GN250 Valve Timing for Dummies; Anyone know how to get the cam shaft just right?

[h0dgep0dge]

I've just got done replacing my timing chain (fucker was loose as all hell) and now I'm having trouble getting my valve timing just right. The manual says to set the cam sprocket locating pin at 1 o'clock, but when set as close as I can get it, the timing just looks completely wrong. That is, the cams don't look like they're where they should be, and the rockers are clamped down real hard on the inlet, and have about 5mm of play on the exhaust. I've done some research, and I've heard a few people talking about marks engraved on the cam sprocket, but I cannot for the life of me find it. Should I just do what the manual says, adjust the valve clearance, and see how it goes? Does anyone have any pro tips for lining the cam shaft up more positively than estimating 1 o'clock?

After going to take another look, I noticed that the magneto rotor indicates TDC twice for every rotation of the cam shaft. Does this mean that there are 2 positions that are correct for the timing? Every 180 degrees around the shaft? I'm so confused

[h0dgep0dge]

After pondering this for a while, I've decided that the cams probably looked wrong because I had rotated the magneto rotor around to top dead center again, but the cam shaft was 180 degrees out from where I expected it to be. I suppose I'll try setting the cam shaft to 30 degrees, and make sure the cam shaft is where it should be before setting the valve clearance.

[spanner spinner]

I've just got done replacing my timing chain (fucker was loose as all hell) and now I'm having trouble getting my valve timing just right. The manual says to set the cam sprocket locating pin at 1 o'clock, but when set as close as I can get it, the timing just looks completely wrong. That is, the cams don't look like they're where they should be, and the rockers are clamped down real hard on the inlet, and have about 5mm of play on the exhaust. I've done some research, and I've heard a few people talking about marks engraved on the cam sprocket, but I cannot for the life of me find it. Should I just do what the manual says, adjust the valve clearance, and see how it goes? Does anyone have any pro tips for lining the cam shaft up more positively than estimating 1 o'clock?

After going to take another look, I noticed that the magneto rotor indicates TDC twice for every rotation of the cam shaft. Does this mean that there are 2 positions that are correct for the timing? Every 180 degrees around the shaft? I'm so confused

have done a few of these a few notes that might help you. the cam runs at half the speed of the crank which is why you see to tdc twice and it doesn't matter which tdc you use. cam lobes should be pointing down when at tdc and there is either a cut out in the end of the cam which doesn't have the sprocket on that lines up with the head face or a line on the cam sprocket that lines up with the head face at tdc. the marks depend on what year your bike is and if it's a Japanese assembled or Chinese assembled. hope this helps, if it doesn't where are you in Hawkes Bay as I could come and have a look if your not to far away. PM me if you need some more help.

[Akzle]

. Should I just do what the manual says, adjust the valve clearance, and see how it goes?

no.

But also yes, because that would be hilarious.

You need tdc on compression stroke.

Turn everything over by hand slowly before hitting it with the starter.

[h0dgep0dge]

You need tdc on compression stroke.

How do I know which stroke it's on?

[Akzle]

How do I know which stroke it's on?

cam lobes will be at rest on both sides.

[h0dgep0dge]

have done a few of these a few notes that might help you. the cam runs at half the speed of the crank which is why you see to tdc twice and it doesn't matter which tdc you use. cam lobes should be pointing down when at tdc and there is either a cut out in the end of the cam which doesn't have the sprocket on that lines up with the head face or a line on the cam sprocket that lines up with the head face at tdc. the marks depend on what year your bike is and if it's a Japanese assembled or Chinese assembled. hope this helps, if it doesn't where are you in Hawkes Bay as I could come and have a look if your not to far away. PM me if you need some more help.

It's an '87, Japanese made. I found a notch on the end of the cam shaft, so I lined that up with the edge of the head (and also put the locator pin at around 1 o'clock), put everything back together, did my valve clearance, put the fuel tank back on. I start trying to crank the thing, and I'm getting no ignition at all. I'd check to see if the spark plug is dirty, but I haven't the tool to get my spark plug out. I've run my battery flat a few times trying to start it, hopefully it just needs a bit more time?

[h0dgep0dge]

Scratch that. Just needed the choke. Took it out for a test drive, and it seems to be running pretty well. It's been off the road for a few months, and I don't know if it's been louder since I've done some work on it, or I just forgot how loud it was. Perhaps I have an exhaust leak. Either way, happy to have it running again.

EDIT: Of course, knock one problem down, a new one pops up. I've now had a complete electrical failure. I suspect it's something to do with the ignition, I'll look into it tomorrow. Might start a thread for it at some stage.

[Big Dog]

My wife's old gn did a total electrical failure once or twice after being neglected for a few weeks. Once it was spiders in the kill switch. Head light went nothing else.
The other it was spiders in the main fuse housing. No lights.

But first check would be I'd your battery holding charge. Bike batteries can be temperamental in winter. Especially if neglected.

Sent via tapatalk.

[h0dgep0dge]

Bike batteries can be temperamental in winter. Especially if neglected.

My battery is most certainly quite neglected, but the problem turned out to be a blown fuse, popped in a new fuse and I was good to go. I should have gone straight to checking fuses, sometimes I try to be too clever for my own good. The bike is now running like it used to, but it has this awful ticking noise (the reason I looked at the timing chain in the first place, there was a thread about it months ago). I'm pretty sure my valve clearance is good, so I'll have to keep tinkering.

[Katman]

The bike is now running like it used to, but it has this awful ticking noise (the reason I looked at the timing chain in the first place, there was a thread about it months ago). I'm pretty sure my valve clearance is good, so I'll have to keep tinkering.

I think this might be your problem.

.....and have about 5mm of play on the exhaust.

[h0dgep0dge]

I think this might be your problem.

That was before I'd redone the valve clearance, it should be right now, but the sound is still there.

[sidecar bob]

That was before I'd redone the valve clearance, it should be right now, but the sound is still there.

I'm highly amused that you only worked out after you put it back together that a cam rotates at half crankshaft speed. Top marks for going headlong into a piece of machinery you didn't understand the fundamentals of at all.
Secondly, it's most likely got a worn rocker & cam lobe.

[h0dgep0dge]

Top marks for going headlong into a piece of machinery you didn't understand the fundamentals of at all.

Not sure if that's intended to be spiteful or friendly. I'll fully concur that I don't know what I'm doing, this is all a learning experience (see the title of the thread), and better to learn it on a 2 bit learner bike than when I get an actually nice bike.

Secondly, it's most likely got a worn rocker & cam lobe.

I'm heading towards an upper end rebuild anyway, I guess I'll start there.

[sidecar bob]

Not sure if that's intended to be spiteful or friendly. I'll fully concur that I don't know what I'm doing, this is all a learning experience (see the title of the thread), and better to learn it on a 2 bit learner bike than when I get an actually nice bike.



I'm heading towards an upper end rebuild anyway, I guess I'll start there.

Not spiteful. I think its awesome that you got right in there. Im betting you're 100 percent Kiwi & at least third generation.
For what its worth, you can buy 300cc kits & heads on Aliexpress for them.