Bike Won't Start

So after replacing the crankshaft in my 03 YZ450 I got it together tonight and could not start it. I bought the bike seized up so I don't know anything about it before hand.

So far I have checked spark and I do have a strong spark. I am wondering if it may be a cam timing issue. Is it possible to have the spark at the wrong time of the cam sequence, could it be 180 off in relationship to the valve events. When setting my timing I aligned the flywheel with the mark and then installed my cams. This is my first time but I am pretty sure the timing between cams is correct (12 pins between them with a dot on each level with the horizon and also a dot pointing straight up).

When trying to start the bike it does not even attempt to fire up. I have tried a little starting fluid also with no luck which leads me to believe that it is bad ignition timing instead of a carburetor problem.

I guess the manual doesn't explain it well but it seems to me that the flywheel passes the pickup every revolution but the spark plug fires every other revolution, so if you are assembling your engine from nothing how do you know if you are at TDC when it should fire vs. TDC when it shouldn't.

Well thanks for any help you can give me, and let me know if you have any questions regarding the situation.

It fires every revolution so it's not possible to have it 180 out. As long as it is at TDC when you set the timing you should be fine.

I just replaced the cylinder and piston on mine last week (also a 03 450), I have 14 pins between the timing marks, not 12. As long as the cam lobes are facing outward on both the intake and exhaust, you should be all set. I've seen pictures of cams with both 13 and 14 pins between on the 450, but never 12 pins.

It is very possible that the valve timing is off, are the marks lined up with the top of the head? I would think it is the same as the 426.The pins are referenced usually because we run cams that arent intended for the 426.

I got it running this morning. The cam timing was off. The crank must have moved while we were setting the timing initially and also I had it stuck in my head that there were supposed to be 12 pins between the gears. So far everything seems good.

This is my first four stroke, I traded my 125 for it and all I can say is wow. I don't think I ever did more than half throtle due to break in, but this thing rips. Is it possible to keep the front wheel on the ground in first gear? Well thanks for the help.

Good deal :thumbsup: And yes the 450 does rip!

Did you happen to notice where you ended up on pin count?

I'm pretty sure it ended up at 13 with a new chain.

13 is the right pin count for a YZ450 using cams for a YZ450, BUT it's the wrong way to time the cams. That will only tell you if the two cams are in the correct relationship to each other. The important thing is that each cam be timed correctly relative to the crankshaft. To do that, the crank is positioned at TDC, and with one mark on the exhaust cam at 9:00 o'clock and the other at 12:00, the 9:00 mark is aligned with the top gasket surface on the head. The front run of chain must have no slack in it when this is done. The intake cam is then positioned so that the I mark aligns with the gasket flange, again with no slack in the chain between the crank and exhaust cam, or between the two cams. Then set the chain tensioner and double check.

The fact that the pin count is correct between cams is wonderful, but means nothing in terms of whether the cam timing is right or not. You could have the same 13 pins showing with the crank at BDC as when it was timed right.

13 is the right pin count for a YZ450 using cams for a YZ450

My cam lobes line up better with 14 pins in between the timing marks. The bike starts (most times one kick) and runs fine. I'm guessing maybe one of the cam gears is off a bit. Either way, the intake lobe seems to be a little high at 13 pins. Next time I have the cover off I'll get a pic of the cam timing in both positions.

What effect would I have trying it with the intake pointing more towards 10 o'clock?

You can't hope to get anywhere near the right timing by eyeballing the cam lobes themselves. And before I forget, how old is your cam chain? Cams that just don't want to align correctly in either of two positions can be an indication of chain wear, and possibly crank sprocket wear, and the crank sprocket is not replaceable separate from the crank.

Anyway, the right way is as I described it above. It's not at all uncommon for the marks not to line up perfectly, but they should be closer to it in one position than in another. The marks, and not the appearance of the lobes is the index that was designed to be used.

Rarely, a cam sprocket will loosen and slip a little on the cam, but when this happens, it always slips in the direction of rotation, which retards the cam. If you suspect this, the cam timing should be verified with a degree wheel. Read here to see how that works.

You asked about rotating the intake toward 10:00. That would advance the cam timing, which usually will increase low end grunt at the expense of top end. But let's keep two things in mind. First, there is very little clearance between the valves and piston, and advancing the intake reduces that further. Second, there are only 32 teeth on the cam gears. That means that moving the cam one tooth shifts the cam timing by 22.5 crankshaft degrees, a huge change, and not to be done without cautiously verifying the clearance. Besides, a change that big intake should have a huge effect on engine performance, and if it was in the wrong place, it very likely would not run well at all.

You can't hope to get anywhere near the right timing by eyeballing the cam lobes themselves.

I only made reference to the cam lobes in relation to the timing marks. I've had the cams in and out many times and I'm aware of the installation procedure. The timing marks really only give a reference to the lobes. After all, it's the lobes that open the valves, not the marks.

I replaced the timing chain last year when I installed a new crank assembly (based on the advice given here).

Thanks for link grayracer! I'll add that to the winter checkup list.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now