Still need decomp lever with auto-cam swap...


8 replies to this topic
  • grassmonkey

Posted January 12, 2007 - 07:21 AM

#1

I bought a bike that has the auto decompression exhaust cam modification. However, I still have to use the decomp lever to start the bike. It will not start without it. Any ideas?

  • Pincushion

Posted January 12, 2007 - 07:24 AM

#2

dishonest seller.

  • grassmonkey

Posted January 12, 2007 - 07:33 AM

#3

I just had the valves checked and they said it was in there. But, clueless why it was not working.

  • Florida_426

Posted January 12, 2007 - 07:35 AM

#4

I would agree with that statement, but I would plyy the cam cover and look before I accused them of screwing me over. If the 450DCM or a Hot Cam is installed check the detent balls and the weights for sticking it in the closed position. I sure hope you get this one straightened out to your advantage.

Bill

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  • Pincushion

Posted January 12, 2007 - 07:37 AM

#5

Does the bike get stuck at TDC when you kick it without using the lever? If so, there is no decomp cam in there. And can you visually confirm the mechanism is actually inserted into the motor and still has spring tension?

  • grassmonkey

Posted January 12, 2007 - 07:50 AM

#6

Does the bike get stuck at TDC when you kick it without using the lever? If so, there is no decomp cam in there. And can you visually confirm the mechanism is actually inserted into the motor and still has spring tension?


The bike very difficult to kick over. It does not get stuck at TDC, you can kick but it takes my 205lbs to turn it over. My mechanic verified that it was in there. He thought that it might be a WR450 cam. Could this be a problem?

Or, could it be a problem with the way that it is installed? The mechanic did not pull the cam to check its functionality. Wouldn't that be a procedure to do when we adjust the valves?

  • Pincushion

Posted January 12, 2007 - 08:05 AM

#7

What year and bike do you have? 400 or 426?

I've never heard of anyone using the WR cam in the YZ, but that doesn't mean its not possible. And you only need to remove the cams if the valves are out of measurement and require new shims under the buckets.

Could you remove the gas tank, top motor mount and pull the valve cover? Pictures would be great right now.

  • grayracer513

Posted January 12, 2007 - 08:48 AM

#8

Either the WR or the YZ cam could be used, and both should be set up the same as to timing. But if the cam is advanced, it would render the AD system useless by virtue of letting it close the exhaust valve too early. The previous owner may also have ground the pin down for one reason or other, or the unit could be malfunctioning, although that rarely happens.

There are a couple of things to understand about this setup. The first is that it is not a full compression release; it's a compression reducer. It doesn't eliminate compression the way that the manual lever does, but it cuts it from 12.5:1 to around 7 or 8:1 so that it can be more easily cranked. Compression will still be felt, and even though it's lowered, healthy rings will still demand that it be kicked through, as opposed to simply pushing.

The way it works is that there is a flyweight attached to the exhaust cam which has a spring on it that retracts the weight at speeds below 600-700 rpm or so. Speeds higher than that force the weight outward. As the weight retracts, the internal linkage extends a pin from the cam. As the engine passes BDC on the start of the compression stroke, this pin comes to bear on the left exhaust valve lifter and raises the valve just off its seat. As the engine nears about 45-60 degrees BTDC, the cam rotates far enough that the pin slips off the lifter, the valve re-seats, and effective compression starts at that point. It's a lot like there was a hole in the cylinder 1/3 of the way down from the top.

You can check all that for function and see what you can determine is not happening. Also check the cam timing. It's important to know what cam was used. After market cams made for a 426 will have the timing marks in the right place for a 426, and they should be timed according to those marks. Yamaha OEM 450 cams, whether YZ or WR must be timed according to the instructions below:

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

  • grassmonkey

Posted January 12, 2007 - 04:19 PM

#9

WOW.....Now there is some knowledge...just as I expect on this forum. Thanks grayracer.

It will be a few days before we can get to this. I will update accordingly.





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