always figured the stock carb and its flap type choke system, as opposed to a true start enrichment circuit, had a lot to do with it too. When kicking from TDC, you immediately have the strongest vacuum signal at the highest rpm of the kick. Then given all the passages and the sheer volume of it all that that starting mix has to get pulled thru to get to the cylinder and still be rich enough to start, not to mention the rpms needing to still be high enough to trigger the cdi to send spark to the coil. All of this needing to happen before the starting kick rpms fall below optimal.

 

All of this is a big reason why I have always suggested, when starting a cold engine, pulling the decomp lever, no throttle, full choke and kicking thru several times to help draw in/prime the system so to have already gotten a starting mix well on its way before giving it an actual starting kick.

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Its been my experience that XRs each seem to have their own little starting routine.  They are the most idiosyncratic bikes I've ever owned.

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So then why do bikes like crf450r or x's not require you to find TDC?

 

Referring to what I said earlier, It could be several things about those engines. Anything from different head design, valve timing, porting, a real start enrichment circuit(choke), different ignition map(cdi), higher compression(in turn, higher vacuum), yada yada.........Those are just things I can think off the top of my head, not knowing those bikes very well.

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One full kick results in the engine turning 2 revolutions which is one full combustion cycle.  If you start the kick at TDC you are guaranteed exhausting gases from the cylinder followed by a full charge of air/fuel then a spark.  This gives the best chance at starting the engine.

 

Once everything is warmed up I don't worry as much about finding TDC.  I can usually give a half-hearted kick from anywhere and it'll start.

 

Ian

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I don't need to find tdc with mine it just starts right up in 2-3 kicks no matter what.

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If I don't kick from TDC I run the risk of kicking it and hitting what I believe is the bottom of the stroke or something like that. The kickstarter suddenly stops and feels like I broke my ankle.

I always find TDC even when hot for fear of the above happening. 2003 XR250R.

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Kicking from TDC you don't risk snapping off the kick starter shaft on the earlier models as much.

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