Auto Decompression Cam Mod Questions


18 replies to this topic
  • Aris

Posted October 03, 2006 - 07:13 PM

#1

I apologize to everyone if these questions have been asked a gazillion times in the past. I swear that I did a search, but I'm still confused. I'm new to this whole thing, and I keep confusing myself. I also downloaded and read the pdf instructions that someone put together here. I just wanted to double check my understanding before moving on. My bike is a 2001 WR426. The previous owner told me that he set it up with the YZ timing.

First, here are pics of my stock cams after I pulled the cover off. All my questions reference these pics:

Posted Image

Posted Image

Here are my questions:

1. Can you tell if I'm at TDC based on this pic? The manual says to align the line with the notches in the hole, but the line in the manual looks different than the actual line. When I look in the hole, I see 'HI' inside. I lined the 'I' with the notch. I'm assuming that's TDC, but I'm not sure.

2. Does the YZ timing mean that the stock exhaust cam was rotated a couple links so that there are only 12 links between the marks on the cam gears?

3. I read that the WR timing should be 14 links between marks. Is this true?

4. I bought the 03 YZ450 autodecompression cam. Is that different than the 03 WR450 cam?

5. Should I install the new cam the same as the existing cam (12 links between dots), or should I put in 14 links in between?

6. What are the clearances supposed to be with the cam rotated like this? Is it the same as stock? (0.10 - 0.15 mm)

I appreciate any answers that y'all can give me.

  • jbrooks26

Posted October 03, 2006 - 08:18 PM

#2

I apologize to everyone if these questions have been asked a gazillion times in the past. I swear that I did a search, but I'm still confused. I'm new to this whole thing, and I keep confusing myself. I also downloaded and read the pdf instructions that someone put together here. I just wanted to double check my understanding before moving on. My bike is a 2001 WR426. The previous owner told me that he set it up with the YZ timing.

First, here are pics of my stock cams after I pulled the cover off. All my questions reference these pics:

Posted Image

Posted Image

Here are my questions:

1. Can you tell if I'm at TDC based on this pic? The manual says to align the line with the notches in the hole, but the line in the manual looks different than the actual line. When I look in the hole, I see 'HI' inside. I lined the 'I' with the notch. I'm assuming that's TDC, but I'm not sure.


Yes, it appears that you are at TDC. You are correct with the I lined up with the notch.

2. Does the YZ timing mean that the stock exhaust cam was rotated a couple links so that there are only 12 links between the marks on the cam gears?


Yes, sort of. The stock WR cam was rotated 1 pin clockwise to get YZ timing on the bikes before 03.

3. I read that the WR timing should be 14 links between marks. Is this true?


No, stock WR timing on the 01 was 13 pins between the timing marks. You are getting confused a little here with the upgrade to the 03 auto decomp cam.

4. I bought the 03 YZ450 autodecompression cam. Is that different than the 03 WR450 cam?


Only slightly, the only real difference is the clocking of the cam gear and a slight length difference in the decompression pin. If you want YZ timing with auto decomp then you need the YZ cam. If you want WR timing with auto decomp then you need the WR cam.

5. Should I install the new cam the same as the existing cam (12 links between dots), or should I put in 14 links in between?


No, the new YZ cam will need to have 14 pins between the links with the intake cam lined up with the top of the head like it states in your manual and the engine at TDC. This will compensate for the slight timing and gear tooth pitch differences between the pre 03 cam and the newer cam.

6. What are the clearances supposed to be with the cam rotated like this? Is it the same as stock? (0.10 - 0.15 mm)


The valve clearances will be the same as stated in your manual.

I appreciate any answers that y'all can give me.


No problem, I hope this helps clear things up for you.

Josh

  • Aris

Posted October 03, 2006 - 09:13 PM

#3

Thank you very much for the quick answers, Josh. That really helps. I think that I understand now. It sounds like I have done everything correctly up to now. I think that I'm good to go now.

By the way, I remember reading in one of the older threads about someone recommending that you stuff a shop rag somewhere as preventative for some reason. Maybe it was to keep stuff from falling in the case? Do you know anything about this?

  • jbrooks26

Posted October 03, 2006 - 09:41 PM

#4

Yes, you want to put a shop rag down in the hole where the cam chain goes. This is to keep the circlips from falling out of the cam caps and down into the case. They can be hard to retrieve. Just be careful and you should be fine, but the rag is a good idea. Follow the steps on this link and you won't have any problems. There is a lot of good information here for you. The site is based on the 250, but most all of it will cross over. Hope this helps.

Josh



http://www.thumperfa...m/yz_timing.htm

  • Aris

Posted October 03, 2006 - 09:54 PM

#5

Thank you very much, Josh!! That link is outstanding!

