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Primary coil failure on 2008 YZ450F?

11 posts in this topic

I have had my -08 YZ450F from new. Today, after 155 hours of riding(total, not today) I opened the throttle and the bike coasted to a stop, like I hit the kill switch. After that, the bike would not start.

After trying things, I checked the electricals. Everything was in spec except for the primary coil in the spark plug boot(or whatever the thing is called). That read 0.03, and the range is 0.08-0.10.

Is this just incidental or something that would cause the bike to die completely (there was no sputter, no rough running before the engine totally shut down)? I will probably try and get a new boot since the only other thing I can think of is the CDI and that is pretty expensive.

There is still something happening electrically, since my hour meter (wire around said boot) still starts flashing the hourglass icon when I push the kick starter.

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I have had my -08 YZ450F from new. Today, after 155 hours of riding(total, not today) I opened the throttle and the bike coasted to a stop, like I hit the kill switch. After that, the bike would not start.

After trying things, I checked the electricals. Everything was in spec except for the primary coil in the spark plug boot(or whatever the thing is called). That read 0.03, and the range is 0.08-0.10.

Is this just incidental or something that would cause the bike to die completely (there was no sputter, no rough running before the engine totally shut down)? I will probably try and get a new boot since the only other thing I can think of is the CDI and that is pretty expensive.

There is still something happening electrically, since my hour meter (wire around said boot) still starts flashing the hourglass icon when I push the kick starter.

I have seen stators check ok and they are actually bad. Usually in most cases with these electrical problems its the stator rather than a cdi or coil cap

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I have seen stators check ok and they are actually bad. Usually in most cases with these electrical problems its the stator rather than a cdi or coil cap
Often, there will end up being a bare spot within one of the coils, or a broken wire that tests OK when cold but moves when heated and causes a fault in real time.

If possible, test the stator, etc., immediately after it quits.

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Long story short, re-tested the plug boot and the result was 0.3, not 0.03. Shorted the multimeter wires and the result was 0.2. Tested for spark in the dark garage and the spark was there.

Pulled the flywheel to check that it was in the correct position and decided to replace the cam chain at the same time. Found out the middle intake valve had broken near the top of the stem. Pulled the head and no visible damage to the head or piston (the valve had stayed in place). Seemed that the valve had failed with no warning (could not see blow-by behind the valve). These were the original valves.

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It did seem a bit light but I was cranking it over standing on the pegs (with the bike on the stand) and a bunch of adrenaline in my system so I thought it was just mental. It takes a pretty light kick anyways and has been starting on the first kick so I don't have much experience.

Now, what should I replace? I'm thinking of getting a steel intake valve set. Should I get new valve guides? Is there reason to assume the exhausts are still ok? All valves were still in spec except for the one that snapped in the middle.

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You need to have your head examined. Your bike's head, that is. The valve head bouncing around in the top end will have done some damage unless you are the world's luckiest motorcyclist. Typical damage can include bashing a dent in the valve seat, bending other valves, damage to valve guides, damage to the piston crown and ring grooves, damage to the cylinder wall, and so on.

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The valve broke at maybe half an inch from the top of the stem. The head stayed attached to the stem. I pulled the head and you can honestly maybe just say that the valve came into contact with the piston (from looking at the piston and head).

But the valve still moves smoothly along its normal range through the guide and what not. I did not pull it because the break is irregular and I fear it might damage the guide in case the guide might be salvageable (my biggest suspicion is that maybe the valve stem did get bent and damaged the guide and seat).

I will get the intake valves replaced and at the same time the shop will take a look at the guides and seats as well. BTW, where can I find the angles for the seats? I could not find them in the user manual.

Thanks for the replies.

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A good machine shop should know the angles already. I'm reasonably certain that the face angle of the valve is 45 (+0, -0.5), and the seat is 45 with a 30 and 60 under/over. Should also be listed in the dealer service manuals.

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A good machine shop should know the angles already. I'm reasonably certain that the face angle of the valve is 45 (+0, -0.5), and the seat is 45 with a 30 and 60 under/over. Should also be listed in the dealer service manuals.
correct...45-60-30. Wow I havent seen a YZ valve on a 450 break there...a yz 250f yes...they had a recall on those....I have seen a valve spring break on a 450 and that bike was ridden for about 15 hours beyond that with no damage.

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