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RD73

450 Cam problems

11 posts in this topic

First, I would like to thank you guys that have contributed to this forum. I recently did the BK/GB Mod by following the instructions. Worked great! :ride: Next, I decided to try the 450 cam in my 2001 yz426.

I put in the 450 cam, installed the proper shims, checked clearances and put it back together. The bike fired for a couple seconds, then died. Now it just backfires when trying to start and does not run.

I thought the timing must be off, so I checked the timing marks and cam positions (lined up with the "I" not "H"). Timing is correct and cams look like those in the picture for the mod. My cams have 14 pins between marks, but I noticed one guy said that he only had 13. So, I turned the exhaust cam so that 13 pins were showing, but that looked way off from the right side of the bike. Moved back to 14 pins and cam positions. The cams look right (I even rechecked clearances and cam positions after installing the cam chain tensioner). One shim was really tight. Could that cause a valve to stick open?

Any ideas??? :applause:

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I ran into the same problem with setting up the 03 cam on my 00 426. The first thing I would do is re-check the clearance on your exhaust and intake to make sure they are within spec. What I did to get my bike to fire up and run was to adjust my fuel screw all the way in and then adjust it out 1 1/2 turns. I kicked it over a couple of times and it fired up. I let the bike warm up and made some fine tune adjust on the fuel screw to get rid of the hanging idle. I would also recommend investing in the easy adjust fuel screw this makes fine tune very easy. I hope this help you get your bike running so you can go tear up some dirt. :applause:

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13 pins for WR timing and 14 for YZ timing. Are you sure they gave you the correct cam? Can you verify the part number?

Whats your valve clearance?

I've had mine apart twice. Zero abnormal wear and no issues reassembling. It's very straightforward if you have the .PDF instructions to follow.

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When doing the 450 cam conversion to a 426, the procedure is to first be sure the intake cam is correctly timed at TDC. Then the exhaust cam is positioned so that there are 14 pins between the two 12:00 o'clock reference marks on the cams. In this position, the cam marks will appear to be about one tooth too far forward. The stock 450 cam timing marks are otherwise useless because the 450 head is so different from that of a 426. Confusion sets in when someone mentions that a 450 sets up with 13 pins across that gap. It does, but only in a 450 head. Then there's the instructions for changing WR timing to YZF timing, but ignore all that. You want your intake timed right, and then 14 pins to the exhaust cam.

If your timing's right, and your clearance is good, it's time to look elsewhere. Drain the carb, let it refill, and drain it a second time. s3706 is probably onto something. The engine may need to be tweaked just a little to get the carb right with the new cam. If when you say "backfire", you mean that it pops out the pipe when you crank it, you might try breaking the throttle open just slightly, or raising the idle a little to get it going. The pop means it's pumping fuel through and into the exhaust without lighting it. It may want just a little more air.

It won't hurt to check or change the plug, either.

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s3706 and grayracer I will give those suggestions a try. Thanks. There are 14 pins betweent the two cams and the exhaust cam looks correct from the right side of the bike.

I thought about the carb, but the bike was running fine before the cam swap so I thought it must be something I did. Grayracer, you are right it is a "pop" out the pipe rather than a backfire. I just put in a new iridium plug.

Pincushion, the part number is 5TA-12180-00-00. Clearance is .21 and .22. On the tight side, but within spec. I followed the PDF instructions on assemble and when I tore it down and reassembled it.

If the shims were too large, could that be the cause? It did take some force to get the feeler gauge under the cam, but I checked the intake cam and used that effort as a guide (since I had not checked clearances before).

Also, I zip tied the cam chain to the intake cam. Could the chain have slipped? Timing looks right, on the "I" the intake cam is correct. After setting the intake cam on TDC, I then installed the exhaust cam per the PDF.

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My bike was running fine before I did the cam swap. I was having the same problem where you would kick it a couple of times and then it would pop out the exhaust. Sometimes i would kick it and it would start i would let it idle for a minute or so and then try and start it again and it would pop. It sounds like everything is set correctly on the timing. I still think it is the carb setting.

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That's the correct part number which eliminates that aspect. When you say 'It did take some force to get the feeler gauge under the cam', how much force are you talking? When I check clearance I always feel it should only be as resistant as pulling a sheet of paper from under a fridge magnet.

I can almost gurantee you that you'll need to reshim the exhaust after swapping them out. I dont think many (if any) people did not go to larger shims. I went from 178 & 180 to 190 & 190 on my 2000. You'll get through this man........

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s3706, hearing your description gives me some hope. The fuel screw will be my first adjustment.

Pincushion, I am not ruling out the shim issue, but I did go from a 181 and 179 to 195 and 200. The shims did need to be larger. When talking about the force, it is fairly easy (as you describe, pulling paper from under a magnet) to pull the gauge out. However, it does take more force than that (but not a tremenous amount more) to get the gauge to go under the cam.

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Sounds like most everything is ruled out except your carb settings. Now the real fun begins :applause:

Write down your baseline settings before you change anything and only change one thing at a time. Keep notes of changes that made an improvement and also changes that did not help.

Start with the fuel screw, then pilot circuit. Do you know whats in there now?

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Thanks guys. Adjusted the fuel screw a few times and it fired right up! Thanks!!!

I can tell the difference in the power with the 450 cam. Nice!

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