  • jbrooks26

Posted October 03, 2006 - 09:58 PM

#6

No problem, if you need anything else just ask. Just go slow and be sure of what you are doing and you will be fine. Make sure to turn the engine over by hand several times before you try to start it to make sure that there are no interference issues. It should roll over freely with no binding or noises. Remove the spark plug to make it easier to turn and hear.

Josh

  • agermaney

Posted October 04, 2006 - 07:41 AM

#7

Some great info there Josh. Without wanting to look as if I'm hijacking the thread, any chance of a brief description on how to find TDC on the firing stroke. I have my head and barrel off at the moment and have just realised that I've moved my crank during various inspections.
I no longer know where firing TDC is.

Andy.

  • byggd

Posted October 04, 2006 - 07:46 AM

#8

Here are the instructions I used and the dyno runs.
http://s18.photobuck...gAnch=imgAnch25
http://s18.photobuck...gAnch=imgAnch16

  • jbrooks26

Posted October 04, 2006 - 09:58 AM

#9

Every stroke is a firing stroke. The difference is when the valves are open, if you set your piston at TDC, and then time your cams off of that you can't go wrong. Hope this helps

Josh

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • agermaney

Posted October 04, 2006 - 11:18 AM

#10

Thanks Josh, I've learnt a lot. I was thinking auto engines.

Andy.

  • Yama4Stroker

Posted October 04, 2006 - 11:37 AM

#11

Josh, I'm considering doing the YZ timing on my WR426. After reading you directions, it makes more sense to me. But what are the differences between the two, as far as riding and performance. And is there any re-jetting involved with this mod?

  • jbrooks26

Posted October 04, 2006 - 03:11 PM

#12

The biggest thing I have noticed is that my bike pulls harder all over. I am not a professional rider, and I can't tell you exactly what the differences will be. I can tell you that I have not had any second thoughts about going back. As for jetting, you may find the need to re-jet and you may not. It just depends on how accurate your jetting is right now. You will not be sorry, I say go ahead. And just to clear things up, some of what you read on this thread is mine, but any link you see here was created by someone else and I just point out the good info there in. If you have any questions about the procedure don't hesitate to ask, I am always glad to help when I can. Hope this helps.

Josh

  • Aris

Posted October 09, 2006 - 08:26 PM

#13

No problem, if you need anything else just ask. Just go slow and be sure of what you are doing and you will be fine. Make sure to turn the engine over by hand several times before you try to start it to make sure that there are no interference issues. It should roll over freely with no binding or noises. Remove the spark plug to make it easier to turn and hear.

Josh




Josh, I just wanted to thank you again for all your help. I finished the cam swap, and everything went in much easier than I expected. The bike starts so much easier now. :devil:

The only mistake I made happened when I put in the Thumper Talk decompression plug. I didn't bother reading the instructions, and I stripped the retaining bolt. Nothing more fun than stripping a bolt then reading the instructions that say to be careful torquing the bolt because it strips easy. :thumbsdn:

  • jbrooks26

Posted October 09, 2006 - 09:17 PM

#14

Congratulations, I am sure you won't be disappointed with the change. Glad it all went well for you, let us know if you ever need anything else.

Josh

  • hozhead

Posted October 09, 2006 - 09:25 PM

#15

Every stroke is a firing stroke. The difference is when the valves are open, if you set your piston at TDC, and then time your cams off of that you can't go wrong. Hope this helps

Josh



Not to nitpick but, that's not true.
It's a four stroke bike.
Stroke 1 = down = intake
Stroke 2 = up = compression
Stroke 3 = down = power
Stroke 4 = up = exhaust

  • Aris

Posted October 09, 2006 - 09:35 PM

#16

I have to admit that I was actually thinking about selling the bike before this mod because I thought that it was just too much of a pain in the ass to start. I'm keeping her now.

  • jbrooks26

Posted October 09, 2006 - 10:49 PM

#17

Not to nitpick but, that's not true.
It's a four stroke bike.
Stroke 1 = down = intake
Stroke 2 = up = compression
Stroke 3 = down = power
Stroke 4 = up = exhaust


You are correct, I should have been more clear. My point remains correct however, the engine fires every revolution. There are 2 strokes per revolution of the crank in a 4 stroke engine. They are as you stated above. However, the ignition coil does not care if it is a compression or exhaust stroke, it fires every revolution, even on the exhaust stroke. I hope this clears up any confusion.

  • hozhead

Posted October 10, 2006 - 05:28 AM

#18

Perfect. That does clear it up. I just didn't want anyone to get confused by the original statement.

  • jbrooks26

Posted October 10, 2006 - 02:05 PM

#19

No problem, I prefer to be corrected when I am wrong. I also prefer for people to tell me when I have confused them so that I can clear it up. Nothing worse than passing on bad information. Thanks

Josh




 
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