Hi Comp. Piston verses Auto Decomp. Cam


9 replies to this topic
  • BergRyder

Posted March 05, 2006 - 10:18 PM

#1

I just had a high compression (13.5 :1) piston put in my YZ426, I have both HotCams with the auto decomp function. The thing is, that with the high comp piston, the auto decomp cam doesn't take away enough of the compression to easily kick over, like it did with the stock piston. Now I have put the manual decompression assembly back on and taken out the thumper talk plug. Is this to be expected, Have other people had the same experience?

  • grayracer513

Posted March 05, 2006 - 11:36 PM

#2

I have heard this complaint from other users of Hot Cams, whether installed with a higher compression piston or not. The timing of the re-seating of the lifted left exhaust valve is important to the auto decomp function, and apparently, they don't always get it right. I would contact them.

  • BergRyder

Posted March 06, 2006 - 12:10 AM

#3

The HotCam Auto Decomp worked fine with the stock piston.......
What happens when you put a high comp piston in the YZF450 ( that has auto decomp cam stock) is it much harder to kick over or doesn't it make any difference?

  • jbrooks26

Posted March 06, 2006 - 01:33 AM

#4

From what I understand, the hotcam has a significantly shorter decompression pin than the stock yamaha autodecomp cam. This means that the stock cam will open the valve sooner and farther and hold it longer. I could see where your problem could occur with a high comp piston, since the compression will start to build sooner I think. I don't know if there is anything you could do about it though? Maybe get a stock cam and try it to see if it is any easier to kick. Hope this helps.

Josh

  • 642MX

Posted March 06, 2006 - 10:41 AM

#5

I've got the high comp piston and the hot cams. It takes a good hard kick to light it, but I don't need the decompression lever put back on. Are you sure its timed right?

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  • BergRyder

Posted March 06, 2006 - 02:59 PM

#6

I've got the high comp piston and the hot cams. It takes a good hard kick to light it, but I don't need the decompression lever put back on. Are you sure its timed right?


When I first put the motor back in the frame I could not get the bike to start so the fact that it was hard to kick over just added to my problem. When I have the jetting sorted and it is easier to start I may not need the lever. However the extra effort required with the high comp piston is significant.
I hope it's timed properly, the local pro engine builder did the work.

  • Chazlom

Posted March 06, 2006 - 10:27 PM

#7

I have had 12.5:1 and 13.5:1 pistons and have had my Hot Cams out numerous times, I have noticed a big difference if the cams are off just a little, not just with the decompression but how easily the bike starts.
I just put a new 13.5:1 piston in and it took several trys to get the timing just right, now the decompression feels the same as stock.

  • Birdy426

Posted March 06, 2006 - 10:49 PM

#8

I have a Wiseco 13.5:1 with both HotCams, and starting is as easy as stock.

  • kelstr

Posted March 07, 2006 - 03:33 AM

#9

I have heard this complaint from other users of Hot Cams, whether installed with a higher compression piston or not. The timing of the re-seating of the lifted left exhaust valve is important to the auto decomp function, and apparently, they don't always get it right. I would contact them.

gray is correct on this issue,( but generally any aftermarket cam will not sit in any motor on the lobe center it should :thumbsup: )
generally the hot cam will not sit in the motor on the correct lobe center as stock , therefore not only the cam timing but also the decomp timing is off .
(this is why you guys need to understand how to check your lobe centers and know where you were stock and were your decomp system came on and off at vs where your new cams are and you will see what is going on)
and most builders just miss the boat on this,
( most shops wheather it be auto or bike have no idea or clue on checking or correcting camshaft lobe centers, nor do they understand port timing on two strokes, it is sad but it is what it is ) if you are savvy and have screwed up many times before ( ask me how i know ) you will play with the stock motor , record the lobe centers on the stock cams and note where the decompresser comes on and off at.

that way when you are building a motor and you have a lobe center in mind you want to start at you at least also know where you are and how far off you are on the decomp system compared to stock.

and it does not take but acoupple degrees one way or the other to dam near make the decomp not work.

i have built some of these over 14.1 and if you get the decomp system timed correctly it will turn over just as easy as stock,

some of the kit yamaha race cams are very hard to get where you want them ( you are trying different lobe centers to get the motor to really work and making lobe center changes) so naturally the decompresser is not going to be where you want it while testing,
so you hook up the manuel release until you really get the cams where you want them and then you go back in and mod the pin to work just where you want it to .
real normal deal :thumbsup:

  • BergRyder

Posted March 07, 2006 - 07:47 PM

#10

Update after hearing from the mechanic: Because the bike is used for supermoto racing the engine builder adjusted the exhaust cam timing for the max top end. If the decomp function is more important to me (it isn't) I could adjust it back. He just figured mo' power over easy kickin'.





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